C++ 和 GNU++ 的区别
    GNU++是标准C++的扩展，如-std=gnu++11，支持C++11标准和GNU扩展特性

    C++20 Support in GCC C++20 features are available since GCC 8. To enable C++20 support, add the command-line parameter (use in GCC 9 and earlier) to your command line. Or, to enable GNU extensions in addition to C++20 features, add .-std=c++20-std=c++2ag++-std=gnu++20