diff --git a/CMakeLists.txt b/CMakeLists.txt
index 28b2771c..df8db81e 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -10,6 +10,7 @@ project(inja LANGUAGES CXX VERSION 3.4.0)
 
 option(INJA_USE_EMBEDDED_JSON "Use the shipped json header if not available on the system" ON)
 option(INJA_INSTALL "Generate install targets for inja" ON)
+option(INJA_INSTALL_SINGLE_HEADER "Install the single header instead" OFF)
 option(INJA_EXPORT "Export the current build tree to the package registry" ON)
 option(BUILD_TESTING "Build unit tests" ON)
 option(INJA_BUILD_TESTS "Build unit tests when BUILD_TESTING is enabled." ON)
@@ -166,11 +167,20 @@ if(INJA_INSTALL)
     NO_CHECK_REQUIRED_COMPONENTS_MACRO
   )
 
-  install(
-    DIRECTORY include/inja
-    DESTINATION ${INJA_INSTALL_INCLUDE_DIR}
-    FILES_MATCHING PATTERN "*.hpp"
-  )
+  if(INJA_INSTALL_SINGLE_HEADER)
+    install(
+      DIRECTORY single_include/inja
+      DESTINATION ${INJA_INSTALL_INCLUDE_DIR}
+      FILES_MATCHING PATTERN "*.hpp"
+    )
+  else()
+    install(
+      DIRECTORY include/inja
+      DESTINATION ${INJA_INSTALL_INCLUDE_DIR}
+      FILES_MATCHING PATTERN "*.hpp"
+    )
+  endif()
+
   if(INJA_USE_EMBEDDED_JSON)
     install(
       DIRECTORY third_party/include/nlohmann
@@ -178,6 +188,7 @@ if(INJA_INSTALL)
       FILES_MATCHING PATTERN "*.hpp"
     )
   endif()
+
   install(
     FILES
       "${CMAKE_CURRENT_BINARY_DIR}/${INJA_CONFIG_PATH}/injaConfig.cmake"
diff --git a/include/inja/environment.hpp b/include/inja/environment.hpp
index e36b6a44..f976f837 100644
--- a/include/inja/environment.hpp
+++ b/include/inja/environment.hpp
@@ -3,17 +3,16 @@
 
 #include <fstream>
 #include <iostream>
-#include <memory>
 #include <sstream>
 #include <string>
 #include <string_view>
 
 #include "config.hpp"
 #include "function_storage.hpp"
+#include "inja.hpp"
 #include "parser.hpp"
 #include "renderer.hpp"
 #include "template.hpp"
-#include "utils.hpp"
 
 namespace inja {
 
diff --git a/include/inja/exceptions.hpp b/include/inja/exceptions.hpp
index f0dc8aa9..3badb9c7 100644
--- a/include/inja/exceptions.hpp
+++ b/include/inja/exceptions.hpp
@@ -1,6 +1,7 @@
 #ifndef INCLUDE_INJA_EXCEPTIONS_HPP_
 #define INCLUDE_INJA_EXCEPTIONS_HPP_
 
+#include <cstddef>
 #include <stdexcept>
 #include <string>
 
diff --git a/include/inja/function_storage.hpp b/include/inja/function_storage.hpp
index 9542d7a2..60bf0734 100644
--- a/include/inja/function_storage.hpp
+++ b/include/inja/function_storage.hpp
@@ -1,9 +1,15 @@
 #ifndef INCLUDE_INJA_FUNCTION_STORAGE_HPP_
 #define INCLUDE_INJA_FUNCTION_STORAGE_HPP_
 
+#include <functional>
+#include <map>
+#include <string>
 #include <string_view>
+#include <utility>
 #include <vector>
 
+#include "inja.hpp"
+
 namespace inja {
 
 using Arguments = std::vector<const json*>;
diff --git a/include/inja/lexer.hpp b/include/inja/lexer.hpp
index 789c61a1..6c744039 100644
--- a/include/inja/lexer.hpp
+++ b/include/inja/lexer.hpp
@@ -2,9 +2,11 @@
 #define INCLUDE_INJA_LEXER_HPP_
 
