C++11 usage of delete specifier vs private functions

Question

I am in the process of boning up on my C++ (as in, attempting to get into more modern-style coding) and am looking at the delete specifier. It is my understanding that it is used to make sure that certain functionality cannot be defined or called. If I understand it correctly, this is primarily within the domain of assignment and copy. I am not quite sure what the difference is between using the delete specifier and just making those functions private.

For instance, what is the difference between:

 class Foo {
 private:
      Foo& operator(const Foo&);
      Foo(const Foo&);     
 };

And

 class Bar {
 public:
      Bar& operator(const Bar&) = delete;
      Bar(const Bar&) = delete;
 };

In other words: what does using the delete specifier gain? Is it just to make things look nicer?

Answer 1

One obvious difference is that if you make the function private, then it is still accessible from within the class and any friends.

An explicitly deleted function is not usable anywhere, so you know simply from that one line that it's never used, without having to inspect the implementation.

You can make the function both private and deleted: then its participation in overload resolution is more consistent.