Approaches to function SFINAE in C++

Question

I am using function SFINAE heavily in a project and am not sure if there are any differences between the following two approaches (other than style):

#include &l         


        

Answer 1

I have seen method 2 used more often in stackoverflow, but I prefer method 1.

Suggestion: prefer method 2.

Both methods work with single functions. The problem arises when you have more than a function, with the same signature, and you want enable only one function of the set.

Suppose that you want enable foo(), version 1, when bar() (pretend it's a constexpr function) is true, and foo(), version 2, when bar() is false.

With

template ()>>
void foo () // version 1
 { }

template ()>>
void foo () // version 2
 { }

you get a compilation error because you have an ambiguity: two foo() functions with the same signature (a default template parameter doesn't change the signature).

But the following solution

template (), bool> = true>
void foo () // version 1
 { }

template (), bool> = true>
void foo () // version 2
 { }

works, because SFINAE modify the signature of the functions.

Unrelated observation: there is also a third method: enable/disable the return type (except for class/struct constructors, obviously)

template 
std::enable_if_t()> foo () // version 1
 { }

template 
std::enable_if_t()> foo () // version 2
 { }

As method 2, method 3 is compatible with selection of alternative functions with same signature.