cpp-yyjson

Ultra-fast and intuitive C++ JSON reader/writer with yyjson backend.

1. Features
2. Requirements
3. Overview
   1. JSON Reader
   2. JSON Writer
   3. Serialization and Deserialization
4. Installation
   1. Using CMake
5. Benchmark
   1. Read performance
   2. Write performance
6. Reference
   1. Namespaces
   2. Immutable JSON classes
   3. Mutable JSON classes
   4. Allocator classes
   5. Serialize and deserialize JSON
   6. Performance best practices
   7. Misc
7. Author

Features

• Header-only
• C++20 range adaption
• STL-like accessors
• Intuitive JSON construction
• Mutual transformation of JSON and C++ classes with
  • compile-time reflection of struct/class field name
  • pre-defined STL casters
  • user-defined casters in two ways
• Minimum overhead compared to yyjson
• Object lifetime safety

Requirements

• C++20 compiler with range supports
  • LLVM >= 16
  • GCC >= 12
  • clang-cl >= 17 (Windows)
  • Visual Studio >= 2022 version 17.5
    • ❌️ MSVC 19.36.32546.0
    • ✅ MSVC 19.38.33141.0
    • ✅ MSVC 19.39.33523.0
    • ✅ MSVC 19.40.33816.0
    • ❌️ MSVC 19.41.34123.0
• yyjson
• {fmt}
• Nameof C++

Overview

The following is an overview of reading and writing JSON using cpp-yyjson. See the reference for details.

JSON Reader

#include "cpp_yyjson.hpp"

using namespace yyjson;

auto json_str = R"(
{
    "id": 1,
    "pi": 3.141592,
    "name": "example",
    "array": [0, 1, 2, 3, 4],
    "currency": {
        "USD": 129.66,
        "EUR": 140.35,
        "GBP": 158.72
    },
    "success": true
})";

// Read JSON string
auto val = read(json_str);

// as_xxx methods return std::optional<T>
auto obj = *val.as_object();

// Key access to the JSON object class
auto id = *obj["id"].as_int();
auto pi = *obj["pi"].as_real();
auto name = *obj["name"].as_string();
auto success = *obj["success"].as_bool();

// JSON array/object classes adapt the range concept
auto list = *obj["array"].as_array();
for (const auto& v : list)
{
    // `write` returns JSON read-only string
    std::cout << v.write() << std::endl;
}

// The range value type of object class is a key-value pair
auto dict = *obj["currency"].as_object();
for (const auto& [k, v] : dict)
{
    std::cout << "{" << k << ": " << v.write() << "}" << std::endl;
}

// JSON array/object to container conversion
auto numbers = cast<std::vector<int>>(list);
auto currency = cast<std::map<std::string_view, double>>(dict);

// Stringify read-only string
std::cout << obj.write() << std::endl;
// -> {"id":1,"pi":3.141592,"name":"example","array":[0,1,2,3,4],
//     "currency":{"USD":129.66,"EUR":140.35,"GBP":158.72},"success":true}

JSON Writer

#include "cpp_yyjson.hpp"

using namespace yyjson;

// Create a new JSON value from primitive types
auto v_null = value();  // Initial value as null
auto v_bool = value(true);
auto v_num = value(3.141592);
auto v_str = value("example");

// Create a new empty JSON array
auto arr = array();
arr.emplace_back(1);
arr.emplace_back("string");

// Create a new empty JSON object
auto obj = object();
obj.emplace("USD", 129.66);
obj.emplace("date", "Wed Feb 1 2023");

// Conversion from range to JSON array class
auto vec = std::vector{1, 2, 3};
auto vec_nst = std::vector<std::vector<int>>{{1, 2}, {3, 4}};
auto arr_vec = array(vec);      // -> [1,2,3]
auto arr_nst = array(vec_nst);  // -> [[1,2],[3,4]]
array arr_rng =     // transformation via range adaptors
    std::vector{1, 2, 3} | std::ranges::views::transform([](auto x) { return x * x; });
    // -> [1,4,9]

// Conversion from key-value-like range to JSON object class
auto kv_map =
    std::map<std::string_view, double>{{"first", 1.0}, {"second", 2.0}, {"third", 3.0}};
auto val_map =
    std::map<std::string_view, value>{{"number", 1.5}, {"error", nullptr}, {"text", "abc"}};
auto obj_map = object(kv_map);
auto obj_kv  = object(val_map);

// Construction by std::initializer_list
auto init_arr = array{nullptr, true, "2", 3.0, {4.0, "5", false}, {{"7", 8}, {"9", {0}}}};
auto init_obj = object{{"id", 1},
                       {"pi", 3.141592},
                       {"name", "example"},
                       {"array", {0, 1, 2, 3, 4}},
                       {"currency", {{"USD", 129.66}, {"EUR", 140.35}, {"GBP", 158.72}}},
                       {"success", true}};

Serialization and Deserialization

As shown above, cpp-yyjson provides conversion between JSON value/array/object classes and C++ ranges and container types recursively. In addition to that, the following additional JSON casters are available (see the reference in detail):

• Pre-defined STL casters (e.g., std::optional, std::variant, std::tuple (C++23 tuple-like)).
• Conversion using compile-time reflection of struct/class if it is available.
• Registration of field names with VISITABLE_STRUCT macro.
• User-defined casters.

Pre-defined STL casters

// cast JSON value from/to std::optional
auto nullable = std::optional<int>(3);
auto serialized = value(nullable);                        // serialize std::optional to JSON value
auto deserialized = cast<decltype(nullable)>(serialized); // deserialize JSON value into std::optional

// cast JSON value from/to std::variant
auto variant = std::variant<std::monostate, int, std::string>("example");
auto serialized = value(variant);                         // serialize std::variant to JSON value
auto deserialized = cast<decltype(variant)>(serialized);  // deserialize JSON value into std::variant

// cast JSON array class from/to tuple-like array type
auto tpl_arr = std::tuple{nullptr, true, "2", 3.0, std::tuple{4.0, "5", false}};
auto serialized = array(tpl_arr);                         // serialize tuple-like array to JSON array
auto deserialized = cast<decltype(tpl_arr)>(serialized);  // deserialize JSON array into tuple-like

// cast JSON object class from/to tuple-like object type
std::tuple tpl_obj = {std::pair{"number", 1.5}, std::pair{"error", nullptr}, std::pair{"text", "abc"}};
auto serialized = object(tpl_obj);                        // serialize tuple-like object to JSON object
auto deserialized = cast<decltype(tpl_obj)>(serialized);  // deserialize JSON object into tuple-like

Automatic casting from/to JSON object with compile-time reflection

(see the reference about available conditions)

struct X
{
    int a;
    std::optional<double> b;
    std::string c = "default";
};

// serialize struxt X to JSON object with field-name reflection
auto reflectable = X{.a = 1, .b = std::nullopt, .c = "x"};
auto serialized = object(visitable);
// -> {"a":1,"b":null,"c":"x"}