 #include <cctype>
-#include <locale>
+#include <cstddef>
+#include <string_view>
 
 #include "config.hpp"
+#include "exceptions.hpp"
 #include "token.hpp"
 #include "utils.hpp"
 
@@ -311,7 +313,7 @@ class Lexer {
       pos += open_start;
 
       // try to match one of the opening sequences, and get the close
-      std::string_view open_str = m_in.substr(pos);
+      const std::string_view open_str = m_in.substr(pos);
       bool must_lstrip = false;
       if (inja::string_view::starts_with(open_str, config.expression_open)) {
         if (inja::string_view::starts_with(open_str, config.expression_open_force_lstrip)) {
diff --git a/include/inja/node.hpp b/include/inja/node.hpp
index 9ad9ef6e..501854e0 100644
--- a/include/inja/node.hpp
+++ b/include/inja/node.hpp
@@ -1,11 +1,15 @@
 #ifndef INCLUDE_INJA_NODE_HPP_
 #define INCLUDE_INJA_NODE_HPP_
 
+#include <cstddef>
+#include <memory>
 #include <string>
 #include <string_view>
-#include <utility>
+#include <tuple>
+#include <vector>
 
 #include "function_storage.hpp"
+#include "inja.hpp"
 #include "utils.hpp"
 
 namespace inja {
diff --git a/include/inja/parser.hpp b/include/inja/parser.hpp
index bda271c8..3c396c9b 100644
--- a/include/inja/parser.hpp
+++ b/include/inja/parser.hpp
@@ -1,20 +1,24 @@
 #ifndef INCLUDE_INJA_PARSER_HPP_
 #define INCLUDE_INJA_PARSER_HPP_
 
-#include <limits>
+#include <cstddef>
+#include <fstream>
+#include <iterator>
+#include <memory>
 #include <stack>
 #include <string>
+#include <string_view>
 #include <utility>
 #include <vector>
 
 #include "config.hpp"
 #include "exceptions.hpp"
 #include "function_storage.hpp"
+#include "inja.hpp"
 #include "lexer.hpp"
 #include "node.hpp"
 #include "template.hpp"
 #include "token.hpp"
-#include "utils.hpp"
 
 namespace inja {
 
@@ -64,7 +68,7 @@ class Parser {
   }
 
   inline void add_literal(Arguments &arguments, const char* content_ptr) {
-    std::string_view data_text(literal_start.data(), tok.text.data() - literal_start.data() + tok.text.size());
+    const std::string_view data_text(literal_start.data(), tok.text.data() - literal_start.data() + tok.text.size());
     arguments.emplace_back(std::make_shared<LiteralNode>(data_text, data_text.data() - content_ptr));
   }
 
@@ -88,8 +92,8 @@ class Parser {
       return;
     }
 
-    std::string original_path = static_cast<std::string>(path);
-    std::string original_name = template_name;
+    const std::string original_path = static_cast<std::string>(path);
+    const std::string original_name = template_name;
 
     if (config.search_included_templates_in_files) {
       // Build the relative path
@@ -103,7 +107,7 @@ class Parser {
         std::ifstream file;
         file.open(template_name);
         if (!file.fail()) {
-          std::string text((std::istreambuf_iterator<char>(file)), std::istreambuf_iterator<char>());
+          const std::string text((std::istreambuf_iterator<char>(file)), std::istreambuf_iterator<char>());
 
           auto include_template = Template(text);
           template_storage.emplace(template_name, include_template);
@@ -472,7 +476,7 @@ class Parser {
           throw_parser_error("expected id, got '" + tok.describe() + "'");
         }
 
-        Token key_token = std::move(value_token);
+        const Token key_token = std::move(value_token);
         value_token = tok;
         get_next_token();
 
@@ -533,7 +537,7 @@ class Parser {
         throw_parser_error("expected variable name, got '" + tok.describe() + "'");
       }
 
