is_empty

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### is_empty

    template <class T>
    struct is_empty : public true_type-or-false_type {};

Inherits: If T is an empty class type then inherits from true_type, otherwise inherits from false_type.

C++ Standard Reference: 10p5.

Header: `#include <boost/type_traits/is_empty.hpp>` or `#include <boost/type_traits.hpp>`

Compiler Compatibility: In order to correctly detect empty classes this trait relies on either:

- the compiler implementing zero sized empty base classes, or
- the compiler providing intrinsics to detect empty classes.

Can not be used with incomplete types.

Can not be used with union types, until is_union can be made to work.

If the compiler does not support partial-specialization of class templates, then this template can not be used with abstract types.

Examples:

> Given: `struct empty_class {};`

> `is_empty<empty_class>` inherits from `true_type`.

> `is_empty<empty_class const>::type` is the type `true_type`.

> `is_empty<empty_class>::value` is an integral constant expression that evaluates to true.

> `is_empty<T>::value_type` is the type `bool`.