// deserialize JSON object into struct X with field-name reflection
auto deserialized = cast<X>(serialized);
// -> X{.a = 1, .b = std::nullopt, .c = "x"}

Field name registration with VISITABLE_STRUCT macro:

// register fields except `c` on purpose
VISITABLE_STRUCT(X, a, b);

// serialize visitable struxt X to JSON object
auto visitable = X{.a = 1, .b = std::nullopt, .c = "x"};
auto serialized = object(visitable);
// -> {"a":1,"b":null}

// deserialize JSON object into struct X
auto deserialized = cast<X>(serialized);
// -> X{.a = 1, .b = std::nullopt, .c = "default"}

User-define caster (see the reference in detail)

template <>
struct yyjson::caster<X>
{
    // convert X to string (serialize)
    inline static auto to_json(const X& x)
    {
        return fmt::format("{} {} {}", x.a, (x.b ? fmt::format("{}", *y.b) : "null"), x.c);
    }
};

// convert struxt X to JSON string with user-defined caster
auto  = X{.a = 1, .b = std::nullopt, .c = "x"};
auto serialized = value(visitable);
// -> "1 null x"

Installation

To use cpp-yyjson, the dependent packages are required to be installed. It is convenient to use vcpkg to install the packages:

$ ./vcpkg install yyjson fmt nameof

Then add the path include/cpp_yyjson.hpp to the include directory of your project.

Using CMake

To integrate cpp-yyjson into your CMake project, simply add the following:

add_subdirectory(<PATH_TO_CLONE_DIR>/cpp-yyjson ${CMAKE_CURRENT_BINARY_DIR}/cpp-yyjson)
target_link_libraries(${PROJECT_NAME} PRIVATE cpp_yyjson::cpp_yyjson)

If you have installed cpp-yyjson via CMake, find_package command is enabled:

find_package(cpp_yyjson CONFIG REQUIRED)
target_link_libraries(${PROJECT_NAME} PRIVATE cpp_yyjson::cpp_yyjson)

Benchmark

Benchmark results are described to compare the cpp-yyjson with other prominent fast C/C++ JSON libraries: yyjson v0.6.0, simdjson v3.0.1, rapidjson #232389d, and nlohmann-json v3.9.1.

The results are obtained on Ubuntu 22.04, INTEL Core i9-12900K with all E cores and HTT disabled, compiled with GCC 12.1.0. The benchmark programs are in the test directory.

Read performance

In each library, the following options are there for reading JSON. By using the appropriate options for your use case, the best performance can be achieved.

In-situ parsing : Modify the input JSON string during parsing. This can be used if the string is writable (and/or padded at the end) and can be discarded after parsing. Therefore, this method cannot be used for a given fixed-length read-only JSON string. Alternatively, you can copy the input string once and use in-situ parsing.
The yyjson and simdjson require some padding at the end of the JSON string for in-situ parsing but the rapidjson does not. For the former libraries, the JSON string must be copied even if it is writable but has a fixed length. The simdjson has two methods for parsing, which are "DOM" and "On Demand". The "On Demand" approach seems to be faster than "DOM" but less flexible because it behaves as a forward iterator like a stream and can only receive padded JSON strings.

Single buffer : Reuse a single pre-allocated buffer or a parser object for multiple parsing. This is suitable for tasks that repeatedly read multiple JSON strings, e.g. API servers.
The yyjson can prepare a pre-allocated buffer and the maximum required size of the buffer can be estimated from the length of the JSON string. The allocator can be given in the same way for the rapidjson, but we need to clear it explicitly after parsing because the buffer will probably not be released automatically (please let me know if I make a wrong manner). For the simdjson, the parser object is reusable to minimize the new allocation cost. Reusing string objects when copying JSON strings for in-situ and padding can also be considered a single buffer.

The benchmarks were performed on each JSON library with all possible patterns with the above options. Classified by the following keywords.

(no mark) : No option.

insitu, pad : In-situ parsing is used/a padded string is input.

dom, ond_pad : "DOM" and "On Demand" parsing for the simdjson, respectively.

single : Reuse the parsing and temporal object as much as possible.

copy : Create a copy of the input string for in-situ parsing or padded string input.

The JSON datasets are from yyjson_benchmark. Measurements are the median time to parse and iterate all elements with 100 repetitions on google benchmark. The time unit is ms and the raw logs are available here.

The cpp-yyjson shows a very good read performance, as same as the original yyjson. A small overhead of cpp-yyjson compared to the yyjson may be from the pointer wrapped by std::shared_ptr.

Write performance

The write performance is measured by the time it takes to create a large array or object and output it as a JSON string. One option when creating a JSON is to make a copy of the string or not. The yyjson and the rapidjson have such as option and make a small difference in speed. The results are obtained by the size of 1,000,000 elements with 100 repetitions on google benchmark. The time unit is ms and the raw logs are available here.

The cpp-yyjson and yyjson show excellent write performance. In some cases, the cpp-yyjson performs slightly better than the original yyjson because it implements an additional range-based conversion to a JSON array and object.

Reference

Namespaces

The yyjson namespace includes the following function and classes. Typically, you will start with them for reading and writing JSON.

Function
yyjson::read	Read a JSON string and return an immutable JSON document class which is alias of yyjson::reader::value

JSON Type
yyjson::value	Mutable JSON value class; alias of yyjson::writer::value
yyjson::array	Mutable JSON array class; alias of yyjson::writer::array
yyjson::object	Mutable JSON object class; alias of yyjson::writer::object

Allocator class
yyjson::dynamic_allocator	Dynamic memory allocator wrapper, available for yyjson >= v0.8.0
yyjson::pool_allocator	Fixed size memory allocator wrapper on the heap
yyjson::stack_pool_allocator	Fixed size memory allocator wrapper on the stack

In internal namespaces, the cpp-yyjson provides JSON value, array, object, reference, and iterator classes. Each internal yyjson::reader and yyjson::writer namespace defines immutable and mutable JSON classes, respectively. Although you rarely need to be aware of the classes provided in the internal namespaces, the reference classes are noted here.

The JSON value, array, and object classes have the corresponding reference (only for mutable classes) and const reference versions of them as shown later, such as value_ref, const_value_ref, array_ref, const_array_ref, and so on. The reference classes have member functions with almost the same signature as the normal versions. The difference between a normal value class and a [const_]value_ref class is whether they have their data ownership or not. The (const) reference JSON classes appear in return types of member functions of the JSON classes. This is to maximize performance by avoiding copying.

Note

The reference class refers to data in the owner, so its lifetime should be noted.

Immutable JSON classes

Immutable JSON classes defined in yyjson::reader namespace are derived from the yyjson::read function that reads a JSON text string. It may not be necessary to use these classes directly.

yyjson::read

Read a JSON string and return an immutable JSON value.

