Can I compile using C++ 20?

kitfox

I have an up-to-date installation of Qt. Is it possible to compile my code with the latest features in C++20? It seems C++17 is the latest Qt supports. I saw a post that suggested putting CONFIG += c++2a in my .pro file, but that doesn't seem to do anything for me.

SGaist

Hi,

Which version of Qt are you using ?
With which compiler ?
On which OS ?

kitfox

Qt Version 5.15.0
mingw-32 compiler
Windows 10

Bonnie

Qt will add -std=gnu++2a to CXXFLAGS after adding CONFIG+=c++2a in your case.
But the final effect is depending on your compiler.

J.Hilk

@kitfox well does your compiler support c++20 ?

Where did you get it, and when (last update)

if you make a simple c++ program with

#include <iostream>

using namespace std;

int main()
{
#if __cplusplus >= 202002L
    // C++20 (and later) code
    cout << "C++20 (and later)" << endl;
#endif
    return 0;
}

Does it then print something ?

kitfox

@J-Hilk
I presume so since it is the mingw that comes with Qt 5.15.0. However, I do not know what the configuration options are that I need to get it to compile for C++20.

When I compile with CONFIG += c++2a, __cplusplus gives the string 201709. CONFIG += c++17 gives 201703 and CONFIG += c++20 gives 201402. I'm not sure what to pass to indicate a different version of C++.

If my compiler does not support c++ 20, how would I get one that does?

Christian Ehrlicher

@kitfox said in Can I compile using C++ 20?:

If my compiler does not support c++ 20, how would I get one that does?

GCC 8 does not support more than 201709L I would guess according to the documentation. Therefore you need to upgrade it. MinGW64 does not provide a later compiler than gcc8: http://mingw-w64.org/doku.php/versions

SGaist

AFAIK, on Windows, the best compiler on Windows with C++20 support is currently VS2019.