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add examples for setting C++ standard
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Thom Troy
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01-basic/L-cpp-standard/i-common-method/README.adoc
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= C++ Standard Common Method
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:toc:
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:toc-placement!:
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toc::[]
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# Introduction
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This example shows a common method to set the C++ Standard. This can be used with most versions of CMake. However, if you are targetting a recent version of CMake there are more convenient methods available.
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The files in this tutorial are below:
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```
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A-hello-cmake$ tree
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.
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├── CMakeLists.txt
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├── main.cpp
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```
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* link:CMakeLists.txt[CMakeLists.txt] - Contains the CMake commands you wish to run
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* link:main.cpp[main.cpp] - A simple "Hello World" cpp file targeting C++11.
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# Concepts
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## Checking Compile flags
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CMake has support for attempting to compile a program with any flags you pass into the function `CMAKE_CXX_COMPILER_FLAG`. The result is then stored in a variable that you pass in.
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For example:
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[source,cmake]
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----
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include(CheckCXXCompilerFlag)
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CHECK_CXX_COMPILER_FLAG("-std=c++11" COMPILER_SUPPORTS_CXX11)
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----
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This example will attempt to compile a program with the flag `=std=c++11` and store the result in the variable `COMPILER_SUPPORTS_CXX11`.
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The line `include(CheckCXXCompilerFlag)` tells CMake to include this function to make it available for use.
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## Adding the flag
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Once you have determined if the compile supports a flag, you can then use the link:../../G-compile-flags/[standard cmake methods] to add this flag to a target. In this example we use the `CMAKE_CXX_FLAGS` to propegate the flag to all targets .
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[source,cmake]
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----
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if(COMPILER_SUPPORTS_CXX11)#
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set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -std=c++11")
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elseif(COMPILER_SUPPORTS_CXX0X)#
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set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -std=c++0x")
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else()
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message(STATUS "The compiler ${CMAKE_CXX_COMPILER} has no C++11 support. Please use a different C++ compiler.")
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endif()
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----
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The above example only checks for the gcc version of the compile flags and supports fallback from C+\+11 to the pre-standardisation C+\+0x flag. In real usage you may want to check for C++14, or add support for different methods of setting the compile, e.g. `-std=gnu++11`
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# Building the Examples
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Below is sample output from building this example.
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[source,bash]
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----
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$ mkdir build
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$ cd build
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$ cmake ..
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-- The C compiler identification is GNU 4.8.4
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-- The CXX compiler identification is GNU 4.8.4
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-- Check for working C compiler: /usr/bin/cc
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-- Check for working C compiler: /usr/bin/cc -- works
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-- Detecting C compiler ABI info
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-- Detecting C compiler ABI info - done
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-- Check for working CXX compiler: /usr/bin/c++
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-- Check for working CXX compiler: /usr/bin/c++ -- works
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-- Detecting CXX compiler ABI info
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-- Detecting CXX compiler ABI info - done
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-- Performing Test COMPILER_SUPPORTS_CXX11
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-- Performing Test COMPILER_SUPPORTS_CXX11 - Success
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-- Performing Test COMPILER_SUPPORTS_CXX0X
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-- Performing Test COMPILER_SUPPORTS_CXX0X - Success
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-- Configuring done
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-- Generating done
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-- Build files have been written to: /data/code/01-basic/L-cpp-standard/i-common-method/build
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$ make VERBOSE=1
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/usr/bin/cmake -H/data/code/01-basic/L-cpp-standard/i-common-method -B/data/code/01-basic/L-cpp-standard/i-common-method/build --check-build-system CMakeFiles/Makefile.cmake 0
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/usr/bin/cmake -E cmake_progress_start /data/code/01-basic/L-cpp-standard/i-common-method/build/CMakeFiles /data/code/01-basic/L-cpp-standard/i-common-method/build/CMakeFiles/progress.marks
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make -f CMakeFiles/Makefile2 all
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make[1]: Entering directory `/data/code/01-basic/L-cpp-standard/i-common-method/build'
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make -f CMakeFiles/hello_cpp11.dir/build.make CMakeFiles/hello_cpp11.dir/depend
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make[2]: Entering directory `/data/code/01-basic/L-cpp-standard/i-common-method/build'
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cd /data/code/01-basic/L-cpp-standard/i-common-method/build && /usr/bin/cmake -E cmake_depends "Unix Makefiles" /data/code/01-basic/L-cpp-standard/i-common-method /data/code/01-basic/L-cpp-standard/i-common-method /data/code/01-basic/L-cpp-standard/i-common-method/build /data/code/01-basic/L-cpp-standard/i-common-method/build /data/code/01-basic/L-cpp-standard/i-common-method/build/CMakeFiles/hello_cpp11.dir/DependInfo.cmake --color=
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Dependee "/data/code/01-basic/L-cpp-standard/i-common-method/build/CMakeFiles/hello_cpp11.dir/DependInfo.cmake" is newer than depender "/data/code/01-basic/L-cpp-standard/i-common-method/build/CMakeFiles/hello_cpp11.dir/depend.internal".
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Dependee "/data/code/01-basic/L-cpp-standard/i-common-method/build/CMakeFiles/CMakeDirectoryInformation.cmake" is newer than depender "/data/code/01-basic/L-cpp-standard/i-common-method/build/CMakeFiles/hello_cpp11.dir/depend.internal".
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Scanning dependencies of target hello_cpp11
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make[2]: Leaving directory `/data/code/01-basic/L-cpp-standard/i-common-method/build'
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make -f CMakeFiles/hello_cpp11.dir/build.make CMakeFiles/hello_cpp11.dir/build
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make[2]: Entering directory `/data/code/01-basic/L-cpp-standard/i-common-method/build'
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/usr/bin/cmake -E cmake_progress_report /data/code/01-basic/L-cpp-standard/i-common-method/build/CMakeFiles 1
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[100%] Building CXX object CMakeFiles/hello_cpp11.dir/main.cpp.o
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/usr/bin/c++ -std=c++11 -o CMakeFiles/hello_cpp11.dir/main.cpp.o -c /data/code/01-basic/L-cpp-standard/i-common-method/main.cpp
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Linking CXX executable hello_cpp11
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/usr/bin/cmake -E cmake_link_script CMakeFiles/hello_cpp11.dir/link.txt --verbose=1
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/usr/bin/c++ -std=c++11 CMakeFiles/hello_cpp11.dir/main.cpp.o -o hello_cpp11 -rdynamic
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make[2]: Leaving directory `/data/code/01-basic/L-cpp-standard/i-common-method/build'
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/usr/bin/cmake -E cmake_progress_report /data/code/01-basic/L-cpp-standard/i-common-method/build/CMakeFiles 1
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[100%] Built target hello_cpp11
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make[1]: Leaving directory `/data/code/01-basic/L-cpp-standard/i-common-method/build'
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/usr/bin/cmake -E cmake_progress_start /data/code/01-basic/L-cpp-standard/i-common-method/build/CMakeFiles 0
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----
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