See the reference of yyjson for the information on reader flags.

yyjson::reader::value read(std::string_view json_string, [std::size_t len,] [Allocator alc,] ReadFlag = ReadFlag::NoFlag);
yyjson::reader::value read(const std::string& json_string, [std::size_t len,] [Allocator alc,] ReadFlag = ReadFlag::NoFlag);
yyjson::reader::value read(const char* json_string, [std::size_t len,] [Allocator alc,] ReadFlag = ReadFlag::NoFlag);
yyjson::reader::value read(std::string& json_string, [std::size_t len,] [Allocator alc,] ReadFlag = ReadFlag::NoFlag);
yyjson::reader::value read(char* json_string, [std::size_t len,] [Allocator alc,] ReadFlag = ReadFlag::NoFlag);

enum class yyjson::ReadFlag : yyjson_read_flag
{
    NoFlag = YYJSON_READ_NOFLAG,
    ReadInsitu = YYJSON_READ_INSITU,
    AllowTrailingCommas = YYJSON_READ_ALLOW_TRAILING_COMMAS,
    AllowComments = YYJSON_READ_ALLOW_COMMENTS,
    AllowInfAndNan = YYJSON_READ_ALLOW_INF_AND_NAN,
    NumberAsRaw = YYJSON_READ_NUMBER_AS_RAW,
    AllowInvalidUnicode = YYJSON_READ_ALLOW_INVALID_UNICODE,
    BignumAsRaw = YYJSON_READ_BIGNUM_AS_RAW // for yyjson >= v0.7.0
};

The read function takes a JSON string and returns an immutable JSON value. The function argument may optionally specify a string length and an allocator to be described later. Otherwise, the length of the JSON string is determined and the yyjson default allocator is used.

If the read option has the ReadInsitu flag, You must specify the JSON string as writable (std::string& or char*) and its length. This writable string must be padded at least YYJSON_PADDING_SIZE bytes to the end. The length of the JSON string should be unpadded.

yyjson::reader::value

The immutable JSON value class is returned from yyjson::read function.

Constructor

yyjson::reader::value() = delete;
yyjson::reader::value(const yyjson::reader::value&) = default;
yyjson::reader::value(yyjson::reader::value&&) = default;

Member function

// Check the type of JSON
bool is_null() const;       // null
bool is_true() const;       // true
bool is_false() const;      // false
bool is_bool() const;       // bool
bool is_uint() const;       // std::uint64_t
bool is_sint() const;       // std::int64_t
bool is_int() const;        // std::uint64_t/std::int64_t
bool is_real() const;       // double
bool is_num() const;        // std::uint64_t/std::int64_t/double
bool is_string() const;     // string
bool is_array() const;      // array
bool is_object() const;     // object
bool is_container() const;  // array/object

// Get the content of the JSON value
std::optional<std::nullptr_t> as_null() const;
std::optional<bool> as_bool() const;
std::optional<std::uint64_t> as_uint() const;
std::optional<std::int64_t> as_sint() const;
std::optional<std::int64_t> as_int() const;
std::optional<double> as_real() const;
std::optional<double> as_num() const;
std::optional<std::string_view> as_string() const&;
std::optional<std::string> as_string() &&;
std::optional<yyjson::reader::const_array_ref> as_array() const&;
std::optional<yyjson::reader::array> as_array() &&;
std::optional<yyjson::reader::const_object_ref> as_object() const&;
std::optional<yyjson::reader::object> as_object() &&;

// Cast
template<typename T>
T cast<T>() const;
template<typename T>
explicit operator T() const;

// Output JSON string
yyjson::json_string write(WriteFlag write_flag = WriteFlag::NoFlag) const;
template <yyjson_allocator Allocator>
yyjson::json_string write(Allocator alc, WriteFlag write_flag = WriteFlag::NoFlag) const;

enum class yyjson::WriteFlag : yyjson_write_flag
{
    NoFlag = YYJSON_WRITE_NOFLAG,
    Pretty = YYJSON_WRITE_PRETTY,
    EscapeUnicode = YYJSON_WRITE_ESCAPE_UNICODE,
    EscapeSlashes = YYJSON_WRITE_ESCAPE_SLASHES,
    AllowInfAndNan = YYJSON_WRITE_ALLOW_INF_AND_NAN,
    InfAndNanAsNull = YYJSON_WRITE_INF_AND_NAN_AS_NULL,
    AllowInvalidUnicode = YYJSON_WRITE_ALLOW_INVALID_UNICODE,
    PrettyTwoSpaces = YYJSON_WRITE_PRETTY_TWO_SPACES    // for yyjson >= v0.7.0
};

The write function returns a read-only string which is inherited from std::string_view.

See the reference of yyjson for the information on writer flags.

Example

See also Overview.

using namespace yyjson;

std::string_view json_str = R"(
{
    "id": 1,
    "pi": 3.141592,
    "emoji": "🫠",
    "array": [0, 1, 2, 3, 4],
    "currency": {
        "USD": 129.66,
        "EUR": 140.35,
        "GBP": 158.72
    },
    "success": true
})";

auto val = read(json_str);
std::cout << val.write(WriteFlag::Pretty) << std::endl;
// {
//     "id": 1,
//     "pi": 3.141592,
//     "emoji": "🫠",
//     "array": [
//         0,
//         1,
//         2,
//         3,
//         4
//     ],
//     "currency": {
//         "USD": 129.66,
//         "EUR": 140.35,
//         "GBP": 158.72
//     },
//     "success": true
// }

yyjson::reader::array, yyjson::reader::const_array_ref

The immutable JSON array class is created by the value::as_array member function. This class adapts std::ranges::input_range concept.

Constructor

yyjson::reader::array() = delete;
yyjson::reader::array(const yyjson::reader::array&) = default;
yyjson::reader::array(yyjson::reader::array&&) = default;
yyjson::reader::array(const yyjson::reader::value&);
yyjson::reader::array(yyjson::reader::value&&);

yyjson::reader::const_array_ref() = delete;
yyjson::reader::const_array_ref(const yyjson::reader::const_array_ref&) = default;
yyjson::reader::const_array_ref(yyjson::reader::const_array_ref&&) = default;

Member function

// STL-like functions
yyjson::reader::const_array_iter cbegin() const;
yyjson::reader::const_array_iter begin() const;
yyjson::reader::const_array_iter cend() const;
yyjson::reader::const_array_iter end() const;
yyjson::reader::const_value_ref front() const;
yyjson::reader::const_value_ref back() const;
yyjson::reader::const_value_ref operator[](std::size_t) const;
std::size_t size() const;
bool empty() const;

// Cast
template<typename T>
T cast<T>() const;
template<typename T>
explicit operator T() const;

// Output JSON string (inherited)
yyjson::json_string write(WriteFlag write_flag = WriteFlag::NoFlag) const;
template <yyjson_allocator Allocator>
yyjson::json_string write(Allocator alc, WriteFlag write_flag = WriteFlag::NoFlag) const;

// Range concept
std::ranges::iterator_t<yyjson::reader::const_array_ref> -> yyjson::reader::const_array_iter
std::ranges::range_value_t<yyjson::reader::const_array_ref> -> yyjson::reader::const_value_ref

Example

See also Overview.

using namespace yyjson;

std::string_view json_str = R"([0, "1", 2.0])";
auto val = read(json_str);
assert(val.is_array());

// NOTE: Prior to C++23 (P2644R1), range-based for loop for temporal std::optional
// instance requires initializer because std::optional<T>::value() returns T&&.
// for (const auto& v : *val.as_array()) { ... }             // 💀 UB
for (const auto arr = *val.as_array(); const auto& v : arr)  // ✅ OK
{
    std::cout << v.write() << std::endl;
}
// 0
// "1"
// 2.0

yyjson::reader::object, yyjson::reader::const_object_ref

The immutable JSON object class is created by the value::as_object member function. This class adapts std::ranges::input_range concept.