-      std::string key = static_cast<std::string>(tok.text);
+      const std::string key = static_cast<std::string>(tok.text);
       get_next_token();
 
       auto set_statement_node = std::make_shared<SetStatementNode>(key, tok.text.data() - tmpl.content.c_str());
@@ -627,7 +631,7 @@ class Parser {
   }
 
   void parse_into_template(Template& tmpl, std::string_view filename) {
-    std::string_view path = filename.substr(0, filename.find_last_of("/\\") + 1);
+    const std::string_view path = filename.substr(0, filename.find_last_of("/\\") + 1);
 
     // StringRef path = sys::path::parent_path(filename);
     auto sub_parser = Parser(config, lexer.get_config(), template_storage, function_storage);
diff --git a/include/inja/renderer.hpp b/include/inja/renderer.hpp
index 6b8b05ec..3622eedb 100644
--- a/include/inja/renderer.hpp
+++ b/include/inja/renderer.hpp
@@ -2,13 +2,23 @@
 #define INCLUDE_INJA_RENDERER_HPP_
 
 #include <algorithm>
+#include <array>
+#include <cctype>
+#include <cmath>
+#include <cstddef>
+#include <memory>
 #include <numeric>
+#include <ostream>
+#include <sstream>
+#include <stack>
 #include <string>
 #include <utility>
 #include <vector>
 
 #include "config.hpp"
 #include "exceptions.hpp"
+#include "function_storage.hpp"
+#include "inja.hpp"
 #include "node.hpp"
 #include "template.hpp"
 #include "utils.hpp"
@@ -69,7 +79,7 @@ class Renderer : public NodeVisitor {
     return buffer;
   }
 
-  void print_data(const std::shared_ptr<json> value) {
+  void print_data(const std::shared_ptr<json>& value) {
     if (value->is_string()) {
       if (config.html_autoescape) {
         *output_stream << htmlescape(value->get_ref<const json::string_t&>());
@@ -107,7 +117,7 @@ class Renderer : public NodeVisitor {
         throw_renderer_error("expression could not be evaluated", expression_list);
       }
 
-      auto node = not_found_stack.top();
+      const auto node = not_found_stack.top();
       not_found_stack.pop();
 
       throw_renderer_error("variable '" + static_cast<std::string>(node->name) + "' not found", *node);
@@ -116,7 +126,7 @@ class Renderer : public NodeVisitor {
   }
 
   void throw_renderer_error(const std::string& message, const AstNode& node) {
-    SourceLocation loc = get_source_location(current_template->content, node.pos);
+    const SourceLocation loc = get_source_location(current_template->content, node.pos);
     INJA_THROW(RenderError(message, loc));
   }
 
@@ -158,7 +168,7 @@ class Renderer : public NodeVisitor {
 
   template <bool throw_not_found = true> Arguments get_argument_vector(const FunctionNode& node) {
     const size_t N = node.arguments.size();
-    for (auto a : node.arguments) {
+    for (const auto& a : node.arguments) {
       a->accept(*this);
     }
 
@@ -184,7 +194,7 @@ class Renderer : public NodeVisitor {
   }
 
   void visit(const BlockNode& node) {
-    for (auto& n : node.nodes) {
+    for (const auto& n : node.nodes) {
       n->accept(*this);
 
       if (break_rendering) {
diff --git a/include/inja/statistics.hpp b/include/inja/statistics.hpp
index da33593b..fa67eb29 100644
--- a/include/inja/statistics.hpp
+++ b/include/inja/statistics.hpp
@@ -10,7 +10,7 @@ namespace inja {
  */
 class StatisticsVisitor : public NodeVisitor {
   void visit(const BlockNode& node) {
-    for (auto& n : node.nodes) {
+    for (const auto& n : node.nodes) {
       n->accept(*this);
     }
   }
@@ -24,7 +24,7 @@ class StatisticsVisitor : public NodeVisitor {
   }
 
   void visit(const FunctionNode& node) {
-    for (auto& n : node.arguments) {
+    for (const auto& n : node.arguments) {
       n->accept(*this);
     }
   }
diff --git a/include/inja/template.hpp b/include/inja/template.hpp
index a396dd52..30638e85 100644
--- a/include/inja/template.hpp
+++ b/include/inja/template.hpp
@@ -4,7 +4,6 @@
 #include <map>
 #include <memory>
 #include <string>
-#include <vector>
 
