C++ SFINAE examples?

Question

I want to get into more template meta-programming. I know that SFINAE stands for "substitution failure is not an error." But can someone show me a good use for SFINAE?

Answer 1

I like using SFINAE to check boolean conditions.

template<int I> void div(char(*)[I % 2 == 0] = 0) {
    /* this is taken when I is even */
}

template<int I> void div(char(*)[I % 2 == 1] = 0) {
    /* this is taken when I is odd */
}

It can be quite useful. For example, i used it to check whether an initializer list collected using operator comma is no longer than a fixed size

template<int N>
struct Vector {
    template<int M> 
    Vector(MyInitList<M> const& i, char(*)[M <= N] = 0) { /* ... */ }
}

The list is only accepted when M is smaller than N, which means that the initializer list has not too many elements.

The syntax char(*)[C] means: Pointer to an array with element type char and size C. If C is false (0 here), then we get the invalid type char(*)[0], pointer to a zero sized array: SFINAE makes it so that the template will be ignored then.

Expressed with boost::enable_if, that looks like this

template<int N>
struct Vector {
    template<int M> 
    Vector(MyInitList<M> const& i, 
           typename enable_if_c<(M <= N)>::type* = 0) { /* ... */ }
}

In practice, i often find the ability to check conditions a useful ability.