Why is my return type meaningless?

Question

I am trying to use a return type of const MyClass * const. However, I get a warning:

Warning: #815-D: type qualifier on return type is m

Answer 1

Why return a const value? Consider the following (and please excuse the unimaginative variable names):

struct A { int a; };

A operator+(const A& a1, const A& a2) 
{
    A a3 = { a1.a + a2.a };
    return a3;
}

Given that declaration of operator+, I can do the following:

A a = {2}, b = {3}, c = {4}, d = ((a+b) = c);

That's right, I just assigned to the temporary A that was returned by operator+. The resulting value of d.a is 4, not 5. Changing the return type of operator+ to const A prevents this assignment, causing the expression (a+b) = c to generate a compiler error.

If I try to assign to a pointer or integer returned from a function, my compiler (MSVC) generates a "left operand must be l-value" error, which seems consistent with the ARM compiler telling you that the constness of the pointer is meaningless--the pointer can't be assigned to anyway. But for classes/structs, apparently it's okay to assign to non-const return values.