Returns the number of shared_ptr objects that share ownership over the same pointer as this object (including it).
If this is an emptyshared_ptr, the function returns zero.
Library implementations are not required to keep a count of any particular set of owners, and thus it may not be efficient to call this function. To check specifically whether use_count is 1, you can use member unique instead, which may be faster.
The number of objects sharing ownership over the same pointer as this.