compile error

[hyq5436]

compile error when compile with nlohmann json.
1>C:\Program Files (x86)\Microsoft Visual Studio\2019\Community\VC\Tools\MSVC\14.29.30133\include\filesystem(499,21): error C2039: 'find_if_not': is not a member of 'std' 1>C:\Program Files (x86)\Microsoft Visual Studio\2019\Community\VC\Tools\MSVC\14.29.30133\include\filesystem(40): message : see declaration of 'std' 1>C:\Program Files (x86)\Microsoft Visual Studio\2019\Community\VC\Tools\MSVC\14.29.30133\include\filesystem(499,32): error C3861: 'find_if_not': identifier not found 1>C:\Program Files (x86)\Microsoft Visual Studio\2019\Community\VC\Tools\MSVC\14.29.30133\include\filesystem(507,42): error C2039: 'find_if_not': is not a member of 'std'

and compile error gone when i modify uuid.h and header file include path to directory upper gsl
#include <span>
to

#ifdef __cpp_lib_span
#include <span>
#else
#include <gsl/span>
#endif

[mariusbancila]

#include <span> should be enough, if you have the folder gsl added to the additional include directories. That's something done with CMake when UUID_USING_CXX20_SPAN is not set (is OFF by default):

# Using span from std
if (NOT UUID_USING_CXX20_SPAN)
    target_include_directories(${PROJECT_NAME} INTERFACE
            $<BUILD_INTERFACE:${CMAKE_CURRENT_SOURCE_DIR}/gsl>
            $<INSTALL_INTERFACE:include/gsl>)
    install(DIRECTORY gsl DESTINATION include)
endif ()

[slav-slavov-schneider]

I am also facing this issue. My code builds well on Linux (g++ (Ubuntu 9.4.0-1ubuntu1~20.04.1) 9.4.0) but fails on Windows (VisualStudio 2019). The proposed ifdef solution works well.

C:/Projects/myProject/ThirdParty/stduuid/include/uuid.h:18:10: fatal error: span: No such file or directory
   18 | #include <span>
      |          ^~~~~~
compilation terminated.

Update:
Our project is configured to use C++17
If I add gsl to the target_link_libraries section of my library I get errors like:

The contents of <span> are available only with C++20 or later.
C:\Projects\miGenieRelease2Controller\ThirdParty\stduuid\include\uuid.h(59): error C2653: 'gsl': is not a class or namespace name
C:\Projects\miGenieRelease2Controller\ThirdParty\stduuid\include\uuid.h(59): error C2061: syntax error: identifier 'span'

[mariusbancila]

How are you building? Is UUID_USING_CXX20_SPAN set to ON or is it OFF?

[slav-slavov-schneider]

It is the default - OFF. I am just setting UUID_BUILD_TESTS to OFF in the upper CMakeLists file

set(UUID_BUILD_TESTS OFF CACHE BOOL "" FORCE)
add_subdirectory(stduuid)

[slav-slavov-schneider]

Here is a small snippet that reproduces the issue. stduuid is downloaded in a folder where main.cpp is.
CMakeLists.txt:

project(stduuid-test)

set(CMAKE_CXX_STANDARD 17)
set(CMAKE_CXX_STANDARD_REQUIRED ON)
set(CMAKE_CXX_EXTENSIONS OFF)

include(FetchContent)

FetchContent_Declare(gsl
    URL "https://github.com/microsoft/GSL/archive/refs/tags/v4.0.0.zip"
    URL_HASH SHA384=a769a51846f3d3336c671f3bf01fa79d8987dbd3bc337ba90e82cf5e88e18366b56a8baa05aa5fb1093c3e70283fab66
)
FetchContent_MakeAvailable(gsl)

set(UUID_BUILD_TESTS OFF CACHE BOOL "" FORCE)
add_subdirectory(stduuid)

add_executable (stduuid-test main.cpp)
target_link_libraries (stduuid-test Microsoft.GSL::GSL stduuid)

main.cpp

#include <uuid.h>

#include <cassert>

int main()
{
   std::random_device rd;
   auto seed_data = std::array<int, std::mt19937::state_size> {};
   std::generate(std::begin(seed_data), std::end(seed_data), std::ref(rd));
   std::seed_seq seq(std::begin(seed_data), std::end(seed_data));
   std::mt19937 generator(seq);
   uuids::uuid_random_generator gen{generator};

   uuids::uuid const id = gen();
   assert(!id.is_nil());
   assert(id.as_bytes().size() == 16);
   assert(id.version() == uuids::uuid_version::random_number_based);
   assert(id.variant() == uuids::uuid_variant::rfc);
}

[slav-slavov-schneider]

And while trying this I decided to fetch stduuid instead of downloading into the project folder and with this change in the CMakeLists file the issue does not appear.

project(stduuid-test)

set(CMAKE_CXX_STANDARD 17)
set(CMAKE_CXX_STANDARD_REQUIRED ON)
set(CMAKE_CXX_EXTENSIONS OFF)

include(FetchContent)

FetchContent_Declare(gsl
    URL "https://github.com/microsoft/GSL/archive/refs/tags/v4.0.0.zip"
    URL_HASH SHA384=a769a51846f3d3336c671f3bf01fa79d8987dbd3bc337ba90e82cf5e88e18366b56a8baa05aa5fb1093c3e70283fab66
)
FetchContent_MakeAvailable(gsl)

set(UUID_BUILD_TESTS OFF CACHE BOOL "" FORCE)
FetchContent_Declare(stduuid
    GIT_REPOSITORY "https://github.com/mariusbancila/stduuid.git"
)
FetchContent_MakeAvailable(stduuid)

add_executable (stduuid-test main.cpp)
target_link_libraries (stduuid-test Microsoft.GSL::GSL stduuid)

[mariusbancila]

Does this solution is suitable for you?

[slav-slavov-schneider]

Yes, I am OK with the fetch method. Thanks.