 #include "node.hpp"
 #include "statistics.hpp"
@@ -23,7 +22,7 @@ struct Template {
   explicit Template(const std::string& content): content(content) {}
 
   /// Return number of variables (total number, not distinct ones) in the template
-  int count_variables() {
+  int count_variables() const {
     auto statistic_visitor = StatisticsVisitor();
     root.accept(statistic_visitor);
     return statistic_visitor.variable_counter;
diff --git a/include/inja/utils.hpp b/include/inja/utils.hpp
index 4de05fd8..e4e28560 100644
--- a/include/inja/utils.hpp
+++ b/include/inja/utils.hpp
@@ -2,7 +2,7 @@
 #define INCLUDE_INJA_UTILS_HPP_
 
 #include <algorithm>
-#include <fstream>
+#include <cstddef>
 #include <string>
 #include <string_view>
 #include <utility>
@@ -19,7 +19,7 @@ inline std::string_view slice(std::string_view view, size_t start, size_t end) {
 }
 
 inline std::pair<std::string_view, std::string_view> split(std::string_view view, char Separator) {
-  size_t idx = view.find(Separator);
+  const size_t idx = view.find(Separator);
   if (idx == std::string_view::npos) {
     return std::make_pair(view, std::string_view());
   }
@@ -34,7 +34,7 @@ inline bool starts_with(std::string_view view, std::string_view prefix) {
 inline SourceLocation get_source_location(std::string_view content, size_t pos) {
   // Get line and offset position (starts at 1:1)
   auto sliced = string_view::slice(content, 0, pos);
-  std::size_t last_newline = sliced.rfind("\n");
+  const std::size_t last_newline = sliced.rfind('\n');
 
   if (last_newline == std::string_view::npos) {
     return {1, sliced.length() + 1};
@@ -44,7 +44,7 @@ inline SourceLocation get_source_location(std::string_view content, size_t pos)
   size_t count_lines = 0;
   size_t search_start = 0;
   while (search_start <= sliced.size()) {
-    search_start = sliced.find("\n", search_start) + 1;
+    search_start = sliced.find('\n', search_start) + 1;
     if (search_start == 0) {
       break;
     }
diff --git a/single_include/inja/inja.hpp b/single_include/inja/inja.hpp
index 1d8d711f..c21db739 100644
--- a/single_include/inja/inja.hpp
+++ b/single_include/inja/inja.hpp
@@ -57,7 +57,6 @@ using json = INJA_DATA_TYPE;
 
 #include <fstream>
 #include <iostream>
-#include <memory>
 #include <sstream>
 #include <string>
 #include <string_view>
@@ -76,23 +75,32 @@ using json = INJA_DATA_TYPE;
 #include <map>
 #include <memory>
 #include <string>
-#include <vector>
 
 // #include "node.hpp"
 #ifndef INCLUDE_INJA_NODE_HPP_
 #define INCLUDE_INJA_NODE_HPP_
 
+#include <cstddef>
+#include <memory>
 #include <string>
 #include <string_view>
-#include <utility>
+#include <tuple>
+#include <vector>
 
 // #include "function_storage.hpp"
 #ifndef INCLUDE_INJA_FUNCTION_STORAGE_HPP_
 #define INCLUDE_INJA_FUNCTION_STORAGE_HPP_
 
+#include <functional>
+#include <map>
+#include <string>
 #include <string_view>
+#include <utility>
 #include <vector>
 
+// #include "inja.hpp"
+
+
 namespace inja {
 
 using Arguments = std::vector<const json*>;
@@ -227,12 +235,14 @@ class FunctionStorage {
 
