My program looks compiling in C++ 20 but cannot include <format> (to call std::format) [duplicate]

Question

I've trained myself on a C++ 17 book on a project made of multiple executables in order to run each of them thing quickly. Today, to handle a new source Copies.cpp I've asked for C++ 20.
(CMakeLists.txt in text format at the end of the question)

However, my code is unable to #include <format> to access std::format as depicted in cppreference.com

It looks compiling with C++ 20, as far as I can see:
(-g -std=c++20 in the log)

/home/lebihan/.local/share/JetBrains/Toolbox/apps/clion/bin/cmake/linux/x64/bin/cmake --build /home/lebihan/dev/apprentissageDev/cpp/teach_yourself/cppChapitre03/cmake-build-debug --target all -j 30
[1/2] Building CXX object CMakeFiles/Copies.dir/Copies.cpp.o
FAILED: CMakeFiles/Copies.dir/Copies.cpp.o 
/usr/bin/c++   -g -std=c++20 -fdiagnostics-color=always -MD -MT CMakeFiles/Copies.dir/Copies.cpp.o -MF CMakeFiles/Copies.dir/Copies.cpp.o.d -o CMakeFiles/Copies.dir/Copies.cpp.o -c /home/lebihan/dev/apprentissageDev/cpp/teach_yourself/cppChapitre03/Copies.cpp
/home/lebihan/dev/apprentissageDev/cpp/teach_yourself/cppChapitre03/Copies.cpp:1:10: fatal error: format: Aucun fichier ou dossier de ce type
    1 | #include <format>
      |          ^~~~~~~~
compilation terminated.
ninja: build stopped: subcommand failed.

What am I missing?

My C++ compiler is:

c++ --version

c++ (Debian 12.2.0-14+deb12u1) 12.2.0
Copyright (C) 2022 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

And my CMakeCache.txt file the CMake .. produces from cmake-build-debug has this content:

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As I'm investigating at the same time I'm asking this question, it seems that std::format isn't available to g++/c++ 12.x?

In my mind, I was taking for sure that for a C++ language version (C++ 11, 14, 17, 20, 23) was a single standard library std with improvments, but it's not the case?

There's a matter of:

I have C++ 20.g++ 1.22 that implements one third of std?
There's a C++ 20 g++ 1.23 that implements two thirds of std?
And a g++ 1.24 (maybe) that would have all the std functions that are mentioned in cppreference for C++ 20?

Am I in front of:

A build that doesn't start a compilation in C++ 20, whatever it says,
Or a compiler that is only partialy implementing C++ 20 std?

cmake_minimum_required(VERSION 3.25)
project(cppChapitre03)

set(CMAKE_CXX_STANDARD 20)
set(CMAKE_CXX_STANDARD_REQUIRED YES)
set(CMAKE_CXX_EXTENSIONS OFF)

add_executable(VariablesDemo_MultiplyTwoNumbers
    "3.1 VariablesDemo_MultiplyTwoNumbers.cpp")

add_executable(VariablesScopeDemo
    "3.2 VariablesScopeDemo.cpp")

add_executable(GlobalVariablesDemo
    "3.3 GlobalVariablesDemo.cpp")

add_executable(Operators_Signed_Unsigned_Overflow
    "3.4 Operators_Signed_Unsigned_Overflow.cpp")

add_executable(Using_sizeof
    "3.5 Using_sizeof.cpp")

add_executable(UsingAuto
    "3.6 UsingAuto.cpp")

add_executable(Demo_Constants
    "3.7 Demo_Constants.cpp")

add_executable(using_constexpr
    "3.8 using constexpr.cpp")

add_executable(Using_enum_Constants
    "3.9 Using_enum_Constants.cpp")

add_executable(Calculate_Area_Circumference
    "3.Ex3 Calculate_Area_Circumference.cpp")

add_executable(Copies
    "Copies.cpp")

As far as I can see from cppreference compiler support std::format is only available from gcc 13 (look for Text formatting (FTM)*) — Pepijn Kramer
– Pepijn Kramer, Commented Jun 2 at 4:42
@PepijnKramer It's what it seems to me too, as I'm still searching. I've been trapped by cppreference that says that std::format is a C++ 20 function, but compilers that handle only a bit of a C++ 20 language version... I've never met that before. It's like the old era of compatible PC being compatible at 97% with IBM PC. I'm using a compiler being just a bit compatible with C++ 20... — Marc Le Bihan
– Marc Le Bihan, Commented Jun 2 at 4:47
Unlike Java, the C++ standards comity is independent from the compiler writers. The comity comes up with a standard, and then the compiler writers have to play catch-up with it. — Some programmer dude
– Some programmer dude, Commented Jun 2 at 5:23
You should avoid pictures of text, they are not friendly to searching and accessibility features like screen readers. — Dominik Kaszewski
– Dominik Kaszewski, Commented Jun 2 at 5:50
Also have a look at cppreference's documentation on feature testing. This can help you determine if your compiler supports a specific feature at compile time. Which will show exactly how far each compiler version caught up with for example the C++20 standard. E.g. the feature you are looking for is : __cpp_lib_format — Pepijn Kramer
– Pepijn Kramer, Commented Jun 2 at 6:03

Dominik Kaszewski · Accepted Answer · 2025-06-02 05:46:44Z

0

Parts of the standard library require language features from a particular standard. For example, you cannot implement std::tuple<Ts...> without language support for variadic templates that came in C++11 (at least not for arbitrary length).

std::format requires consteval and other constexpr extensions to provide format and argument validation at compile time, that were not available in C++17 and older standards.

Since older compilers may only partially support a newer language standard (kinda like a beta), they can also only partially support a newer library standard and/or be missing some files. Trying to copy missing code from a newer version without updating the compiler itself would end up in compilation errors.

You can also take a look at fmtlib which is the original library that was later standardized as std::format. It only requires C++11, but be warned that some compiler errors become runtime errors then, so you should compensate with good unit test coverage.

edited Jun 2 at 5:46

answered Jun 2 at 5:40

Dominik Kaszewski

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4 Comments

Alan Birtles Jun 2 at 6:33

Gccs c++20 language support was completed in gcc 11 so that's not the reason it doesn't support format until gcc 13

Marc Le Bihan Jun 2 at 8:13

Ok so It asks the question of knowing the list of versions of compilers that are complete for a given C++ language version. As for Java, you have some versions (from 12 to 16, from 18 to 20) that are for feature preview, you have also such C++ compilers. →g++ 13.1 is able to handle C++ 20 perfectly, but g++ 12.x has only few of its features implemented (because C++ comes with stdlib). Like Java 12 to 16 or 18 to 20 g++ 12.x should not be used to compile C++ 20 targeting a production environment, I guess. g++/c++ 13.1 is the "LTS", let's say, for C++ 20. Before, it's only preview versions.

sweenish Jun 2 at 12:20

Let's not assign LTS labels to C++ compilers. It makes no sense. The most current stable version of gcc is 15. If you want newer features, use newer compilers. That's all there is to it. Java, python, Rust, etc. are not different in this regard, they're just better self-contained ecosystems.

sweenish Jun 2 at 12:25

As for splitting standard support between the compiler and standard library implementation, that's an incredibly fine hair to split, no matter how technically true it is.

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