Merge pull request #1559 from theodelrieu/feat/explicit_conversion_operator

Feat/explicit conversion operator

Author: nlohmann

.travis.yml

@@ -145,6 +145,11 @@ matrix:
   - os: osx
     osx_image: xcode12
 
+  - os: osx
+    osx_image: xcode12
+    env:
+      - IMPLICIT_CONVERSIONS=OFF
+
   # Linux / GCC
 
   - os: linux
@@ -203,6 +208,16 @@ matrix:
         sources: ['ubuntu-toolchain-r-test']
         packages: ['g++-9', 'ninja-build']
 
+  - os: linux
+    compiler: gcc
+    env:
+      - COMPILER=g++-9
+      - IMPLICIT_CONVERSIONS=OFF
+    addons:
+      apt:
+        sources: ['ubuntu-toolchain-r-test']
+        packages: ['g++-9', 'ninja-build']
+
   - os: linux
     compiler: gcc
     env:
@@ -315,10 +330,12 @@ script:
   - if [[ "${COMPILER}" != "" ]]; then export CXX=${COMPILER}; fi
   # by default, use the single-header version
   - if [[ "${MULTIPLE_HEADERS}" == "" ]]; then export MULTIPLE_HEADERS=OFF; fi
+  # by default, use implicit conversions
+  - if [[ "${IMPLICIT_CONVERSIONS}" == "" ]]; then export IMPLICIT_CONVERSIONS=ON; fi
 
   # compile and execute unit tests
   - mkdir -p build && cd build
-  - cmake .. ${CMAKE_OPTIONS} -DJSON_MultipleHeaders=${MULTIPLE_HEADERS} -DJSON_BuildTests=On -GNinja && cmake --build . --config Release
+  - cmake .. ${CMAKE_OPTIONS} -DJSON_MultipleHeaders=${MULTIPLE_HEADERS} -DJSON_ImplicitConversions=${IMPLICIT_CONVERSIONS} -DJSON_BuildTests=On -GNinja && cmake --build . --config Release
   - ctest -C Release --timeout 2700 -V -j
   - cd ..
 

CMakeLists.txt

@@ -24,6 +24,7 @@ endif ()
 option(JSON_BuildTests "Build the unit tests when BUILD_TESTING is enabled." ON)
 option(JSON_Install "Install CMake targets during install step." ON)
 option(JSON_MultipleHeaders "Use non-amalgamated version of the library." OFF)
+option(JSON_ImplicitConversions "Enable implicit conversions" ON)
 
 ##
 ## CONFIGURATION
@@ -60,6 +61,12 @@ else()
     target_compile_features(${NLOHMANN_JSON_TARGET_NAME} INTERFACE cxx_std_11)
 endif()
 
+target_compile_definitions(
+    ${NLOHMANN_JSON_TARGET_NAME}
+    INTERFACE
+    JSON_USE_IMPLICIT_CONVERSIONS=$<BOOL:${JSON_ImplicitConversions}>
+)
+
 target_include_directories(
     ${NLOHMANN_JSON_TARGET_NAME}
     INTERFACE

appveyor.yml

@@ -7,27 +7,31 @@ environment:
       platform: x86
       CXX_FLAGS: ""
       LINKER_FLAGS: ""
+      CMAKE_OPTIONS: ""
       GENERATOR: Visual Studio 14 2015
 
     - APPVEYOR_BUILD_WORKER_IMAGE: Visual Studio 2017
       configuration: Debug
       platform: x86
       CXX_FLAGS: ""
       LINKER_FLAGS: ""
+      CMAKE_OPTIONS: ""
       GENERATOR: Visual Studio 15 2017
 
     - APPVEYOR_BUILD_WORKER_IMAGE: Visual Studio 2019
       configuration: Debug
       platform: x86
       CXX_FLAGS: ""
       LINKER_FLAGS: ""
+      CMAKE_OPTIONS: ""
       GENERATOR: Visual Studio 16 2019
 
     - APPVEYOR_BUILD_WORKER_IMAGE: Visual Studio 2019
       configuration: Debug
       platform: x64
       CXX_FLAGS: ""
       LINKER_FLAGS: ""
+      CMAKE_OPTIONS: ""
       GENERATOR: Visual Studio 16 2019
 