 #endif // INCLUDE_INJA_FUNCTION_STORAGE_HPP_
 
+// #include "inja.hpp"
+
 // #include "utils.hpp"
 #ifndef INCLUDE_INJA_UTILS_HPP_
 #define INCLUDE_INJA_UTILS_HPP_
 
 #include <algorithm>
-#include <fstream>
+#include <cstddef>
 #include <string>
 #include <string_view>
 #include <utility>
@@ -241,6 +251,7 @@ class FunctionStorage {
 #ifndef INCLUDE_INJA_EXCEPTIONS_HPP_
 #define INCLUDE_INJA_EXCEPTIONS_HPP_
 
+#include <cstddef>
 #include <stdexcept>
 #include <string>
 
@@ -297,7 +308,7 @@ inline std::string_view slice(std::string_view view, size_t start, size_t end) {
 }
 
 inline std::pair<std::string_view, std::string_view> split(std::string_view view, char Separator) {
-  size_t idx = view.find(Separator);
+  const size_t idx = view.find(Separator);
   if (idx == std::string_view::npos) {
     return std::make_pair(view, std::string_view());
   }
@@ -312,7 +323,7 @@ inline bool starts_with(std::string_view view, std::string_view prefix) {
 inline SourceLocation get_source_location(std::string_view content, size_t pos) {
   // Get line and offset position (starts at 1:1)
   auto sliced = string_view::slice(content, 0, pos);
-  std::size_t last_newline = sliced.rfind("\n");
+  const std::size_t last_newline = sliced.rfind('\n');
 
   if (last_newline == std::string_view::npos) {
     return {1, sliced.length() + 1};
@@ -322,7 +333,7 @@ inline SourceLocation get_source_location(std::string_view content, size_t pos)
   size_t count_lines = 0;
   size_t search_start = 0;
   while (search_start <= sliced.size()) {
-    search_start = sliced.find("\n", search_start) + 1;
+    search_start = sliced.find('\n', search_start) + 1;
     if (search_start == 0) {
       break;
     }
@@ -725,7 +736,7 @@ namespace inja {
  */
 class StatisticsVisitor : public NodeVisitor {
   void visit(const BlockNode& node) {
-    for (auto& n : node.nodes) {
+    for (const auto& n : node.nodes) {
       n->accept(*this);
     }
   }
@@ -739,7 +750,7 @@ class StatisticsVisitor : public NodeVisitor {
   }
 
   void visit(const FunctionNode& node) {
-    for (auto& n : node.arguments) {
+    for (const auto& n : node.arguments) {
       n->accept(*this);
     }
   }
@@ -802,7 +813,7 @@ struct Template {
   explicit Template(const std::string& content): content(content) {}
 
   /// Return number of variables (total number, not distinct ones) in the template
-  int count_variables() {
+  int count_variables() const {
     auto statistic_visitor = StatisticsVisitor();
     root.accept(statistic_visitor);
     return statistic_visitor.variable_counter;
@@ -893,13 +904,19 @@ struct RenderConfig {
 
 // #include "function_storage.hpp"
 
+// #include "inja.hpp"
+
 // #include "parser.hpp"
 #ifndef INCLUDE_INJA_PARSER_HPP_
 #define INCLUDE_INJA_PARSER_HPP_
 
-#include <limits>
+#include <cstddef>
+#include <fstream>
+#include <iterator>
+#include <memory>
 #include <stack>
 #include <string>
+#include <string_view>
 #include <utility>
 #include <vector>
 
@@ -909,15 +926,20 @@ struct RenderConfig {
 
 // #include "function_storage.hpp"
 
+// #include "inja.hpp"
+
 // #include "lexer.hpp"
 #ifndef INCLUDE_INJA_LEXER_HPP_
 #define INCLUDE_INJA_LEXER_HPP_
 
 #include <cctype>
-#include <locale>
+#include <cstddef>
+#include <string_view>
 
 // #include "config.hpp"
 
