how to use c++20 in cmake project

Question

I want to use the header available in c++20.

I am using the most up to date release of cmake.

my CMakeFiles looks like

set(CMAKE_CXX_STANDARD 20)
set(CMAKE_CXX_STANDARD_REQUIRED ON)
set(CMAKE_BUILD_TYPE debug)
set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -std=c++20")

i am using clang 9 as my compiler.

however i am getting the following error when including :

fatal error: 'format' file not found
#include <format>

i have also used the flag -std=c++2a, with no effect. In short, i feel i have missed something important here. Im a little new to cmake, any help?

Solution

According to the C++ compiler support page (archive) on cppreference, none of the mainstream compilers currently support the C++20 <format> functionalities.

Therefore, you can't currently use #include <format>. Consider using the {fmt} library for now, on which the C++20 <format> library is based. It is famous for its high safety and efficiency.

Comments

[Hunter Kohler]

Good news! MSVC now supports text formatting.

[Ayxan Haqverdili]

I wonder why this is the case. Is it that hard to implement <format>? More complex parts like "concepts" seem to be already supported by most compilers.

[L. F.]

@Ayxan Concepts started as a TS; many compilers have worked on it several years ago. <format>, however, is mostly just the fmt library standardized.

[Martin]

I came accross this post, searching for a different c++20 feature being supported and followed your link. The table changed a little, but Text formatting is still all red. @Tak Makoni: in case you use MSVC, here is a list of supported features per MSVC version: docs.microsoft.com/en-us/cpp/overview/…