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json/CMakeLists.txt
Niels Lohmann 3b6ae43c53 Add JSON_DISABLE_TUPLE_REFERENCE_CONVERSION to fix std::tuple conversions (#5598) 
* Add JSON_DISABLE_TUPLE_REFERENCE_CONVERSION to fix std::tuple conversions

basic_json can be constructed from std::tuple<json&>, which it turns into
a one-element array. Because of this, std::tuple picks its converting
constructor that converts the whole source tuple instead of the
element-wise one. As a result, std::tuple<const json&> built from
std::forward_as_tuple(j) binds to a temporary (a compile error with libc++,
a dangling reference with other standard libraries), and std::tuple<json>
built the same way holds [j] instead of a copy of j.

The new opt-in macro JSON_DISABLE_TUPLE_REFERENCE_CONVERSION (CMake option
JSON_DisableTupleReferenceConversion) removes the conversion from a
one-element tuple holding a reference to the same basic_json type, so
std::tuple converts element-wise. It is off by default, so existing
behavior is unchanged. It does not change any function body and therefore
is not part of the ABI tag.

Fixes #2226

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

* Convert one-element tuples to arrays on every compiler

to_json for std::tuple assigns a braced list, j = { std::get<Idx>(t)... }.
With a single element that is itself a basic_json, Apple clang 15 and 16
treat j = {x} as a copy of x, so std::tuple<json>{true} became true
instead of [true]. The macOS jobs (Xcode 15.1, 16.1) failed the new
checks in unit-disable-tuple-reference-conversion and unit-regression2.

The one-element overload that already handles
JSON_BRACE_INIT_COPY_SEMANTICS builds the array (or object, for a
[string, value] element) explicitly, the same way the initializer-list
constructor does. Use it unconditionally. The output is unchanged on
compilers that already wrapped the element.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

* Skip json reference tuple tests on clang < 4 and GCC < 5

ci_test_compilers_gcc_old (4.8) and ci_test_compilers_clang (3.4) could
not compile the new tuple tests. Creating a std::tuple of basic_json
references, e.g. std::forward_as_tuple(j), makes these compilers
instantiate basic_json's conversion operator for libstdc++'s internal
tuple bases, which fails hard. This happens with and without
JSON_DISABLE_TUPLE_REFERENCE_CONVERSION, so it is a limitation of these
compilers, not of the new option.

Tested with the CI images: clang 3.4 to 3.9 and GCC 4.8 and 4.9 fail,
clang 4, 5, and 6 and GCC 5 and 6 compile all cases. Skip only the
checks that create such tuples; the is_constructible checks still run.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