+// #include "exceptions.hpp"
+
 // #include "token.hpp"
 #ifndef INCLUDE_INJA_TOKEN_HPP_
 #define INCLUDE_INJA_TOKEN_HPP_
@@ -1299,7 +1321,7 @@ class Lexer {
       pos += open_start;
 
       // try to match one of the opening sequences, and get the close
-      std::string_view open_str = m_in.substr(pos);
+      const std::string_view open_str = m_in.substr(pos);
       bool must_lstrip = false;
       if (inja::string_view::starts_with(open_str, config.expression_open)) {
         if (inja::string_view::starts_with(open_str, config.expression_open_force_lstrip)) {
@@ -1428,8 +1450,6 @@ class Lexer {
 
 // #include "token.hpp"
 
-// #include "utils.hpp"
-
 
 namespace inja {
 
@@ -1479,7 +1499,7 @@ class Parser {
   }
 
   inline void add_literal(Arguments &arguments, const char* content_ptr) {
-    std::string_view data_text(literal_start.data(), tok.text.data() - literal_start.data() + tok.text.size());
+    const std::string_view data_text(literal_start.data(), tok.text.data() - literal_start.data() + tok.text.size());
     arguments.emplace_back(std::make_shared<LiteralNode>(data_text, data_text.data() - content_ptr));
   }
 
@@ -1503,8 +1523,8 @@ class Parser {
       return;
     }
 
-    std::string original_path = static_cast<std::string>(path);
-    std::string original_name = template_name;
+    const std::string original_path = static_cast<std::string>(path);
+    const std::string original_name = template_name;
 
     if (config.search_included_templates_in_files) {
       // Build the relative path
@@ -1518,7 +1538,7 @@ class Parser {
         std::ifstream file;
         file.open(template_name);
         if (!file.fail()) {
-          std::string text((std::istreambuf_iterator<char>(file)), std::istreambuf_iterator<char>());
+          const std::string text((std::istreambuf_iterator<char>(file)), std::istreambuf_iterator<char>());
 
           auto include_template = Template(text);
           template_storage.emplace(template_name, include_template);
@@ -1887,7 +1907,7 @@ class Parser {
           throw_parser_error("expected id, got '" + tok.describe() + "'");
         }
 
-        Token key_token = std::move(value_token);
+        const Token key_token = std::move(value_token);
         value_token = tok;
         get_next_token();
 
@@ -1948,7 +1968,7 @@ class Parser {
         throw_parser_error("expected variable name, got '" + tok.describe() + "'");
       }
 
-      std::string key = static_cast<std::string>(tok.text);
+      const std::string key = static_cast<std::string>(tok.text);
       get_next_token();
 
       auto set_statement_node = std::make_shared<SetStatementNode>(key, tok.text.data() - tmpl.content.c_str());
@@ -2042,7 +2062,7 @@ class Parser {
   }
 
   void parse_into_template(Template& tmpl, std::string_view filename) {
-    std::string_view path = filename.substr(0, filename.find_last_of("/\\") + 1);
+    const std::string_view path = filename.substr(0, filename.find_last_of("/\\") + 1);
 
     // StringRef path = sys::path::parent_path(filename);
     auto sub_parser = Parser(config, lexer.get_config(), template_storage, function_storage);
@@ -2069,7 +2089,15 @@ class Parser {
 #define INCLUDE_INJA_RENDERER_HPP_
 
 #include <algorithm>
+#include <array>
+#include <cctype>
+#include <cmath>
+#include <cstddef>
+#include <memory>
 #include <numeric>
+#include <ostream>
+#include <sstream>
+#include <stack>
 #include <string>
 #include <utility>
 #include <vector>
@@ -2078,6 +2106,10 @@ class Parser {
 
 // #include "exceptions.hpp"
 
+// #include "function_storage.hpp"
+
+// #include "inja.hpp"
+
 // #include "node.hpp"
 
 // #include "template.hpp"
@@ -2141,7 +2173,7 @@ class Renderer : public NodeVisitor {
     return buffer;
   }
 
