is that unique_ptr is lighter than shared_ptr and has no runtime burden, so the deleter of unique_ptr is determined at compile time.

two unique_ptr belong to different types even if they point to the same type and if the deleter is different. The deleter of unique_ptr is already built into the type, so you don't need to store a deleter object to know where the deleter is. The "type embedding" process is determined at compile time, and the code for the deletion process is, of course, run at run time.

shared_ptr , the constructor passes in a real object that is stored for use. The "object Storage" procedure is determined by the run time, and the code that deletes the process is, of course, run at run time. shared_ptr is more flexible.

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template< class Y, class Deleter > 
shared_ptr( Y* ptr, Deleter d );

template<
    class T,
    class Deleter = std::default_delete<T>
> class unique_ptr;