---------

Signed-off-by: Niels Lohmann <mail@nlohmann.me>
2026-09-30 20:53:56 +02:00

251 lines
9.0 KiB
CMake
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cmake_minimum_required(VERSION 3.5...4.0)
##
## PROJECT
## name and version
##
project(nlohmann_json VERSION 3.12.0 LANGUAGES CXX)
##
## MAIN_PROJECT CHECK
## determine if nlohmann_json is built as a subproject (using add_subdirectory) or if it is the main project
##
set(MAIN_PROJECT OFF)
if (CMAKE_CURRENT_SOURCE_DIR STREQUAL CMAKE_SOURCE_DIR)
set(MAIN_PROJECT ON)
endif()
##
## INCLUDE
##
##
set(CMAKE_MODULE_PATH ${CMAKE_CURRENT_SOURCE_DIR}/cmake ${CMAKE_MODULE_PATH})
if (POLICY CMP0077)
# Allow CMake 3.13+ to override options when using FetchContent / add_subdirectory.
cmake_policy(SET CMP0077 NEW)
endif ()
# ---- C++ Modules Support (optional) ----
option(NLOHMANN_JSON_BUILD_MODULES "Build C++ modules support" OFF)
if(NLOHMANN_JSON_BUILD_MODULES)
if(CMAKE_VERSION VERSION_GREATER_EQUAL 3.28)
message(STATUS "Building nlohmann.json C++ module")
add_subdirectory(src/modules)
else()
message(WARNING "Skipping nlohmann.json C++ module (requires CMake 3.28+, found ${CMAKE_VERSION})")
endif()
endif()
##
## OPTIONS
##
# VERSION_GREATER_EQUAL is not available in older CMake (< 3.7)
if(${MAIN_PROJECT} AND (${CMAKE_VERSION} VERSION_EQUAL 3.13 OR ${CMAKE_VERSION} VERSION_GREATER 3.13))
set(JSON_BuildTests_INIT ON)
else()
set(JSON_BuildTests_INIT OFF)
endif()
option(JSON_BuildTests "Build the unit tests when BUILD_TESTING is enabled." ${JSON_BuildTests_INIT})
option(JSON_CI "Enable CI build targets." OFF)
option(JSON_Diagnostics "Use extended diagnostic messages." OFF)
option(JSON_Diagnostic_Positions "Enable diagnostic positions." OFF)
option(JSON_GlobalUDLs "Place user-defined string literals in the global namespace." ON)
option(JSON_ImplicitConversions "Enable implicit conversions." ON)
option(JSON_DisableEnumSerialization "Disable default integer enum serialization." OFF)
option(JSON_DisableTupleReferenceConversion "Disable conversion from a one-element tuple of a JSON reference." OFF)
option(JSON_LegacyDiscardedValueComparison "Enable legacy discarded value comparison." OFF)
option(JSON_Install "Install CMake targets during install step." ${MAIN_PROJECT})
option(JSON_MultipleHeaders "Use non-amalgamated version of the library." ON)
option(JSON_SystemInclude "Include as system headers (skip for clang-tidy)." OFF)
option(JSON_StrictNulHandling "Build with strict NUL-byte handling enabled." OFF)
if (JSON_CI)
include(ci)
endif ()
##
## CONFIGURATION
##
include(GNUInstallDirs)
if (NOT DEFINED NLOHMANN_JSON_TARGET_NAME)
# Allow overriding the target name when using FetchContent / add_subdirectory.
set(NLOHMANN_JSON_TARGET_NAME ${PROJECT_NAME})
endif()
set(NLOHMANN_JSON_CONFIG_INSTALL_DIR "${CMAKE_INSTALL_DATADIR}/cmake/${PROJECT_NAME}" CACHE INTERNAL "")
