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 <type_traits>

template<typename T>
auto add_or_double(T t1, T t2) noexcept {
    if constexpr (std::is_same_v<T, int>)
        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.

Answer 2

If the parameter is definitely unused, [[maybe_unused]] is not particularly useful, unnamed parameters and comments work just fine for that.

[[maybe_unused]] is mostly useful for things that are potentially unused, like in

void fun(int i, int j) {
    assert(i < j);
    // j not used here anymore
}

This can't be handled with unnamed parameters, but if NDEBUG is defined, will produce a warning because j is unused.

Similar situations can occur when a parameter is only used for (potentially disabled) logging.