-  void print_data(const std::shared_ptr<json> value) {
+  void print_data(const std::shared_ptr<json>& value) {
     if (value->is_string()) {
       if (config.html_autoescape) {
         *output_stream << htmlescape(value->get_ref<const json::string_t&>());
@@ -2179,7 +2211,7 @@ class Renderer : public NodeVisitor {
         throw_renderer_error("expression could not be evaluated", expression_list);
       }
 
-      auto node = not_found_stack.top();
+      const auto node = not_found_stack.top();
       not_found_stack.pop();
 
       throw_renderer_error("variable '" + static_cast<std::string>(node->name) + "' not found", *node);
@@ -2188,7 +2220,7 @@ class Renderer : public NodeVisitor {
   }
 
   void throw_renderer_error(const std::string& message, const AstNode& node) {
-    SourceLocation loc = get_source_location(current_template->content, node.pos);
+    const SourceLocation loc = get_source_location(current_template->content, node.pos);
     INJA_THROW(RenderError(message, loc));
   }
 
@@ -2230,7 +2262,7 @@ class Renderer : public NodeVisitor {
 
   template <bool throw_not_found = true> Arguments get_argument_vector(const FunctionNode& node) {
     const size_t N = node.arguments.size();
-    for (auto a : node.arguments) {
+    for (const auto& a : node.arguments) {
       a->accept(*this);
     }
 
@@ -2256,7 +2288,7 @@ class Renderer : public NodeVisitor {
   }
 
   void visit(const BlockNode& node) {
-    for (auto& n : node.nodes) {
+    for (const auto& n : node.nodes) {
       n->accept(*this);
 
       if (break_rendering) {
@@ -2732,8 +2764,6 @@ class Renderer : public NodeVisitor {
 
 // #include "template.hpp"
 
-// #include "utils.hpp"
-
 
 namespace inja {
 
diff --git a/test/test-files.cpp b/test/test-files.cpp
index e95f3961..32eb9b44 100644
--- a/test/test-files.cpp
+++ b/test/test-files.cpp
@@ -1,5 +1,10 @@
 // Copyright (c) 2020 Pantor. All rights reserved.
 
+#include <doctest/doctest.h>
+
+#include "inja/environment.hpp"
+#include "inja/inja.hpp"
+
 TEST_CASE("loading") {
   inja::Environment env;
   inja::json data;
diff --git a/test/test-functions.cpp b/test/test-functions.cpp
index 7a680899..5249fba8 100644
--- a/test/test-functions.cpp
+++ b/test/test-functions.cpp
@@ -1,5 +1,11 @@
 // Copyright (c) 2020 Pantor. All rights reserved.
 
+#include <doctest/doctest.h>
+#include <string>
+
+#include "inja/environment.hpp"
+#include "inja/inja.hpp"
+
 TEST_CASE("functions") {
   inja::Environment env;
 
diff --git a/test/test-renderer.cpp b/test/test-renderer.cpp
index 68268e7f..41430f51 100644
--- a/test/test-renderer.cpp
+++ b/test/test-renderer.cpp
@@ -1,5 +1,12 @@
 // Copyright (c) 2020 Pantor. All rights reserved.
 
+#include <doctest/doctest.h>
+#include <string>
+
+#include "inja/environment.hpp"
+#include "inja/inja.hpp"
+#include "inja/template.hpp"
+
 TEST_CASE("types") {
   inja::Environment env;
   inja::json data;
@@ -160,7 +167,7 @@ TEST_CASE("templates") {
 
   SUBCASE("reuse") {
     inja::Environment env;
-    inja::Template temp = env.parse("{% if is_happy %}{{ name }}{% else %}{{ city }}{% endif %}");
+    const inja::Template temp = env.parse("{% if is_happy %}{{ name }}{% else %}{{ city }}{% endif %}");
 
     CHECK(env.render(temp, data) == "Peter");
 
