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cxx-standard.cmake
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cxx-standard.cmake
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#
# Copyright (C) 2020 LAAS-CNRS
#
# Author: Guilhem Saurel
#
# .rst: .. ifmode:: internal
#
# .. variable:: ENFORCE_MINIMAL_CXX_STANDARD
#
# When this is ON, every call to :cmake:command:`CHECK_MINIMAL_CXX_STANDARD`
# updates the :cmake:variable:`CMAKE_CXX_STANDARD`.
option(ENFORCE_MINIMAL_CXX_STANDARD
"Set CMAKE_CXX_STANDARD if a dependency require it" OFF)
# .rst: .. ifmode:: user
#
# .. command:: CHECK_MINIMAL_CXX_STANDARD(STANDARD [ENFORCE])
#
# Ensure that a minimal C++ standard will be used.
#
# This will check the default standard of the current compiler, and set
# :cmake:variable:`CMAKE_CXX_STANDARD` if necessary, and `ENFORCE` is provided,
# or :cmake:variable:`ENFORCE_MINIMAL_CXX_STANDARD` is `ON`. Multiple calls to
# this macro will keep the highest standard.
#
# Supported values are 98, 11, 14, 17, and 20.
#
macro(CHECK_MINIMAL_CXX_STANDARD STANDARD)
set(options ENFORCE)
set(oneValueArgs)
set(multiValueArgs)
cmake_parse_arguments(MINIMAL_CXX_STANDARD "${options}" "${oneValueArgs}"
"${multiValueArgs}" ${ARGN})
# Get compiler default cxx standard, by printing "__cplusplus" (only once)
if(NOT DEFINED _COMPILER_DEFAULT_CXX_STANDARD
AND (NOT CMAKE_CROSSCOMPILING OR (CMAKE_CROSSCOMPILING
AND CMAKE_CROSSCOMPILING_EMULATOR)))
if(MSVC)
# See
# https://devblogs.microsoft.com/cppblog/msvc-now-correctly-reports-__cplusplus/
string(APPEND CMAKE_CXX_FLAGS " /Zc:__cplusplus")
endif()
write_file(
${CMAKE_CURRENT_BINARY_DIR}/cmake/tmp-cxx-standard.cpp
"#include <iostream>\nint main(){std::cout << __cplusplus << std::endl;return 0;}"
)
try_run(
_cxx_standard_run_status _cxx_standard_build_status
${CMAKE_CURRENT_BINARY_DIR}
${CMAKE_CURRENT_BINARY_DIR}/cmake/tmp-cxx-standard.cpp
RUN_OUTPUT_VARIABLE _COMPILER_DEFAULT_CXX_STANDARD)
if(_cxx_standard_run_status EQUAL FAILED_TO_RUN
OR NOT _cxx_standard_build_status)
message(
WARNING
"Impossible to build or run the script to retrive the _COMPILER_DEFAULT_CXX_STANDARD quantity from current compiler. Setting _COMPILER_DEFAULT_CXX_STANDARD to 199711"
)
set(_COMPILER_DEFAULT_CXX_STANDARD "199711")
endif()
string(STRIP "${_COMPILER_DEFAULT_CXX_STANDARD}"
_COMPILER_DEFAULT_CXX_STANDARD)
message(STATUS "Default C++ standard: ${_COMPILER_DEFAULT_CXX_STANDARD}")
endif()
# Check if we need to upgrade the current minimum
if(NOT DEFINED _MINIMAL_CXX_STANDARD
OR (NOT ${STANDARD} EQUAL "98"
AND (_MINIMAL_CXX_STANDARD EQUAL "98" OR _MINIMAL_CXX_STANDARD LESS
${STANDARD})))
set(_MINIMAL_CXX_STANDARD
"${STANDARD}"
CACHE INTERNAL "")
message(STATUS "Minimal C++ standard upgraded to ${_MINIMAL_CXX_STANDARD}")
endif()
