What does it mean when one says something is SFINAE-friendly?

Question

I can\'t clearly get the grasp of what it means when one mentions that a particular function, struct or ... is SFINAE-friendly.

Would someone please

Answer 1

When it allows substitution failure without hard error (as static_assert).

for example

template 
void call_f(const T& t)
{
    t.f();
}

The function is declared for all T, even those with don't have f, so you cannot do SFINAE on call_f as the method does exist. (Demo of non compiling code).

With following change:

template 
auto call_f(const T& t) ->decltype(t.f(), void())
{
    t.f();
}

The method exists only for valid T. so you can use SFINAE as

template
auto call_f_if_available_impl(const T& t, int) -> decltype(call_f(t))
{
    call_f(t);
}

template
auto call_f_if_available_impl(const T& t, ...)
{
    // Do nothing;
}

 template
 auto call_f_if_available(const T& t)
 {
    call_f_if_available_impl(t, 0);
 }

Note the int = 0 and ... is to order the overload. Demo

--

An other case is when the template add special parameter to apply SFINAE for specialization:

template  struct S;

And then

// Specialization only available for T which respect the traits.
template 
struct S::value>>
{
};