@@ -171,10 +178,10 @@ TEST_CASE("templates") {
 
   SUBCASE("include") {
     inja::Environment env;
-    inja::Template t1 = env.parse("Hello {{ name }}");
+    const inja::Template t1 = env.parse("Hello {{ name }}");
     env.include_template("greeting", t1);
 
-    inja::Template t2 = env.parse("{% include \"greeting\" %}!");
+    const inja::Template t2 = env.parse("{% include \"greeting\" %}!");
     CHECK(env.render(t2, data) == "Hello Peter!");
     CHECK_THROWS_WITH(env.parse("{% include \"does-not-exist\" %}!"), "[inja.exception.file_error] failed accessing file at 'does-not-exist'");
 
@@ -189,13 +196,13 @@ TEST_CASE("templates") {
     env.set_search_included_templates_in_files(false);
     env.set_include_callback([&env](const std::string&, const std::string&) { return env.parse("Hello {{ name }}"); });
 
-    inja::Template t1 = env.parse("{% include \"greeting\" %}!");
+    const inja::Template t1 = env.parse("{% include \"greeting\" %}!");
     CHECK(env.render(t1, data) == "Hello Peter!");
 
     env.set_search_included_templates_in_files(true);
     env.set_include_callback([&env](const std::string&, const std::string& name) { return env.parse("Bye " + name); });
 
-    inja::Template t2 = env.parse("{% include \"Jeff\" %}!");
+    const inja::Template t2 = env.parse("{% include \"Jeff\" %}!");
     CHECK(env.render(t2, data) == "Bye Jeff!");
   }
 
@@ -211,9 +218,9 @@ TEST_CASE("templates") {
 
   SUBCASE("count variables") {
     inja::Environment env;
-    inja::Template t1 = env.parse("Hello {{ name }}");
-    inja::Template t2 = env.parse("{% if is_happy %}{{ name }}{% else %}{{ city }}{% endif %}");
-    inja::Template t3 = env.parse("{% if at(name, test) %}{{ name }}{% else %}{{ city }}{{ upper(city) }}{% endif %}");
+    const inja::Template t1 = env.parse("Hello {{ name }}");
+    const inja::Template t2 = env.parse("{% if is_happy %}{{ name }}{% else %}{{ city }}{% endif %}");
+    const inja::Template t3 = env.parse("{% if at(name, test) %}{{ name }}{% else %}{{ city }}{{ upper(city) }}{% endif %}");
 
     CHECK(t1.count_variables() == 1);
     CHECK(t2.count_variables() == 3);
diff --git a/test/test-units.cpp b/test/test-units.cpp
index bbef1908..acf79efd 100644
--- a/test/test-units.cpp
+++ b/test/test-units.cpp
@@ -1,5 +1,13 @@
 // Copyright (c) 2020 Pantor. All rights reserved.
 
+#include <doctest/doctest.h>
+#include <string>
+
+#include "inja/environment.hpp"
+#include "inja/function_storage.hpp"
+#include "inja/inja.hpp"
+#include "inja/utils.hpp"
+
 TEST_CASE("source location") {
   std::string content = R""""(Lorem Ipsum
   Dolor
@@ -42,7 +50,7 @@ TEST_CASE("copy environment") {
   CHECK(copy.render(test_tpl, inja::json()) == "4");
 
   // overwrite template in source env
-  inja::Template t2 = env.parse("{{ double(4) }}");
+  const inja::Template t2 = env.parse("{{ double(4) }}");
   env.include_template("tpl", t2);
   REQUIRE(env.render(test_tpl, inja::json()) == "8");
 
diff --git a/test/test.cpp b/test/test.cpp
index f933b761..9b1fa59f 100644
--- a/test/test.cpp
+++ b/test/test.cpp
@@ -2,7 +2,7 @@
 
 #define DOCTEST_CONFIG_IMPLEMENT_WITH_MAIN
 
-#include "doctest/doctest.h"
+#include <doctest/doctest.h>
 
 #define JSON_USE_IMPLICIT_CONVERSIONS 0
 #define JSON_NO_IO 1