使用gcc -lstdc++编译C++文件时，如何查看所用C++版本？

Great question! When you're compiling C++ code with gcc -g test.cpp -lstdc++ -o out, figuring out which C++ standard is being used is super useful—here are a few reliable ways to do it:

1. Check GCC's default C++ standard based on its version #

First, find out your GCC version by running:

gcc --version

GCC uses different default C++ standards depending on its release. Here's a quick breakdown of the defaults:

• GCC 4.7 or older: Defaults to C++98
• GCC 4.8–4.9: Defaults to C++11
• GCC 5–6: Defaults to C++14
• GCC 7–10: Defaults to C++17
• GCC 11 and newer: Defaults to C++20

If you aren't adding any -std= flags to your compile command, this is the standard you're using.

2. Use verbose output to see the exact standard during compilation #

Add the -v flag to your compile command to get detailed build logs:

gcc -g test.cpp -lstdc++ -o out -v

Scroll through the output and look for a line starting with something like -std=gnu++xx—the xx here indicates the version (the gnu prefix means it's using GNU extensions alongside the standard C++ version). For example, -std=gnu++17 means you're using C++17 with GNU-specific additions.

3. Detect the version directly in your code #

This is probably the most straightforward method—add a small snippet to your test.cpp to print the C++ standard being used:

#include <iostream>

int main() {
    #if __cplusplus == 202002L
        std::cout << "Using C++20" << std::endl;
    #elif __cplusplus == 201703L
        std::cout << "Using C++17" << std::endl;
    #elif __cplusplus == 201402L
        std::cout << "Using C++14" << std::endl;
    #elif __cplusplus == 201103L
        std::cout << "Using C++11" << std::endl;
    #elif __cplusplus == 199711L
        std::cout << "Using C++98" << std::endl;
    #else
        std::cout << "Unknown C++ version" << std::endl;
    #endif
    return 0;
}

Compile and run the program, and it'll directly tell you which standard is in use. This works because the __cplusplus macro is defined by the compiler to match the current C++ standard version.

Bonus: How to manually specify a C++ version #

If you want to override the default, just add the -std=c++xx flag to your command. For example:

• Use pure C++17: gcc -g test.cpp -lstdc++ -o out -std=c++17
• Use C++20 with GNU extensions: gcc -g test.cpp -lstdc++ -o out -std=gnu++20

内容的提问来源于stack exchange，提问作者xksyvone