When should I use [[maybe_unused]]?

Question

What is good about using [[maybe_unused]]? Consider

int winmain(int instance, int /*prevInstance*/, const char */*cmdline*/, int show);

int win         


        

Answer 1

Baum mit Augen's answer is the definitive and undisputed explanation. I just want to present another example, which doesn't require macros. Specifically, C++17 introduced the constexpr if construct. So you may see template code like this (bar the stupid functionality):

#include 

template
auto add_or_double(T t1, T t2) noexcept {
    if constexpr (std::is_same_v)
        return t1 + t2;
    else
        return t1 * 2.0;
}

int main(){
    add_or_double(1, 2);
    add_or_double(1.0, 2.0);
}

As of writing this, GCC 8.0.1 warns me about t2 being unused when the else branch is the instantiated one. The attribute is indispensable in a case like this too.