Constructor

yyjson::reader::object() = delete;
yyjson::reader::object(const yyjson::reader::object&) = default;
yyjson::reader::object(yyjson::reader::object&&) = default;
yyjson::reader::object(const yyjson::reader::value&);
yyjson::reader::object(yyjson::reader::value&&);

yyjson::reader::const_object_ref() = delete;
yyjson::reader::const_object_ref(const yyjson::reader::const_object_ref&) = default;
yyjson::reader::const_object_ref(yyjson::reader::const_object_ref&&) = default;

Member function

// STL-like functions
yyjson::reader::const_object_iter cbegin() const;
yyjson::reader::const_object_iter begin() const;
yyjson::reader::const_object_iter cend() const;
yyjson::reader::const_object_iter end() const;
yyjson::reader::const_object_iter find(std::string_view key) const;     // Note: O(N)
yyjson::reader::const_value_ref operator[](std::string_view key) const; // Note: O(N)
std::size_t size() const;
bool empty() const;
bool contains(std::string_view key) const;  // Note: O(N)

// Cast
template<typename T>
T cast<T>() const;
template<typename T>
explicit operator T() const;

// Output JSON string (inherited)
yyjson::json_string write(WriteFlag write_flag = WriteFlag::NoFlag) const;
template <yyjson_allocator Allocator>
yyjson::json_string write(Allocator alc, WriteFlag write_flag = WriteFlag::NoFlag) const;

// Range concept
using yyjson::reader::const_key_value_ref_pair = std::pair<std::string_view, yyjson::reader::const_value_ref>;
std::ranges::iterator_t<yyjson::reader::const_object_ref> -> yyjson::reader::const_object_iter
std::ranges::range_value_t<yyjson::reader::const_object_ref> -> yyjson::reader::const_key_value_ref_pair

Example

See also Overview.

using namespace yyjson;

std::string_view json_str = R"({"USD": 129.66, "EUR": 140.35, "GBP": 158.72})";
auto val = read(json_str);
assert(val.is_object());

auto obj = *val.as_object();
std::cout << *obj["USD"].as_real() << std::endl;
std::cout << *obj["EUR"].as_real() << std::endl;
std::cout << *obj["GBP"].as_real() << std::endl;
// 129.66
// 140.35
// 158.72

Mutable JSON classes

Mutable JSON classes are defined in yyjson::writer namespace. The following user-constructible classes are exported in the top yyjson namespace,

using yyjson::value = yyjson::writer::value;
using yyjson::array = yyjson::writer::array;
using yyjson::object = yyjson::writer::object;

yyjson::value

The mutable JSON value class yyjson::value constructs JSON values such as null, bool, number, string, array, and object.

Constructor

// Concepts (not defined in the library, but described for explanation)
template <typename T>
concept value_constructible = std::constructible_from<yyjson::value, T>;
template <typename T>
concept array_constructible = std::ranges::input_range<T> && value_constructible<std::ranges::range_value_t<T>>;
template <typename T>
concept object_constructible = std::ranges::input_range<T> && key_value_like<std::ranges::range_value_t<T>> &&
                               value_constructible<std::tuple_element_t<1, std::ranges::range_value_t<T>>>;

// Conversion to primitive types
yyjson::value(std::nullptr_t);
yyjson::value(bool);
yyjson::value(std::unsigned_integral);
yyjson::value(std::signed_integral);
yyjson::value(std::floating_point);
yyjson::value(const std::string&, [yyjson::copy_string_t]);
yyjson::value(std::string_view, [yyjson::copy_string_t]);
yyjson::value(const char*, [yyjson::copy_string_t]);

// Conversion to array types
template <array_constructible T>
yyjson::value(T&&, [yyjson::copy_string_t]);

// Conversion to object types
template <object_constructible T>
yyjson::value(T&&, [yyjson::copy_string_t]);

// std::initializer_list
yyjson::value(std::initialize_list<yyjson::value>);
yyjson::value(std::initialize_list<yyjson::writer::key_value_pair>, [yyjson::copy_string_t]);

// Copy from other JSON classes
using reference_types = yyjson::{reader,writer}::[const_]{value,array,object}[_ref];
yyjson::value(const reference_types&);
yyjson::value(reference_types&);
yyjson::value(reference_types&&);

// Default constructor (null by default)
yyjson::value();

// Copy constructor
yyjson::value(const yyjson::value&) = default;
yyjson::value(yyjson::value&&) = default;

Member function

// Check the type of JSON
bool is_null() const;       // null
bool is_true() const;       // true
bool is_false() const;      // false
bool is_bool() const;       // bool
bool is_uint() const;       // std::uint64_t
bool is_sint() const;       // std::int64_t
bool is_int() const;        // std::uint64_t/std::int64_t
bool is_real() const;       // double
bool is_num() const;        // std::uint64_t/std::int64_t/double
bool is_string() const;     // string
bool is_array() const;      // array
bool is_object() const;     // object
bool is_container() const;  // array/object

// Get the content of the JSON value
std::optional<std::nullptr_t> as_null() const;
std::optional<bool> as_bool() const;
std::optional<std::uint64_t> as_uint() const;
std::optional<std::int64_t> as_sint() const;
std::optional<std::int64_t> as_int() const;
std::optional<double> as_real() const;
std::optional<double> as_num() const;
std::optional<std::string_view> as_string() const;
std::optional<yyjson::writer::const_array_ref> as_array() const&;
std::optional<yyjson::writer::array_ref> as_array() &;
std::optional<yyjson::writer::array> as_array() &&;
std::optional<yyjson::writer::const_object_ref> as_object() const&;
std::optional<yyjson::writer::object_ref> as_object() &;
std::optional<yyjson::writer::object> as_object() &&;

// operator=
yyjson::value& yyjson::value::operator=(const yyjson::value&);
yyjson::value& yyjson::value::operator=(yyjson::value&&) noexcept;
template <value_constructible T>
yyjson::value& yyjson::value::operator=(T&&);
template <value_constructible T>
yyjson::value& yyjson::value::operator=(pair_like<T, yyjson::copy_string_t>);

// Cast
template<typename T>
T cast<T>() const;
template<typename T>
explicit operator T() const;

// Output JSON string
yyjson::json_string write(WriteFlag write_flag = WriteFlag::NoFlag) const;
template <yyjson_allocator Allocator>
yyjson::json_string write(Allocator alc, WriteFlag write_flag = WriteFlag::NoFlag) const;

Concepts value_constructible, array_constructible, and object_constructible are NOT defined in the library but are described in the above for explanation hereafter.

