How to get CMake to pass either std=c++14/c++1y or c++17/c++1z based on GCC version?

Question

When wanting to specify a particular C++ version, the recommended way to do this with CMake 3.1 and later is to use the CXX_STANDARD, CXX_STANDARD_REQUIRED and CXX_EXTENSIONS target properties, or their variable equivalents to specify target defaults. Full details can be found here, but the short version goes something like this:

cmake_minimum_required(VERSION 3.1)
project(Example)

set(CMAKE_CXX_STANDARD 14)
set(CMAKE_CXX_STANDARD_REQUIRED ON)
set(CMAKE_CXX_EXTENSIONS OFF)
# ... Define targets, etc. as usual

CMake should then select the appropriate compiler flag for the requested C++ standard based on what the compiler supports, or error out if it doesn't support the requested standard.

It should also be noted that CMake may upgrade the target to use a later language standard than the one specified by its CXX_STANDARD target property. The use of compile feature requirements (as mentioned in @FlorianWolters answer) can raise the language standard requirement. In fact, CMake will always pick the stronger language requirement specified by either the CXX_STANDARD target property or the compile feature requirements set on the target. Note also that the CMake documentation as of 3.10.1 does not accurately reflect the way CXX_EXTENSIONS interacts with compile features, as CXX_EXTENSIONS only takes effect if CXX_STANDARD is also specified for most common compilers (since they are specified together with the one compiler flag).

Modern CMake code should use the target_compile_features command to request a specific C++ standard. This can be specified as build requirement only (PRIVATE), usage requirement only (INTERFACE) or build and usage requirement (PUBLIC).

Example:

cmake_minimum_required(VERSION 3.9.4)

project(cpp-snippets)
add_executable(cpp-snippets "main.cpp")
target_compile_features(cpp-snippets PRIVATE cxx_std_17)

Refer to the section Requiring Language Standards in the official CMake documentation for cmake-compile-features to learn more.

Check if the compiler supports the flags? Perhaps something like

include(CheckCXXCompilerFlag)

# Check for standard to use
check_cxx_compiler_flag(-std=c++17 HAVE_FLAG_STD_CXX17)
if(HAVE_FLAG_STD_CXX17)
    # Have -std=c++17, use it
else()
    check_cxx_compiler_flag(-std=c++1z HAVE_FLAG_STD_CXX1Z)
    if(HAVE_FLAG_STD_CXX1Z)
        # Have -std=c++1z, use it
    else()
        # And so on and on...
    endif()
endif()