set(NLOHMANN_JSON_INCLUDE_INSTALL_DIR "${CMAKE_INSTALL_INCLUDEDIR}")
set(NLOHMANN_JSON_TARGETS_EXPORT_NAME "${PROJECT_NAME}Targets")
set(NLOHMANN_JSON_CMAKE_CONFIG_TEMPLATE "cmake/config.cmake.in")
set(NLOHMANN_JSON_CMAKE_CONFIG_DIR "${CMAKE_CURRENT_BINARY_DIR}")
set(NLOHMANN_JSON_CMAKE_VERSION_CONFIG_FILE "${NLOHMANN_JSON_CMAKE_CONFIG_DIR}/${PROJECT_NAME}ConfigVersion.cmake")
set(NLOHMANN_JSON_CMAKE_PROJECT_CONFIG_FILE "${NLOHMANN_JSON_CMAKE_CONFIG_DIR}/${PROJECT_NAME}Config.cmake")
set(NLOHMANN_JSON_CMAKE_PROJECT_TARGETS_FILE "${NLOHMANN_JSON_CMAKE_CONFIG_DIR}/${PROJECT_NAME}Targets.cmake")
set(NLOHMANN_JSON_PKGCONFIG_INSTALL_DIR "${CMAKE_INSTALL_DATADIR}/pkgconfig")
if (JSON_MultipleHeaders)
set(NLOHMANN_JSON_INCLUDE_BUILD_DIR "${PROJECT_SOURCE_DIR}/include/")
message(STATUS "Using the multi-header code from ${NLOHMANN_JSON_INCLUDE_BUILD_DIR}")
else()
set(NLOHMANN_JSON_INCLUDE_BUILD_DIR "${PROJECT_SOURCE_DIR}/single_include/")
message(STATUS "Using the single-header code from ${NLOHMANN_JSON_INCLUDE_BUILD_DIR}")
endif()
if (NOT JSON_ImplicitConversions)
message(STATUS "Implicit conversions are disabled (JSON_USE_IMPLICIT_CONVERSIONS=0)")
endif()
if (JSON_DisableEnumSerialization)
message(STATUS "Enum integer serialization is disabled (JSON_DISABLE_ENUM_SERIALIZATION=1)")
endif()
if (JSON_DisableTupleReferenceConversion)
message(STATUS "Tuple reference conversion is disabled (JSON_DISABLE_TUPLE_REFERENCE_CONVERSION=1)")
endif()
if (JSON_LegacyDiscardedValueComparison)
message(STATUS "Legacy discarded value comparison enabled (JSON_USE_LEGACY_DISCARDED_VALUE_COMPARISON=1)")
endif()
if (JSON_Diagnostics)
message(STATUS "Diagnostics enabled (JSON_DIAGNOSTICS=1)")
endif()
if (JSON_StrictNulHandling)
message(STATUS "Strict NUL-byte handling enabled (JSON_STRICT_NUL_HANDLING=1)")
endif()
if (JSON_Diagnostic_Positions)
message(STATUS "Diagnostic positions enabled (JSON_DIAGNOSTIC_POSITIONS=1)")
endif()
if (NOT JSON_GlobalUDLs)
message(STATUS "User-defined string literals are not put in the global namespace (JSON_USE_GLOBAL_UDLS=0)")
endif()
if (JSON_SystemInclude)
set(NLOHMANN_JSON_SYSTEM_INCLUDE "SYSTEM")
endif()
##
## TARGET
## create target and add include path
##
add_library(${NLOHMANN_JSON_TARGET_NAME} INTERFACE)
add_library(${PROJECT_NAME}::${NLOHMANN_JSON_TARGET_NAME} ALIAS ${NLOHMANN_JSON_TARGET_NAME})
if (${CMAKE_VERSION} VERSION_LESS "3.8.0")
target_compile_features(${NLOHMANN_JSON_TARGET_NAME} INTERFACE cxx_range_for)
else()
target_compile_features(${NLOHMANN_JSON_TARGET_NAME} INTERFACE cxx_std_11)
endif()
target_compile_definitions(
${NLOHMANN_JSON_TARGET_NAME}
INTERFACE
$<$<NOT:$<BOOL:${JSON_GlobalUDLs}>>:JSON_USE_GLOBAL_UDLS=0>
$<$<NOT:$<BOOL:${JSON_ImplicitConversions}>>:JSON_USE_IMPLICIT_CONVERSIONS=0>
$<$<BOOL:${JSON_DisableEnumSerialization}>:JSON_DISABLE_ENUM_SERIALIZATION=1>
$<$<BOOL:${JSON_DisableTupleReferenceConversion}>:JSON_DISABLE_TUPLE_REFERENCE_CONVERSION=1>