Since yyjson does not copy (but refer) strings to JSON documents by default, it is possible to explicitly specify that strings will be copied to JSON by giving the tag type value yyjson::copy_string as the second argument to the constructor. For safety, the implicit copy occurs when std::string&& is converted to the JSON string without specifying yyjson::copy_string.

JSON arrays and objects are constructed recursively from ranges as long as the range's value type can be constructed to JSON value. The yyjson::copy_string is also passed recursively if it is given. The same manner applies to the other functions of mutable JSON classes.

Example

See also Overview.

using namespace yyjson;
using namespace std::literals;

// Conversion constructors
auto v_bool = value(true);
auto v_null = value(nullptr);
auto v_int = value(-1);
auto v_real = value(3.14);

// String types
auto strlit = "string literal";
auto stdstr = "string example"s;
auto strviw = std::string_view(stdstr);

// Explicit copy
auto v_lit_cp = value(strlit, copy_string);
auto v_str_cp = value(stdstr, copy_string);
auto v_viw_cp = value(strviw, copy_string);

// No copy: the life time of a string must be managed
auto v_lit = value(strlit);
auto v_str = value(stdstr);
auto v_viw = value(strviw);

// If the original string is modified, the uncopied JSON string seems to be also modified.
// (but string length is not changed; it may occur memory access violation)
stdstr = "modified string";
std::cout << v_str_cp.write() << std::endl;
// "string example"
std::cout << v_str.write() << std::endl;
// "modified strin"

// Implicitly copy string if the argument type is `std::string&&` for safety
auto v_str_cp = value(std::move(stdstr));

yyjson::array

The mutable JSON array class is created by the range of JSON value constructible or the yyjson::value::as_array member function. This class adapts std::ranges::input_range concept.

Constructor

// Conversion from range
template <array_constructible T>
yyjson::array(array_constructible, [yyjson::copy_string]);

// std::initializer_list
yyjson::array(std::initialize_list<value>);

// Copy from other JSON classes
// throw yyjson::bad_cast if the JSON value is not convertible to a JSON array
using reference_types = yyjson::{reader,writer}::[const_]{value,array}[_ref];
yyjson::array(const reference_types&);
yyjson::array(reference_types&);
yyjson::array(reference_types&&);

// Default constructor (empty by default)
yyjson::array();

// Copy constructor
yyjson::array(const yyjson::array&) = default;
yyjson::array(yyjson::array&&) = default;

Member function

// STL-like functions
const_array_iter cbegin() const;
const_array_iter cend() const;
const_array_iter begin() const;
const_array_iter end() const;
array_iter begin();
array_iter end();
yyjson::writer::const_value_ref front() const;
yyjson::writer::value_ref front();
yyjson::writer::const_value_ref back() const;
yyjson::writer::value_ref back();
yyjson::writer::const_value_ref operator[](std::size_t) const;  // Note: O(N)
yyjson::writer::value_ref operator[](std::size_t);              // Note: O(N)
std::size_t size() const;
bool empty() const;
void erase(std::size_t);
void clear();

// Insert value_constructible
template <value_constructible T>
array_iter emplace_back(T&&, [yyjson::copy_string_t]);
template <value_constructible T>
array_iter emplace_front(T&&, [yyjson::copy_string_t]);
template <value_constructible T>
array_iter emplace(std::size_t, T&&, [yyjson::copy_string_t]);  // Note: O(N)

// Insert empty array/object
yyjson::writer::array_ref  emplace_back(yyjson::empty_array_t);
yyjson::writer::object_ref emplace_back(yyjson::empty_object_t);
yyjson::writer::array_ref  emplace_front(yyjson::empty_array_t);
yyjson::writer::object_ref emplace_front(yyjson::empty_object_t);
yyjson::writer::array_ref  emplace(std::size_t, yyjson::empty_array_t);     // Note: O(N)
yyjson::writer::object_ref emplace(std::size_t, yyjson::empty_object_t);    // Note: O(N)

// operator=
yyjson::array& operator=(const yyjson::array&);
yyjson::array& operator=(yyjson::array&&) noexcept;
template <array_constructible T>
yyjson::array& operator=(T&&);
template <array_constructible T>
yyjson::array& operator=(pair_like<T, yyjson::copy_string_t>);

// Cast
template<typename T>
T cast<T>() const;
template<typename T>
explicit operator T() const;

// Output JSON string
yyjson::json_string write(WriteFlag write_flag = WriteFlag::NoFlag) const;
template <yyjson_allocator Allocator>
yyjson::json_string write(Allocator alc, WriteFlag write_flag = WriteFlag::NoFlag) const;

// Range concept
std::ranges::range_value_t<yyjson::array&>       -> yyjson::writer::value_ref
std::ranges::range_value_t<const yyjson::array&> -> yyjson::writer::const_value_ref

The yyjson::array is designed to be implemented like STL containers but extended to allow easy insertion of an empty array and object with yyjson::empty_array and yyjson::empty_object tag type values, respectively. These special modifiers also have a different return type, which is not an array iterator but a reference to a newly inserted empty array/object of type yyjson::writer::array_ref or yyjson::writer::object_ref. These are introduced to optimize the performance by not creating a new JSON array/object; see also the performance best practices section.

Iterator classes are not exposed. They are tentatively described as array_iter and const_array_iter in the above.

Example

See also Overview.

using namespace yyjson;

// Create a new mutable JSON array from range
auto arr = array(std::vector{1, 2, 3});

// Insert a new value
arr.emplace_back("4");

// Insert an empty array and insert values into it
auto nested = arr.emplace_back(empty_array);
nested.emplace_back(5.0);
nested.emplace_back("6");
nested.emplace_back(7);

std::cout << arr.write() << std::endl;
// [1,2,3,"4",[5.0,"6",7]]

// Range-based for loop
for (auto&& v : arr)
{
    if (v.is_int()) v = *v.as_int() * 2;
}

std::cout << arr.write() << std::endl;
// [2,4,6,"4",[5.0,"6",7]]

yyjson::object

The mutable JSON object class is created by the key-value range of JSON value constructible or the yyjson::value::as_object member function. This class adapts std::ranges::input_range concept.

