...one of the most highly
regarded and expertly designed C++ library projects in the
world.
— Herb Sutter and Andrei
Alexandrescu, C++
Coding Standards
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> empty_value(empty_init_t, Args&&... args); const T& get() const noexcept; T& get() noexcept; }; inline constexpr empty_init_t empty_init{ }; } /* boost */
T
The type of value to store
Index
Optional: Specify to create a distinct base type
Empty
Optional: Specify to force inheritance from type
type
The template parameter T
empty_value()
= default;
Default initialize the value
template<class... Args>
empty_value(empty_init_t,
Args&&...
args);
Initialize the value with std::forward<Args>(args)...
const T& get() const noexcept;
Returns the value
T&
get()
noexcept;
Returns the value