     - APPVEYOR_BUILD_WORKER_IMAGE: Visual Studio 2015
@@ -36,6 +40,7 @@ environment:
       platform: x86
       CXX_FLAGS: ""
       LINKER_FLAGS: ""
+      CMAKE_OPTIONS: ""
       GENERATOR: Ninja
 
     - APPVEYOR_BUILD_WORKER_IMAGE: Visual Studio 2015
@@ -44,13 +49,15 @@ environment:
       platform: x86
       CXX_FLAGS: ""
       LINKER_FLAGS: ""
+      CMAKE_OPTIONS: ""
       GENERATOR: Ninja
 
     - APPVEYOR_BUILD_WORKER_IMAGE: Visual Studio 2015
       configuration: Release
       platform: x86
       CXX_FLAGS: ""
       LINKER_FLAGS: ""
+      CMAKE_OPTIONS: ""
       GENERATOR: Visual Studio 14 2015
 
     - APPVEYOR_BUILD_WORKER_IMAGE: Visual Studio 2015
@@ -59,41 +66,55 @@ environment:
       name: with_win_header
       CXX_FLAGS: ""
       LINKER_FLAGS: ""
+      CMAKE_OPTIONS: ""
       GENERATOR: Visual Studio 14 2015
 
     - APPVEYOR_BUILD_WORKER_IMAGE: Visual Studio 2017
       configuration: Release
       platform: x86
       CXX_FLAGS: "/permissive- /std:c++latest /utf-8"
       LINKER_FLAGS: ""
+      CMAKE_OPTIONS: ""
       GENERATOR: Visual Studio 15 2017
 
     - APPVEYOR_BUILD_WORKER_IMAGE: Visual Studio 2019
       configuration: Release
       platform: x86
       CXX_FLAGS: ""
       LINKER_FLAGS: ""
+      CMAKE_OPTIONS: "-DJSON_ImplicitConversions=OFF"
+      GENERATOR: Visual Studio 16 2019
+
+    - APPVEYOR_BUILD_WORKER_IMAGE: Visual Studio 2019
+      configuration: Release
+      platform: x64
+      CXX_FLAGS: ""
+      LINKER_FLAGS: ""
+      CMAKE_OPTIONS: ""
       GENERATOR: Visual Studio 16 2019
 
     - APPVEYOR_BUILD_WORKER_IMAGE: Visual Studio 2015
       configuration: Release
       platform: x64
       CXX_FLAGS: ""
       LINKER_FLAGS: ""
+      CMAKE_OPTIONS: ""
       GENERATOR: Visual Studio 14 2015
 
     - APPVEYOR_BUILD_WORKER_IMAGE: Visual Studio 2017
       configuration: Release
       platform: x64
       CXX_FLAGS: "/permissive- /std:c++latest /Zc:__cplusplus /utf-8 /F4000000"
       LINKER_FLAGS: "/STACK:4000000"
+      CMAKE_OPTIONS: ""
       GENERATOR: Visual Studio 15 2017
 
     - APPVEYOR_BUILD_WORKER_IMAGE: Visual Studio 2019
       configuration: Release
       platform: x64
       CXX_FLAGS: ""
       LINKER_FLAGS: ""
+      CMAKE_OPTIONS: ""
       GENERATOR: Visual Studio 16 2019
 
