std::call_once vs std::mutex for thread-safe initialization

Question

I\'m a bit confused about the purpose of std::call_once. To be clear, I understand exactly what std::call_once does, and how to use it. It\'

Answer 1

Slight variation on standard C++ solution is to use lambda inside the usual one:

// header.h
namespace dbj_once {

    struct singleton final {};

    inline singleton & instance()
    {
        static singleton single_instance = []() -> singleton {
            // this is called only once
            // do some more complex initialization
            // here
            return {};
        }();
        return single_instance;
    };

 } // dbj_once

Please observe

• anonymous namespace implies default static linkage for variables inside. Thus do not put this inside it. This is header code.
• worth repeating: this is safe in presence of multiple threads (MT) and is supported as a such by all major compilers
• inside is a lambda which is guaranteed to be called only once
• this pattern is also safe to use in header only situations