Question

What behaviour is being caused by the ‘const’ in the following code, in particular, the cast:

const void *foo()
{
   void *bar = NULL;

   // Assign bar to some address here.

   return (const void *)bar;
}

Is the cast necessary? The compiler gives a warning that the type does not match the return type, unless I perform the cast, but is the resultant behaviour any different?

Answer 1

The cast is not required and is even useless. C does not require any warning if you don’t cast.

You can return a char * when a const char * return value is expected.
This is also true for other types: you can return a T * when a const T * is expected.

C says that for return statement:

(C99, 6.8.6.4p3) “If the expression has a type different from the return type of the function in which it appears, the value is converted as if by assignment to an object having the return type of the function.”

You can assign a char * to a const char * (note that the opposite is not valid).
C says for pointer assignment:

(C99, 6.5.16.1p1) “One of the following shall hold: […] both operands are pointers to qualified or unqualified versions of compatible types, and the type pointed to by the left has all the qualifiers of the type pointed to by the right; […]”