 init:
@@ -112,7 +133,7 @@ before_build:
   # for with_win_header build, inject the inclusion of Windows.h to the single-header library
   - ps: if ($env:name -Eq "with_win_header") { $header_path = "single_include\nlohmann\json.hpp" }
   - ps: if ($env:name -Eq "with_win_header") { "#include <Windows.h>`n" + (Get-Content $header_path | Out-String) | Set-Content $header_path }
-  - if "%GENERATOR%"=="Ninja" (cmake . -G "%GENERATOR%" -DCMAKE_BUILD_TYPE="%configuration%" -DCMAKE_CXX_FLAGS="%CXX_FLAGS%" -DCMAKE_EXE_LINKER_FLAGS="%LINKER_FLAGS%" -DCMAKE_IGNORE_PATH="C:/Program Files/Git/usr/bin" -DJSON_BuildTests=On) else (cmake . -G "%GENERATOR%" -A "%GENERATOR_PLATFORM%" -DCMAKE_CXX_FLAGS="%CXX_FLAGS%" -DCMAKE_EXE_LINKER_FLAGS="%LINKER_FLAGS%" -DCMAKE_IGNORE_PATH="C:/Program Files/Git/usr/bin" -DJSON_BuildTests=On)
+  - if "%GENERATOR%"=="Ninja" (cmake . -G "%GENERATOR%" -DCMAKE_BUILD_TYPE="%configuration%" -DCMAKE_CXX_FLAGS="%CXX_FLAGS%" -DCMAKE_EXE_LINKER_FLAGS="%LINKER_FLAGS%" -DCMAKE_IGNORE_PATH="C:/Program Files/Git/usr/bin" -DJSON_BuildTests=On "%CMAKE_OPTIONS%") else (cmake . -G "%GENERATOR%" -A "%GENERATOR_PLATFORM%" -DCMAKE_CXX_FLAGS="%CXX_FLAGS%" -DCMAKE_EXE_LINKER_FLAGS="%LINKER_FLAGS%" -DCMAKE_IGNORE_PATH="C:/Program Files/Git/usr/bin" -DJSON_BuildTests=On "%CMAKE_OPTIONS%")
 
 build_script:
   - cmake --build . --config "%configuration%"

include/nlohmann/detail/conversions/from_json.hpp

@@ -128,7 +128,7 @@ void from_json(const BasicJsonType& j, EnumType& e)
 
 // forward_list doesn't have an insert method
 template<typename BasicJsonType, typename T, typename Allocator,
-         enable_if_t<std::is_convertible<BasicJsonType, T>::value, int> = 0>
+         enable_if_t<is_getable<BasicJsonType, T>::value, int> = 0>
 void from_json(const BasicJsonType& j, std::forward_list<T, Allocator>& l)
 {
     if (JSON_HEDLEY_UNLIKELY(!j.is_array()))
@@ -145,15 +145,19 @@ void from_json(const BasicJsonType& j, std::forward_list<T, Allocator>& l)
 
 // valarray doesn't have an insert method
 template<typename BasicJsonType, typename T,
-         enable_if_t<std::is_convertible<BasicJsonType, T>::value, int> = 0>
+         enable_if_t<is_getable<BasicJsonType, T>::value, int> = 0>
 void from_json(const BasicJsonType& j, std::valarray<T>& l)
 {
     if (JSON_HEDLEY_UNLIKELY(!j.is_array()))
     {
         JSON_THROW(type_error::create(302, "type must be array, but is " + std::string(j.type_name())));
     }
     l.resize(j.size());
-    std::copy(j.begin(), j.end(), std::begin(l));
+    std::transform(j.begin(), j.end(), std::begin(l),
+                   [](const BasicJsonType & elem)
+    {
+        return elem.template get<T>();
+    });
 }
 
 template<typename BasicJsonType, typename T, std::size_t N>

include/nlohmann/detail/macro_scope.hpp

@@ -284,3 +284,13 @@
 #define NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE(Type, ...)  \
     void to_json(nlohmann::json& j, const Type& t) { NLOHMANN_JSON_EXPAND(NLOHMANN_JSON_PASTE(NLOHMANN_JSON_TO, __VA_ARGS__)) } \
     void from_json(const nlohmann::json& j, Type& t) { NLOHMANN_JSON_EXPAND(NLOHMANN_JSON_PASTE(NLOHMANN_JSON_FROM, __VA_ARGS__)) }
+
+#ifndef JSON_USE_IMPLICIT_CONVERSIONS
+    #define JSON_USE_IMPLICIT_CONVERSIONS 1
+#endif
+
+#if JSON_USE_IMPLICIT_CONVERSIONS
+    #define JSON_EXPLICIT
+#else
+    #define JSON_EXPLICIT explicit
+#endif

include/nlohmann/detail/macro_unscope.hpp

@@ -18,5 +18,6 @@
 #undef JSON_HAS_CPP_17
 #undef NLOHMANN_BASIC_JSON_TPL_DECLARATION
 #undef NLOHMANN_BASIC_JSON_TPL
+#undef JSON_EXPLICIT
 
