Exploring C++ types with puts(__PRETTY_FUNCTION__)

Today on Slack I noticed that some people still don’t know this one, so it should be shared more widely.

template<class T>
void f() {
    puts(__PRETTY_FUNCTION__);
}

You can use this magic formula to explore what the compiler is doing to your types, either for metaprogramming purposes or just to learn more about auto, auto&&, and decltype(auto). For example:

#define EXPLORE(expr) \
    printf("decltype(" #expr ") is... "); \
    f<decltype(expr)>();

int main() {
    auto x = 5;
    auto&& y = 5;
    decltype(auto) z = 5;

    EXPLORE(x); EXPLORE((x));
    EXPLORE(y); EXPLORE((y));
    EXPLORE(z); EXPLORE((z));
}

The MSVC (Visual Studio) equivalent of __PRETTY_FUNCTION__ is __FUNCSIG__.

Sample GCC/EDG __PRETTY_FUNCTION__:

decltype(x) is... void f() [with T = int]
decltype((x)) is... void f() [with T = int &]

Sample Clang __PRETTY_FUNCTION__:

decltype(x) is... void f() [T = int]
decltype((x)) is... void f() [T = int &]

Sample MSVC __FUNCSIG__:

decltype(x) is... void __cdecl f<int>(void)
decltype((x)) is... void __cdecl f<int&>(void)
Posted 2018-08-22
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