do once for IF

if_once.cpp

if (static bool do_once = true; std::exchange(do_once, false)) {
}

if (static bool do_once; !std::exchange(do_once, false)) {
}

struct Once {
    bool b = true;
    explicit operator bool() { return std::exchange(b, false); }
};

if (static Once _; _) {
}

// gcc, clang, icc only; use [[unlikely]] in C++20 instead
#define unlikely(x) __builtin_expect(!!(x), 0)

struct Once {
    bool b = false;
    explicit operator bool()
    {
        if (unlikely(not b)) {
            b = true;
            return true;
        }

        return false;
    }
};


// C++17
if (static int i; i == 0 && (i = 1)){
}

// C++11
if ([]{static int i; return i == 0 && (i = 1);}()){
}