Set C++ standard

The easiest way to quickly set a the C++ standard for your project globally is to use the set(CMAKE_CXX_STANDARD) directive. You can also require that version (instead of making it a recommendation) using set(CMAKE_CXX_STANDARD_REQUIRED TRUE).

cmake_minimum_required(VERSION 3.28)
project(my-project)
set(CMAKE_CXX_STANDARD 20)
set(CMAKE_CXX_STANDARD_REQUIRED TRUE)
add_executable(my-app main.cpp)

#include <iostream>
int main() {
  std::cout << __cplusplus << "\n";
  return 0;
}

cmake -B build
cmake --build build
./build/my-app
#=> 202002

__cplusplus: denotes the version of C++ standard that is being used, expands to value 199711L(until C++11), 201103L(C++11), 201402L(C++14), 201703L(C++17), 202002L(C++20), or 202302L(C++23)

— Replacing text macros - cppreference.com

Set C standard

You can do a similar thing to set the C standard which is different than the C++ standard. Use CMAKE_C_STANDARD instead:

set(CMAKE_C_STANDARD 17)
set(CMAKE_C_STANDARD_REQUIRED TRUE)
add_executable(app1 src/main1.c)

🆕 The better way

The newer way to set the C++ standard for a specific target (like add_executable()) is to use target_compile_features() to only switch on/off C++XY version features per-target instead of globally.

cmake_minimum_required(VERSION 3.28)
project(my-project)
add_executable(my-app main.cpp)
target_compile_features(my-app PRIVATE cxx_std_20)

Note that now we have to declare the target up-front and then switch on C++20 for that specific target. This is generally superior to turning C++XY features on globally so that your CMake configuration doesn't affect any other targets without explicitly meaning to.

You can use target_compile_features() for C standard versions too:

cmake_minimum_required(VERSION 3.28)
project(my-project)
add_executable(my-app main.c)
target_compile_features(my-app PRIVATE c_std_17)

There's a list of enum values that can used in target_compile_features() across three pages: CMAKE_C_KNOWN_FEATURES, CMAKE_CUDA_KNOWN_FEATURES, and CMAKE_CXX_KNOWN_FEATURES. The most useful ones are cxx_std_${XY} where ${XY} indicates a C++ standard number like 20. and c_std_${XY} where ${XY} indicates a C standard number like 17.