# C++ enable_if via return type

I found SFINAE or “Substitution Failure Is Not An Error” quite fascinating. At first, it looked kind of cryptic to me (what do all these “typenames” mean?), so I tended to avoid it.
But when used right, `std::enable_if`

(which leverages SFINAE) really helps simplifying the code. So I started to depend on it.

Recently I wrote a function based on the example provided by Cppreference std::void_t article.
Basically, I wanted to reset a variable, which can be either a scalar type (int, float, etc) or a container. If it is a container, I wanted to call `Container::clear()`

.
Otherwise, I can simply set it to zero.

```
template <typename T, typename = void>
struct is_clearable : std::false_type {};
template <typename T>
struct is_clearable<T, std::void_t<decltype(std::declval<T>().clear())> > : std::true_type {};
template <typename T>
inline constexpr bool is_clearable_v = is_clearable<T>::value;
template <typename T>
typename std::enable_if_t<is_clearable_v<T> > reset(T* t) { // #1
t->clear();
}
template <typename T>
typename std::enable_if_t<!is_clearable_v<T> > reset(T* t) { // #2
*t = 0;
}
```

In my example, `std::void_t`

is used to detect whether `T`

has the member function `clear()`

. `is_clearable<T>::value`

yields `true`

or `false`

based on the result. `is_clearable_v<T>`

is defined as a compile-time boolean constant that takes the `is_clearable<T>::value`

.

Then, the `reset(T* t)`

function is defined separately for the two cases. The first version is enabled (via the return type) when `T`

has the member function `clear()`

; the second version is enabled when `T`

does not.

It turns out to work as advertised for me. But to apply this enable_if idiom, one would have to figure out what any of these (`decltype`

, `declval`

, `constexpr`

, `void_t`

, `enable_if`

) are, and I think that’s not trivial without the help of some good examples.