What's the use of second parameter of std::enable_if?

Question

I am confused about the second parameter of std::enable_if. In using of a return type of int, we can make it using:

template 
typename std::en         


        

Answer 1

what's the difference of too functions below:

One is a template that can be called for any CopyConstructible type, the enable_if only constrains it when the default template argument is used:

#include 

template::value>::type >
T too(T t) { std::cout << "here" << std::endl; return t; }

int too(int t) { std::cout << "there" << std::endl; return t; }

int main()
{
    too(1.0);
}