Constructor

// Conversion from key-value-like range
template <object_constructible T>
yyjson::object(object_constructible, [yyjson::copy_string]);

// std::initializer_list
yyjson::object(std::initialize_list<yyjson::writer::key_value_pair>);

// Copy from other JSON classes
// throw yyjson::bad_cast if the JSON value is not convertible to a JSON object
using reference_types = yyjson::{reader,writer}::[const_]{value,object}[_ref];
yyjson::object(const reference_types&);
yyjson::object(reference_types&);
yyjson::object(reference_types&&);

// Default constructor (empty by default)
yyjson::object();

// Copy constructor
yyjson::object(const yyjson::object&) = default;
yyjson::object(yyjson::object&&) = default;

Member function

// STL-like functions
const_object_iter cbegin() const;
const_object_iter cend() const;
const_object_iter begin() const;
const_object_iter end() const;
const_object_iter find(std::string_view key) const; // Note: O(N)
object_iter begin();
object_iter end();
object_iter find(std::string_view key); // Note: O(N)
std::size_t size() const;
bool empty() const;
void erase(std::string_view);
void clear();
bool contains(std::string_view key) const;  // Note: O(N)

// Insert value_constructible, no duplicate check
template <value_constructible ValueType>
object_iter emplace(KeyType&&, ValueType&&, [yyjson::copy_string_t]);
// Note: O(N), throw std::out_of_range if a key is not found
yyjson::writer::const_value_ref operator[](std::string_view) const;
// Note: O(N), construct default value if a key is not found
yyjson::writer::value_ref operator[](std::string_view);

// Insert empty array/object
yyjson::writer::array_ref  emplace(KeyType&&, yyjson::empty_array_t,  [yyjson::copy_string_t]);
yyjson::writer::object_ref emplace(KeyType&&, yyjson::empty_object_t, [yyjson::copy_string_t]);

// Try emplace, O(N) for duplicate check
template <value_constructible ValueType>
std::pair<object_iter, bool> try_emplace(KeyType&&, ValueType&&, [yyjson::copy_string_t]);

// operator=
yyjson::object& operator=(const yyjson::object&);
yyjson::object& operator=(yyjson::object&&) noexcept;
template <object_constructible T>
yyjson::object& operator=(T&&);
template <object_constructible T>
yyjson::object& operator=(pair_like<T, yyjson::copy_string_t>);

// Cast
template<typename T>
T cast<T>() const;
template<typename T>
explicit operator T() const;

// Output JSON string
yyjson::json_string write(WriteFlag write_flag = WriteFlag::NoFlag) const;
template <yyjson_allocator Allocator>
yyjson::json_string write(Allocator alc, WriteFlag write_flag = WriteFlag::NoFlag) const;

// Range concept
using yyjson::writer::key_value_ref_pair = std::pair<std::string_view, yyjson::reader::value_ref>;
using yyjson::writer::const_key_value_ref_pair = std::pair<std::string_view, yyjson::reader::const_value_ref>;
std::ranges::range_value_t<yyjson::object&>       -> yyjson::writer::key_value_ref_pair
std::ranges::range_value_t<const yyjson::object&> -> yyjson::writer::const_key_value_ref_pair

The yyjson::object is also designed to be implemented like STL map containers and to allow easy insertion of an empty array and object with yyjson::empty_array and yyjson::empty_object tag type values, respectively. These special modifiers also have a different return type, which is not an object iterator but a reference to a newly inserted empty array/object of type yyjson::writer::array_ref or yyjson::writer::object_ref. These are introduced to optimize the performance by not creating a new JSON array/object; see also the performance best practices section.

Iterator classes are not exposed. They are tentatively described as obejct_iter and const_object_iter in the above.

Note

The emplace member functions do NOT check for key duplication.

Example

See also Overview.

using namespace yyjson;

// Create a new mutable JSON object from a key-value-like range
auto obj = object(std::map<std::string, std::map<std::string, int>>{{"key0", {{"a", 0}, {"b", 1}}},
                                                                    {"key1", {{"c", 2}, {"d", 3}}}});

// Insert a new key-value pair
obj.emplace("key2", std::vector{4, 5});

// Insert an empty object and insert key-value pairs into it
auto nested = obj.emplace("key3", empty_object);
nested.emplace("g", 6);
nested.emplace("h", 7);

std::cout << obj.write() << std::endl;
// {"key0":{"a":0,"b":1},"key1":{"c":2,"d":3},"key2":[4,5],"key3":{"g":6,"h":7}}

// Key access and modify
obj["key2"] = std::map<std::string, int>{{"e", 4}, {"f", 5}};

std::cout << obj.write() << std::endl;
// {"key0":{"a":0,"b":1},"key1":{"c":2,"d":3},"key2":{"e":4,"f":5},"key3":{"e":6,"f":7}}

Allocator classes

This library provides the following memory allocator wrappers for reading and writing JSON strings. They are useful to improve performance for tasks that parse or stringify multiple JSON strings to avoid multiple memory allocations and deallocations.

yyjson::dynamic_allocator

This is a smart pointer wrapper for the dynamic allocator of yyjson. The lifetime of the allocator's internal smart pointer will be tied to JSON objects and strings created by read/write (member) functions, respectively. This allocator is for general purposes since it can automatically expand the buffer.

Note

This allocator is only available for yyjson >= v0.8.0.

Constructor

// Default constructors
dynamic_allocator() = default;
dynamic_allocator(const dynamic_allocator&) = default;
dynamic_allocator(dynamic_allocator&&) noexcept = default;

Member function

// Reset to a new dynamic allocator
void reset() const;

Example

using namespace yyjson;

auto alc = dynamic_allocator();

// Read JSON string using dynamic allocator
auto read_json(const string& json)
{
    return read(json, alc);
}

// Create JSON string using dynamic allocator
auto write_json(const auto& json)
{
    return json.write(alc);
}

yyjson::pool_allocator

This is a smart pointer wrapper for the fixed-size memory allocator of yyjson. The lifetime of the allocator's internal smart pointer will be tied to JSON objects and strings created by read/write (member) functions, respectively. This allocator is useful when the required memory size is known; for reading JSON strings. Since the buffer size is fixed, the read function using the single buffer would be faster than the dynamic allocator.

If the free space of the buffer is not large enough, the read or write (member) function will throw an exception. The reset function re-creates the buffer with the specified size and the reserve function can be used to expand the buffer size to be required.

