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## empty_value

### Authors

- Glen Fernandes

### Overview

The header <boost/core/empty_value.hpp> provides the class template `boost::empty_value` for library authors to conveniently leverage the Empty Base Optimization to store objects of potentially empty types.

### Examples

The following example shows `boost::empty_value` used to create a type that stores a pointer, comparer, and allocator, where the comparer and allocator could be empty types.

    template<class Ptr, class Compare, class Allocator>
    class storage
        : empty_value<Compare, 0>
        , empty_value<Allocator, 1> {
    public:
        storage()
            : empty_value<Compare, 0>(empty_init_t())
            , empty_value<Allocator, 1>(empty_init_t())
            , ptr_() { }

        storage(const Compare& c, const Allocator& a)
            : empty_value<Compare, 0>(empty_init_t(), c)
            , empty_value<Allocator, 1>(empty_init_t(), a)
            , ptr_() { }

        const Ptr& pointer() const {
            return ptr_;
        }

        Ptr& pointer() {
            return ptr_;
        }

        const Compare& compare() const {
            return empty_value<Compare, 0>::get();
        }

        Compare& compare() {
            return empty_value<Compare, 0>::get();
        }

        const Allocator& allocator() const {
            return empty_value<Allocator, 1>::get();
        }

        Allocator& allocator() {
            return empty_value<Allocator, 1>::get();
        }

    private:
        Ptr ptr_;
    };

### Reference

    namespace boost {

    struct empty_init_t { };

    template<class T, unsigned Index = 0, bool Empty = see below>
    class empty_value {
    public:
        typedef T type;

        empty_value() = default;

        template<class... Args>
        constepxr empty_value(empty_init_t, Args&&... args);

        constepxr const T& get() const noexcept;

        constepxr T& get() noexcept;
    };

    inline constexpr empty_init_t empty_init{ };

    } /* boost */

#### Template parameters

`T`

  The type of value to store

`Index`

  Optional: Specify to create a distinct base type

`Empty`

  Optional: Specify to force inheritance from type

#### Member types

`type`

  The template parameter `T`

#### Constructors

`constepxr empty_value() = default;`

  Default initialize the value

`template<class... Args> constepxr empty_value(empty_init_t, Args&&... args);`

  Initialize the value with `std::forward<Args>(args)...`

#### Member functions

`constepxr const T& get() const noexcept;`

  Returns the value

`constepxr T& get() noexcept;`

  Returns the value