Can non-special C++ member functions be deleted (or defaulted)?

Question

A lot of attention has been received by = default and = delete with respect to the special members (default constructor, destructor, copy and move). Can = default and = delete be used with other functions; member functions, free functions and operators etc.?

I can understand that = default would probably not be allowed outside the special members; since it is basically saying use the compiler generated default. The default would need to be clearly defined before the compiler is able to generate it. As far as I know, only the special members have these pre-defined defaults.

What about = delete; it is basically saying that the function is declared, but the implementation is explicitly not defined.

• Can = delete be used with functions other than the special members?
• What function types can it be used on (members, non-members, operators etc.)?
• Or conversely, where (or when) is = delete prohibited from being used? Are there any restrictions on its use?

Answer 1

Any function whatsoever can be declared as deleted (using = delete). The standard does not impose any restrictions. This is covered by C++11[dcl.fct.def.delete]. It even gives an example of a deleted operator new.

Defaulted functions, on the other hand, are limited by [dcl.fct.def.default]§1 as follows:

1 ... A function that is explicitly defaulted shall

• be a special member function,
• have the same declared function type (except for possibly differing ref-qualifiers and except that in the case of a copy constructor or copy assignment operator, the parameter type may be “reference to non-const T”, where T is the name of the member function’s class) as if it had been implicitly declared, and
• not have default arguments.

The "special member functions" referenced in the first bullet point are (as per [special]§1):

• default constructor
• copy constructor
• move constructor
• copy assignment operator
• move assignment operator
• destructor