$<$<BOOL:${JSON_Diagnostics}>:JSON_DIAGNOSTICS=1>
$<$<BOOL:${JSON_Diagnostic_Positions}>:JSON_DIAGNOSTIC_POSITIONS=1>
$<$<BOOL:${JSON_LegacyDiscardedValueComparison}>:JSON_USE_LEGACY_DISCARDED_VALUE_COMPARISON=1>
$<$<BOOL:${JSON_StrictNulHandling}>:JSON_STRICT_NUL_HANDLING=1>
)
target_include_directories(
${NLOHMANN_JSON_TARGET_NAME}
${NLOHMANN_JSON_SYSTEM_INCLUDE} INTERFACE
$<BUILD_INTERFACE:${NLOHMANN_JSON_INCLUDE_BUILD_DIR}>
$<INSTALL_INTERFACE:${NLOHMANN_JSON_INCLUDE_INSTALL_DIR}>
)
## add debug view definition file for msvc (natvis)
if (MSVC)
set(NLOHMANN_ADD_NATVIS TRUE)
set(NLOHMANN_NATVIS_FILE "nlohmann_json.natvis")
target_sources(
${NLOHMANN_JSON_TARGET_NAME}
INTERFACE
$<INSTALL_INTERFACE:${NLOHMANN_NATVIS_FILE}>
$<BUILD_INTERFACE:${CMAKE_CURRENT_SOURCE_DIR}/${NLOHMANN_NATVIS_FILE}>
)
endif()
# Install a pkg-config file, so other tools can find this.
configure_file(
"${CMAKE_CURRENT_SOURCE_DIR}/cmake/pkg-config.pc.in"
"${CMAKE_CURRENT_BINARY_DIR}/${PROJECT_NAME}.pc"
@ONLY
)
##
## TESTS
## create and configure the unit test target
##
# Only build tests when JSON_BuildTests is set and testing has not been
# disabled by a parent project via BUILD_TESTING (see the CTest module, which
# also calls enable_testing()).
if (JSON_BuildTests AND (NOT DEFINED BUILD_TESTING OR BUILD_TESTING))
include(CTest)
add_subdirectory(tests)
endif()
##
## INSTALL
## install header files, generate and install cmake config files for find_package()
##
include(CMakePackageConfigHelpers)
# use a custom package version config file instead of
# write_basic_package_version_file to ensure that it's architecture-independent
# https://github.com/nlohmann/json/issues/1697
configure_file(
"cmake/nlohmann_jsonConfigVersion.cmake.in"
${NLOHMANN_JSON_CMAKE_VERSION_CONFIG_FILE}
@ONLY
)
configure_file(
${NLOHMANN_JSON_CMAKE_CONFIG_TEMPLATE}
${NLOHMANN_JSON_CMAKE_PROJECT_CONFIG_FILE}
@ONLY
)
if(JSON_Install)
install(
DIRECTORY ${NLOHMANN_JSON_INCLUDE_BUILD_DIR}
DESTINATION ${NLOHMANN_JSON_INCLUDE_INSTALL_DIR}
)
install(
FILES ${NLOHMANN_JSON_CMAKE_PROJECT_CONFIG_FILE} ${NLOHMANN_JSON_CMAKE_VERSION_CONFIG_FILE}
DESTINATION ${NLOHMANN_JSON_CONFIG_INSTALL_DIR}
)
if (NLOHMANN_ADD_NATVIS)
install(
FILES ${NLOHMANN_NATVIS_FILE}
DESTINATION .
)
endif()
export(
TARGETS ${NLOHMANN_JSON_TARGET_NAME}
NAMESPACE ${PROJECT_NAME}::
FILE ${NLOHMANN_JSON_CMAKE_PROJECT_TARGETS_FILE}
)
install(
TARGETS ${NLOHMANN_JSON_TARGET_NAME}
EXPORT ${NLOHMANN_JSON_TARGETS_EXPORT_NAME}
INCLUDES DESTINATION ${NLOHMANN_JSON_INCLUDE_INSTALL_DIR}
)
install(
EXPORT ${NLOHMANN_JSON_TARGETS_EXPORT_NAME}
NAMESPACE ${PROJECT_NAME}::
DESTINATION ${NLOHMANN_JSON_CONFIG_INSTALL_DIR}
)
install(
FILES "${CMAKE_CURRENT_BINARY_DIR}/${PROJECT_NAME}.pc"
DESTINATION ${NLOHMANN_JSON_PKGCONFIG_INSTALL_DIR}
)
include(CPack)
endif()
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