Boost C++ Libraries

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.

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_;
};

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>
    explicit empty_value(empty_init_t, Args&&... args);

    const T& get() const noexcept;

    T& get() noexcept;
};

} /* boost */