Constructor

// Default constructors
pool_allocator() = default;
pool_allocator(const pool_allocator&) = default;
pool_allocator(pool_allocator&&) noexcept = default;

// Allocate specified bytes of buffer
explicit pool_allocator(std::size_t size_byte);
// Allocate a buffer large enough to read the specified JSON string with read flags
explicit pool_allocator(std::string_view json, ReadFlag flag = ReadFlag::NoFlag);

Member function

// Return the buffer size
std::size_t size() const;
// Reset to a new fixed size allocator
void reset(std::size_t size_byte = 0);
void reset(std::string_view json, ReadFlag flag = ReadFlag::NoFlag);
// Expand the buffer to required size
void reserve(std::size_t size_byte);
void reserve(std::string_view json, ReadFlag flag = ReadFlag::NoFlag);
// Check if the buffer, when empty, is large enough to read the specified JSON string with read flags
bool check_capacity(std::string_view json, ReadFlag = ReadFlag::NoFlag) const;

Example

using namespace yyjson;

// Create a single buffer allocator
auto alc = pool_allocator();

// Read JSON string using pool allocator multiple times
for (const auto& json : json_strings)
{
    // Expand the buffer required to read JSON string
    alc.reserve(json);
    auto val = read(json, alc);
    // ...
}

yyjson::stack_pool_allocator<std::size_t Byte>

This allocator is a fixed-size and stack-allocated buffer. The buffer size is specified by the template parameter and thus the buffer size cannot be changed. The required buffer size must be known in advance.

Note

The lifetime of stack_pool_allocator is not managed automatically unlike other allocators. Not to destroy the allocator while an instance of the JSON object/string is in use.

Constructor

// Default constructors
stack_pool_allocator() = default;
stack_pool_allocator(const stack_pool_allocator&) = default;
stack_pool_allocator(stack_pool_allocator&&) noexcept = default;

Member function

// Return the buffer size
constexpr std::size_t size() const;
// Reset to a new fixed size allocator
void reset();
// Check if the buffer, when empty, is large enough to read the specified JSON string with read flags
bool check_capacity(std::string_view json, ReadFlag = ReadFlag::NoFlag) const

Serialize and deserialize JSON

In the cpp-yyjson, conversion from C++ objects to JSON values is very flexible and vice versa.

Conversion to JSON values, arrays, and objects is provided by their respective constructors. Conversely, to convert from a JSON value, call the yyjson::cast function or static_cast.

using namespace yyjson;

auto val = value(3);
// -> 3
auto num = cast<int>(val);
// -> 3

auto arr = array(std::vector<std::vector<int>>{{1, 2}, {3, 4}});
// -> [[1, 2], [3, 4]]
auto vec = cast<std::vector<std::vector<int>>>(arr);
// -> {{1, 2}, {3, 4}}

auto obj = object(std::unordered_map<std::string, double>{{"first", 1.5}, {"second", 2.5}, {"third", 3.5}});
// -> {"first": 1.5, "second": 2.5, "third": 3.5}
auto map = cast<std::unordered_map<std::string, double>>(obj);
// -> {{"first", 1.5}, {"second", 2.5}, {"third", 3.5}}

If the conversion fails, the yyjson::bad_cast exception is thrown or a compile error occurs.

Note that the cast function and static_cast may give a different result. The cast function tries to convert JSON directly to a given type, while static_cast follows the C++ conversion rules. For example, cast<std::optional<int>>(value(nullptr)) succeeds but static_cast<std::optional<int>>(value(nullptr)) does not. This is because std::optional<int> has a conversion constructor from int, but value(nullptr) cannot be converted to int.

In addition to the above, the following four methods are provided for converting arbitrary C++ types from/to JSON value, array and object:

1. Pre-defined STL casters.
2. Conversion using compile-time reflection.
3. Registration of field names with VISITABLE_STRUCT macro.
4. User-defined casters.

Pre-defined STL casters

Several casters from/to STL classes to JSON are pre-defined in the library for convenience. Currently, available STL types are as follows:

• std::optional
• std::variant
• std::tuple-like
• std::vector<bool> (specialized)

For example, if you receive elements of multiple types, casts from/to std::optional or std::variant are useful.

using namespace yyjson;

auto val = value(std::optional<int>(3));    // serialize
// -> 3
auto num = cast<std::optional<int>>(val);   // deserialize
// -> 3

auto val = value(std::optional<int>(std::nullopt)); // serialize
// -> null
auto num = cast<std::optional<int>>(val);           // deserialize
// -> std::nullopt

auto val = value(std::variant<std::monostate, int, std::string>());     // serialize
// -> null
auto var = cast<std::variant<std::monostate, int, std::string>>(val);   // deserialize
// -> std::monostate()

auto val = value(std::variant<std::monostate, int, std::string>(1));    // serialize
// -> 1
auto var = cast<std::variant<std::monostate, int, std::string>>(val);   // deserialize
// -> 1

auto val = value(std::variant<std::monostate, int, std::string>("a"));  // serialize
// -> "a"
auto var = cast<std::variant<std::monostate, int, std::string>>(val);   // deserialize
// -> "a"

For a multi-type JSON array or object, using the caster for std::tuple is valuable and efficient.

using namespace yyjson;

auto tpl_arr = std::tuple{nullptr, true, "2", 3.0, std::tuple{4.0, "5", false}};
auto json_arr = array(tpl_arr);                             // serialize
// -> [null, true, "2", 3.0, [4.0, "5", false]]
auto tpl_arr2 = cast<decltype(tpl_arr)>(arr);               // deserialize
// -> {nullptr, true, "2", 3.0, {4.0, "5", false}}

auto tpl_obj = std::tuple{std::pair{"number", 1.5}, std::pair{"error", nullptr}, std::pair{"text", "abc"}};
auto json_obj = object(tpl_obj);                            // serialize
// -> {"number": 1.5, "error": null, "text": "abc"}
auto tpl_obj2 = cast<decltype(tpl_obj)>(json_obj);          // deserialize
// -> {{"number", 1.5}, {"error", nullptr}, {"text", "abc"}}

Automatic casting with compile-time reflection

The compile-time reflection is supported to automatically convert C++ struct/class to JSON object and vice versa. This feature is provided by using field-reflection and is included in this library.

If a C++ struct/class satisfies the field_reflection::field_namable concept (see field-reflection), it is possible to automatically convert from/to JSON objects with its field names and no need to write a caster definition or registration of field names with macros explained later.

In practice, the following conditions are required for C++ types for automatic conversion:

• default initializable
• aggregate type
• not a derived class
• no reference member

Example:

using namespace yyjson;

struct X
{
    int a;
    std::optional<double> b;
    std::string c = "default";
};

// serialize struxt X to JSON object with field-name reflection
auto reflectable = X{.a = 1, .b = std::nullopt, .c = "x"};
auto serialized = object(visitable);
// -> {"a":1,"b":null,"c":"x"}

// deserialize JSON object into struct X with field-name reflection
auto deserialized = cast<X>(serialized);
// -> X{.a = 1, .b = std::nullopt, .c = "x"}

Registration of field names of struct/class

Even if the compile-time reflection is NOT available for a C++ type or you want to partially select the fields to be converted, it is possible to register the field names of the C++ type with the VISITABLE_STRUCT macro:

// register fields except `c` on purpose
VISITABLE_STRUCT(X, a, b);

// serialize visitable struxt X to JSON object
auto visitable = X{.a = 1, .b = std::nullopt, .c = "x"};
auto serialized = object(visitable);
// -> {"a":1,"b":null}

// deserialize JSON object into struct X
auto deserialized = cast<X>(serialized);
// -> X{.a = 1, .b = std::nullopt, .c = "default"}

User-defined casters

The most flexible way to convert C++ types to/from JSON is to define custom casters. The custom conversion always has the highest priority among the other conversions. To define a custom caster, specialize the yyjson::caster struct template and implement the from_json and/or to_json template functions.

