= C++ Standard Common Method
:toc:
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toc::[]

# Introduction

This example shows a common method to set the C++ Standard. This can be used with most versions of CMake. However, if you are targeting a recent version of CMake there are more convenient methods available.

The files in this tutorial are below:

```
A-hello-cmake$ tree
.
├── CMakeLists.txt
├── main.cpp
```

  * link:CMakeLists.txt[CMakeLists.txt] - Contains the CMake commands you wish to run
  * link:main.cpp[main.cpp] - A simple "Hello World" cpp file targeting C++11.

# Concepts

## Checking Compile flags

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.

For example:

[source,cmake]
----
include(CheckCXXCompilerFlag)
CHECK_CXX_COMPILER_FLAG("-std=c++11" COMPILER_SUPPORTS_CXX11)
----

This example will attempt to compile a program with the flag `=std=c++11` and store the result in the variable `COMPILER_SUPPORTS_CXX11`.

The line `include(CheckCXXCompilerFlag)` tells CMake to include this function to make it available for use.

## Adding the flag

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 .

[source,cmake]
----
if(COMPILER_SUPPORTS_CXX11)#
    set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -std=c++11")
elseif(COMPILER_SUPPORTS_CXX0X)#
    set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -std=c++0x")
else()
    message(STATUS "The compiler ${CMAKE_CXX_COMPILER} has no C++11 support. Please use a different C++ compiler.")
endif()
----

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`

# Building the Examples

Below is sample output from building this example.

[source,bash]
----
$ mkdir build
$ cd build

$ cmake ..
-- The C compiler identification is GNU 4.8.4
-- The CXX compiler identification is GNU 4.8.4
-- Check for working C compiler: /usr/bin/cc
-- Check for working C compiler: /usr/bin/cc -- works
-- Detecting C compiler ABI info
-- Detecting C compiler ABI info - done
-- Check for working CXX compiler: /usr/bin/c++
-- Check for working CXX compiler: /usr/bin/c++ -- works
-- Detecting CXX compiler ABI info
-- Detecting CXX compiler ABI info - done
-- Performing Test COMPILER_SUPPORTS_CXX11
-- Performing Test COMPILER_SUPPORTS_CXX11 - Success
-- Performing Test COMPILER_SUPPORTS_CXX0X
-- Performing Test COMPILER_SUPPORTS_CXX0X - Success
-- Configuring done
-- Generating done
-- Build files have been written to: /data/code/01-basic/L-cpp-standard/i-common-method/build

$ make VERBOSE=1
/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
/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
make -f CMakeFiles/Makefile2 all
make[1]: Entering directory `/data/code/01-basic/L-cpp-standard/i-common-method/build'
make -f CMakeFiles/hello_cpp11.dir/build.make CMakeFiles/hello_cpp11.dir/depend
make[2]: Entering directory `/data/code/01-basic/L-cpp-standard/i-common-method/build'
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=
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".
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".
Scanning dependencies of target hello_cpp11
make[2]: Leaving directory `/data/code/01-basic/L-cpp-standard/i-common-method/build'
make -f CMakeFiles/hello_cpp11.dir/build.make CMakeFiles/hello_cpp11.dir/build
make[2]: Entering directory `/data/code/01-basic/L-cpp-standard/i-common-method/build'
/usr/bin/cmake -E cmake_progress_report /data/code/01-basic/L-cpp-standard/i-common-method/build/CMakeFiles 1
[100%] Building CXX object CMakeFiles/hello_cpp11.dir/main.cpp.o
/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
Linking CXX executable hello_cpp11
/usr/bin/cmake -E cmake_link_script CMakeFiles/hello_cpp11.dir/link.txt --verbose=1
/usr/bin/c++    -std=c++11    CMakeFiles/hello_cpp11.dir/main.cpp.o  -o hello_cpp11 -rdynamic
make[2]: Leaving directory `/data/code/01-basic/L-cpp-standard/i-common-method/build'
/usr/bin/cmake -E cmake_progress_report /data/code/01-basic/L-cpp-standard/i-common-method/build/CMakeFiles  1
[100%] Built target hello_cpp11
make[1]: Leaving directory `/data/code/01-basic/L-cpp-standard/i-common-method/build'
/usr/bin/cmake -E cmake_progress_start /data/code/01-basic/L-cpp-standard/i-common-method/build/CMakeFiles 0
----