 #include <nlohmann/thirdparty/hedley/hedley_undef.hpp>

include/nlohmann/detail/meta/type_traits.hpp

@@ -94,6 +94,16 @@ using get_template_function = decltype(std::declval<T>().template get<U>());
 template<typename BasicJsonType, typename T, typename = void>
 struct has_from_json : std::false_type {};
 
+// trait checking if j.get<T> is valid
+// use this trait instead of std::is_constructible or std::is_convertible,
+// both rely on, or make use of implicit conversions, and thus fail when T
+// has several constructors/operator= (see https://github.com/nlohmann/json/issues/958)
+template <typename BasicJsonType, typename T>
+struct is_getable
+{
+    static constexpr bool value = is_detected<get_template_function, const BasicJsonType&, T>::value;
+};
+
 template<typename BasicJsonType, typename T>
 struct has_from_json < BasicJsonType, T,
            enable_if_t < !is_basic_json<T>::value >>

include/nlohmann/json.hpp

@@ -3229,7 +3229,7 @@ class basic_json
 #endif
                    && detail::is_detected<detail::get_template_function, const basic_json_t&, ValueType>::value
                    , int >::type = 0 >
-    operator ValueType() const
+    JSON_EXPLICIT operator ValueType() const
     {
         // delegate the call to get<>() const
         return get<ValueType>();
@@ -3784,8 +3784,9 @@ class basic_json
 
     @since version 1.0.0
     */
+    // using std::is_convertible in a std::enable_if will fail when using explicit conversions
     template < class ValueType, typename std::enable_if <
-                   std::is_convertible<basic_json_t, ValueType>::value
+                   detail::is_getable<basic_json_t, ValueType>::value
                    && !std::is_same<value_t, ValueType>::value, int >::type = 0 >
     ValueType value(const typename object_t::key_type& key, const ValueType& default_value) const
     {
@@ -3796,7 +3797,7 @@ class basic_json
             const auto it = find(key);
             if (it != end())
             {
-                return *it;
+                return it->template get<ValueType>();
             }
 
             return default_value;
@@ -3858,7 +3859,7 @@ class basic_json
     @since version 2.0.2
     */
     template<class ValueType, typename std::enable_if<
-                 std::is_convertible<basic_json_t, ValueType>::value, int>::type = 0>
+                 detail::is_getable<basic_json_t, ValueType>::value, int>::type = 0>
     ValueType value(const json_pointer& ptr, const ValueType& default_value) const
     {
         // at only works for objects
@@ -3867,7 +3868,7 @@ class basic_json
             // if pointer resolves a value, return it or use default value
             JSON_TRY
             {
-                return ptr.get_checked(this);
+                return ptr.get_checked(this).template get<ValueType>();
             }
             JSON_INTERNAL_CATCH (out_of_range&)
             {
@@ -8341,8 +8342,8 @@ class basic_json
             }
 
             // collect mandatory members
-            const std::string op = get_value("op", "op", true);
-            const std::string path = get_value(op, "path", true);
+            const auto op = get_value("op", "op", true).template get<std::string>();
+            const auto path = get_value(op, "path", true).template get<std::string>();
             json_pointer ptr(path);
 
             switch (get_op(op))
@@ -8368,7 +8369,7 @@ class basic_json
 
                 case patch_operations::move:
                 {
-                    const std::string from_path = get_value("move", "from", true);
+                    const auto from_path = get_value("move", "from", true).template get<std::string>();
                     json_pointer from_ptr(from_path);
 
                     // the "from" location must exist - use at()
@@ -8385,7 +8386,7 @@ class basic_json
 
                 case patch_operations::copy:
                 {
-                    const std::string from_path = get_value("copy", "from", true);
+                    const auto from_path = get_value("copy", "from", true).template get<std::string>();
                     const json_pointer from_ptr(from_path);
 
                     // the "from" location must exist - use at()