Looking in the header file for std::unique_ptr (g++ version 5.3.1) I can see that unique_ptr stores a tuple of the raw pointer it contains and the struct used to handle deletion (typically the default_deleter defined above in the same file).
template <typename _Tp, typename _Dp = default_delete<_Tp> >
class unique_ptr
{
// use SFINAE to determine whether _Del::pointer exists
class _Pointer
{
template<typename _Up>
static typename _Up::pointer __test(typename _Up::pointer*);
template<typename _Up>
static _Tp* __test(...);
typedef typename remove_reference<_Dp>::type _Del;
public:
typedef decltype(__test<_Del>(0)) type;
};
typedef std::tuple<typename _Pointer::type, _Dp> __tuple_type;
__tuple_type _M_t;
// etc, etc...
I know that in C++ the size of an empty struct is nonzero, so if a unique_ptr stores a pointer (8 bytes) and an empty struct (>0 bytes), how come sizeof(std::unique_ptr<T>) is only 8 bytes?