The example implementation of the from_json template function is as follows:

template <>
struct yyjson::caster<X>
{
    template <typename Json>
    static X from_json(const Json& json)
    {
        if (const auto obj = json.as_object(); obj.has_value())
        {
            auto result = X();
            for (const auto& kv : *obj)
            {
                if (kv.first == "a")
                    result.a = yyjson::cast<decltype(result.a)>(kv.second);
                else if (kv.first == "b")
                    result.b = yyjson::cast<decltype(result.b)>(kv.second);
                else if (kv.first == "c")
                    result.c = yyjson::cast<decltype(result.c)>(kv.second);
            }
            return result;
        }
        throw bad_cast(fmt::format("{} is not constructible from JSON", NAMEOF_TYPE(X)));
    }
};

The from_json template function has a JSON value as an argument template and must return type X.

For the to_json function, there are two ways. The first way is to define a translator for a value_constructible type and return it. The return type must be value_constructible:

template <>
struct yyjson::caster<X>
{
    template <typename... Ts>
    static auto to_json(const X& x, Ts...)
    {
        // Convert X object to JSON string
        return fmt::format("{} {} {}", x.a, (x.b ? fmt::format("{}", *x.b) : "null"), x.c);
    }
};

The to_json function takes a copy_string in the last argument, but this can be ignored if not needed.

The second way is to create a JSON value/array/object directly in the to_json function. The example for the class X equivalent to automatic casting or VISITABLE_STRUCT registration is as follows.

template <>
struct yyjson::caster<X>
{
    template <typename... Ts>
    static void to_json(writer::object_ref& obj, const X& x, Ts... ts)
    {
        obj.emplace("a", x.a, ts...);
        obj.emplace("b", x.b, ts...);
        obj.emplace("c", x.c, ts...);
    }
};

The first argument type is a target JSON class to create. There are 3 options, writer::object_ref&, writer::array_ref& and writer::value_ref&; if you want to convert the class X to JSON array, the first argument should be writer::array_ref&.

The casters are applied recursively to convert from/to JSON classes including custom casters. It is not always necessary to implement both from_json and to_json functions, and the two conversions do not have to be symmetric.

Performance best practices

Creating a new yyjson::value, yyjson::array, or yyjson::object is expensive. This is one point we should be aware of when building JSON to maximize performance.

(1) JSON array/object construction

Although JSON arrays and objects can be constructed from std::initializer_list<value>, which is useful and intuitive, using std::tuple and std::pair is more efficient as it avoids the construction of yyjson::value. The drawback is that you have to write the type in every bracket.

using namespace yyjson;

// 🙁 Construction from std::initializer_list is costly
object json = {{"id", 1},
               {"pi", 3.141592},
               {"name", "example"},
               {"array", {0, 1, 2, 3, 4}},
               {"currency", {{"USD", 129.66}, {"EUR", 140.35}, {"GBP", 158.72}}},
               {"success", true}};

// 🙄 Construction from std::tuple is efficient, but it seems tedious to have to write the type every time.
object json = std::tuple{std::pair{"id", 1},
                         std::pair{"pi", 3.141592},
                         std::pair{"name", "example"},
                         std::pair{"array", std::tuple{0, 1, 2, 3, 4}},
                         std::pair{"currency", std::tuple{std::pair{"USD", 129.66}, std::pair{"EUR", 140.35},
                                                          std::pair{"GBP", 158.72}}},
                         std::pair{"success", true}};

(2) JSON array/object insertion

When creating a nested JSON array or object, it is more efficient to insert an empty array/object rather than to construct a new array/object.

using namespace yyjson;

auto obj = object();

// 🙁 Create a new object and insert it
auto nested = object();
obj.emplace("currency", nested);

// 😀 Insert an empty object and use returned object reference
auto nested = obj.emplace("currency", empty_object);

nested.emplace("USD", 129.66);
nested.emplace("EUR", 140.35);
nested.emplace("GBP", 158.72);

(3) Multi-threaded JSON construction

On the other hand, creating a new array/object may be useful for the multi-threaded construction of large JSON. The following example creates a 1000x1000000 JSON array with 4 threads using BS::thread_pool. In this example, a speedup of about 3x compared to single threading was measured.

#include "BS_thread_pool.hpp"

using namespace yyjson;

auto nums = std::vector<double>(1000000);
std::iota(nums.begin(), nums.end(), 0);

// Multi-threaded construction of 1000x1000000 JSON array
auto tp = BS::thread_pool(4);
auto parallel_results = tp.parallelize_loop(0, 1000,
                                            [&nums](const int a, const int b)
                                            {
                                                auto result = std::vector<array>();
                                                result.reserve(b - a);
                                                for (int i = a; i < b; ++i) result.emplace_back(nums);
                                                return result;
                                            }).get();
auto arr = yyjson::array();
for (auto&& vec : parallel_results)
    for (auto&& a : vec) arr.emplace_back(std::move(a));

// Single-threaded version equivalent to the above
auto arr = yyjson::array();
for (auto i = 0; i < 1000; ++i) arr.emplace_back(nums);

Misc

Support for {fmt} format

The cpp-yyjson defines a specialization of fmt::formatter of the {fmt} library. The JSON classes are formattable as follows:

using namespace yyjson;

auto obj = object(...);
auto json_str = fmt::format("{}", obj);
fmt::print("{}", obj);

Object dependence safety

Unlike the original yyjson, cpp-yyjson can safely add the same JSON value/array/object to the container multiple times.

using namespace yyjson;

auto src_vec = std::vector{value("0"), value(1), value(2.0)};
auto arr = array();
for (auto&& v : src_vec)
{
    // the first addition
    arr.emplace_back(v);
}
for (auto&& v : src_vec)
{
    // the second addition: makes a copy and append it
    arr.emplace_back(v);
}

Conversion from immutable to mutable

If you want to make an immutable JSON instance writable, convert it to a mutable type.

using namespace yyjson;

std::string_view json_str = R"(
{
    "id": 1,
    "pi": 3.141592,
    "name": "🫠",
    "array": [0, 1, 2, 3, 4],
    "currency": {
        "USD": 129.66,
        "EUR": 140.35,
        "GBP": 158.72
    },
    "success": true
})";

// Read JSON string and make mutable
auto obj = object(read(json_str));
obj["name"] = "❤️";

Author

Yoshiki Matsuda (@yosh-matsuda)