Why is it possible to link against C++ Dynamic Libraries when C++ does not have a stable ABI?

Question

Taking Boost as an example, why am I able to link against boost::filesystem from my C++ program, when C++ does not have a stable ABI.

I have Boost installed in my system, and a toy program is able to link using -lboost_filesystem, even though boost::filesystem exposes a C++ API ( no extern 'C' ).

So, is it possible to create C++ shared libraries that can be linked by various compiler versions? How does Boost achieve this without "extern C"?

I tried:

#include <iostream>
#include <boost/filesystem.hpp>

namespace fs = boost::filesystem;

int main() {
    // Replace "/path/to/directory" with the path to the directory you want to list
    std::string directory_path = "/path/to/directory";

    try {
        // Check if the given path exists and is a directory
        if (fs::exists(directory_path) && fs::is_directory(directory_path)) {
            std::cout << "Listing files in directory: " << directory_path << std::endl;

            // Iterate through the files in the directory
            for (const auto& entry : fs::directory_iterator(directory_path)) {
                std::cout << entry.path() << std::endl;
            }
        } else {
            std::cerr << "Error: Invalid directory path." << std::endl;
            return 1;
        }
    } catch (const fs::filesystem_error& ex) {
        std::cerr << "Error: " << ex.what() << std::endl;
        return 1;
    }

    return 0;
}

g++ -o fs fs.cpp -I/usr/include/boost -I/usr/include/boost/filesystem -I/usr/include/boost/system -lboost_system -lboost_filesystem -std=c++14

Expectation: Should get linking errors as C++ does not have a stable ABI

Got: Compilation Succeeded.

Answer 1

I'm not sure about ABI per se, but the real crux of linking a C++ library built by one compiler with a program compiled with another compiler would rely on both compilers sharing the same name mangling schema.

And indeed, g++ and clang++ work the same and have had the same schema for years.

But as this question and answer reveal, trying to do something similar by compiling a library on Windows with MinGW and expecting it to link with Visual Studio, doesn't work. Probably not possible unless something has changed in recent years.