# Check if a non-trivial minimum has been requested
if(DEFINED _MINIMAL_CXX_STANDARD AND NOT _MINIMAL_CXX_STANDARD EQUAL 98)
if(DEFINED CMAKE_CXX_STANDARD)
set(_CURRENT_STANDARD ${CMAKE_CXX_STANDARD})
elseif(DEFINED _COMPILER_DEFAULT_CXX_STANDARD)
# ref
# https://en.cppreference.com/w/cpp/preprocessor/replace#Predefined_macros
# for constants
if(_COMPILER_DEFAULT_CXX_STANDARD EQUAL 199711)
set(_CURRENT_STANDARD 98)
elseif(_COMPILER_DEFAULT_CXX_STANDARD EQUAL 201103)
set(_CURRENT_STANDARD 11)
elseif(_COMPILER_DEFAULT_CXX_STANDARD EQUAL 201402)
set(_CURRENT_STANDARD 14)
elseif(_COMPILER_DEFAULT_CXX_STANDARD EQUAL 201703)
set(_CURRENT_STANDARD 17)
# C++20: g++-9 defines c++2a with literal 201709, g++-11 & clang-10
# define c++2a with literal 202002
elseif(_COMPILER_DEFAULT_CXX_STANDARD EQUAL 201709
OR _COMPILER_DEFAULT_CXX_STANDARD EQUAL 202002)
set(_CURRENT_STANDARD 20)
else()
message(
FATAL_ERROR
"Unknown current C++ standard ${_COMPILER_DEFAULT_CXX_STANDARD} while trying to check for >= ${_MINIMAL_CXX_STANDARD}"
)
endif()
else()
set(_CURRENT_STANDARD 98)
endif()
# Check that the requested minimum is higher than the currently selected
if(_CURRENT_STANDARD EQUAL 98 OR _CURRENT_STANDARD LESS
_MINIMAL_CXX_STANDARD)
message(
STATUS
"Incompatible C++ standard detected: upgrade required from ${_CURRENT_STANDARD} to >= ${_MINIMAL_CXX_STANDARD}"
)
# Check that the requested minimum is higher than any pre-existing
# CMAKE_CXX_STANDARD
if(NOT CMAKE_CXX_STANDARD
OR CMAKE_CXX_STANDARD EQUAL 98
OR CMAKE_CXX_STANDARD LESS _MINIMAL_CXX_STANDARD)
# Throw error if a specific version is required and the currently
# desired one is incompatible
if(CMAKE_CXX_STANDARD_REQUIRED)
message(
FATAL_ERROR
"CMAKE_CXX_STANDARD_REQUIRED set - cannot upgrade incompatible standard"
)
endif()
# Enforcing a standard version is required - check if we can upgrade
# automatically
if(ENFORCE_MINIMAL_CXX_STANDARD OR MINIMAL_CXX_STANDARD_ENFORCE)
set(CMAKE_CXX_STANDARD ${_MINIMAL_CXX_STANDARD})
message(
STATUS
"CMAKE_CXX_STANDARD automatically upgraded from ${_CURRENT_STANDARD} to ${CMAKE_CXX_STANDARD}"
)
else()
message(
FATAL_ERROR
"CMAKE_CXX_STANDARD upgrade from ${_CURRENT_STANDARD} to >= ${_MINIMAL_CXX_STANDARD} required"
)
endif()
endif()
else() # requested minimum is higher than the currently selected
message(
STATUS
"C++ standard sufficient: Minimal required ${_MINIMAL_CXX_STANDARD}, currently defined: ${_CURRENT_STANDARD}"
)
# current C++ standard was not set in CMake but we enforce it
if(NOT DEFINED CMAKE_CXX_STANDARD AND (ENFORCE_MINIMAL_CXX_STANDARD
OR MINIMAL_CXX_STANDARD_ENFORCE))
message(
STATUS
"CMAKE_CXX_STANDARD was not set: automatically set to currently defined standard ${_CURRENT_STANDARD}"
)
set(CMAKE_CXX_STANDARD ${_CURRENT_STANDARD})
endif()
endif() # requested minimum is higher than the currently selected
endif()
endmacro()