What is std::decay and when it should be used?

Question

What are the reasons for the existence of std::decay? In what situations is std::decay useful?

Answer 1

When dealing with template functions that take parameters of a template type, you often have universal parameters. Universal parameters are almost always references of one sort or another. They're also const-volatile qualified. As such, most type traits don't work on them as you'd expect:

template
void func(T&& param) {
    if (std::is_same::value) 
        std::cout << "param is an int\n";
    else 
        std::cout << "param is not an int\n";
}

int main() {
    int three = 3;
    func(three);  //prints "param is not an int"!!!!
}

http://coliru.stacked-crooked.com/a/24476e60bd906bed

The solution here is to use std::decay:

template
void func(T&& param) {
    if (std::is_same::type,int>::value) 
        std::cout << "param is an int\n";
    else 
        std::cout << "param is not an int\n";
}

http://coliru.stacked-crooked.com/a/8cbd0119a28a18bd