Enable support for C++23 in esp-idf (IDFGH-9684)

[0xFEEDC0DE64]

std::expected, std::to_underlying, modules and concepts were missing in the esp-idf and this change enables all of them.

[0xFEEDC0DE64]

I tried putting the following statements in my CMakeLists.txt:

set(CMAKE_CXX_STANDARD 23)
set(CMAKE_CXX_STANDARD_REQUIRED ON)
set(CMAKE_CXX_EXTENSIONS ON)

But esp-idf keeps appending a -std=gnu20 at all compiler calls which destroys my switch to c++23.

This has to be changed in idf itself.

[0xjakob]

@0xFEEDC0DE64 Thanks for the suggestion! We are considering this PR but we first need to run tests on it and make sure that it won't affect anyone else.

[0xFEEDC0DE64]

Can we make it a sdkconfig instead so you dont have to do testing? With defaults set to cpp20

[igrr]

@0xFEEDC0DE64 To set a custom value of C++ standard for your component (without having to change the default value used for other components) you can use set_property CMake command on your component library target. For example, in component CMakeLists.txt file:

idf_component_register(
    SRCS main.cpp
)

set_property(TARGET ${COMPONENT_LIB} PROPERTY CXX_STANDARD 23)

Then the component library will be compiled with an additional -std=gnu++23 argument, which will override the -std=gnu++20 argument added by default.

I've tried to compile the following example main.cpp file with the above CMakeLists.txt and it worked as expected:

#include <expected>

using std::expected;

extern "C" void app_main(void)
{
}

(Not saying that we won't update the default C++ language standard to C++23; just giving you a suggestion how to solve this problem without having to patch IDF or having to wait for us to do this update.)

[0xjakob]

sha=a96f5bd5b098feb0e3ac39180bd8b19183f80907

[CLAassistant]

All committers have signed the CLA.

[0xFEEDC0DE64]

@igrr we just rebased to the latest esp-idf again, and the compiler is called again with -std=gnu++23 and -std=gnu++20. which results in compiler errors.

We are setting the c++ version with

set(CMAKE_CXX_STANDARD 23)
set(CMAKE_CXX_STANDARD_REQUIRED ON)
set(CMAKE_CXX_EXTENSIONS ON)

in the topmost CMakeLists.txt and I added your set_property() to each component in our firmware project

set_property(TARGET ${COMPONENT_LIB} PROPERTY CXX_STANDARD 23)

But still I have to patch build.cmake in esp-idf to finally make it work. What has been merged here?

[0xFEEDC0DE64]

Maybe COMPONENT_LIB has to be replaced with COMPONENT_TARGET ?

[igrr]

I think build.cmake has been updated in a96f5bd already. Could you please verify you have this commit in your fork?

[0xFEEDC0DE64]

Yes we have that commit, but for some reason the if branch for clang was needed to patch to get it to compile again. I dont understand why this happens.

0xFEEDC0DE64@8e9e12a

[0xjakob]

@0xFEEDC0DE64 Sorry for this. We tried to fix an incompatibility with our version of clang, but the code was faulty. We'll take a look ASAP.

[0xjakob]

@0xFEEDC0DE64 Meanwhile, could you try the following in your component's CMakeLists.txt?

target_compile_options(${COMPONENT_LIB} PRIVATE -std=gnu++23)

When I use this, the standard is set twice

• First to gnu++20, by IDF general build commands,
• The second time to gnu++23, from the component's CMakeLists.txt, which is the one prevailing.

I can then compile C++23 code (which fails compiling without setting the option in the component's CMakeLists.txt).