Question

If the size of an empty class can’t be 0, what magic is doing std::tuple so the sizeof of unique_ptr is returning 8 in a 64 bit machine?

In unique_ptr the member is defined as:

  typedef std::tuple<typename _Pointer::type, _Dp>  __tuple_type;                 
  __tuple_type  _M_t;

Where _Dp is the deleter class.

Compiler is gcc version 4.7.1 (Debian 4.7.1-7)

Answer 1

The reason is that the typename _Dp = default_delete<_Tp> is an empty class and the tuple template employs empty base class optimization.

If you instantiate the unique_ptr with a non-default delete, you should see the size increase.