const B and const A* are incompatible, even when B is aliased to A*

Question

Why aren't const B and const A* indistinguishable, when B is typedef'ed to A*? When compiling this simple example:

struct A {};

typedef A* B;

void f1(const A* a1);
void f2(const B a2);

int main()
{
    const A a;
    f1(&a);
    f2(&a);
}

I get the following compiler output (G++ 6.3.1):

test.cpp: In function ‘int main()’:
test.cpp:12:8: error: invalid conversion from ‘const A*’ to ‘B {aka A*}’ [-fpermissive]
     f2(&a);

Note that the call to f1 is fine (since &a is-an A*), but f2 is not, even though const B seems to me like it should be equivalent to const A*.

Answer 1

This is a case of "leading const" being misleading.

If you have

struct A {};

typedef A* B;

Then a const B is in fact A * const. The const-qualifier is applied on the pointer-type, not the pointee-type as it is in const A*.

---

Side note, but the reason I favor "cv-qualifiers on the right" is that reading the type name from right to left in English, matches the actual type defined. So for instance:

int const * * const * const p;

When read from right to left becomes "p is a constant pointer, to a constant pointer, to a pointer, to a constant integer".

Answer 2

const B means A * const that is the pointer itself is a constant but the object pointed to by the pointer is not constant.

On the other hand const A * means that the pointer itself is not constant but the object pointed to by the pointer is constant.

So const B that is A * const is not an alias for const A *

You could write

typedef const A* B;

void f1(const A* a1);
void f2( B a2);

Answer 3

even though const B seems to me like it should be equivalent to const A*.

No, for const B, const qualifies B, i.e. the pointer itself, not the pointee. Then const B is equivalent to A* const, i.e. a const pointer to non-const A, not a pointer to const A; which can't be converted from const A* without explicit conversion.

Answer 4

A const A* is a pointer to a const A. The pointer is mutable, but the object it points to is not.
A const B is a const pointer to an A. The pointer is immutable, but the object it points to is mutable.

The second function is equivalent to void f2(A* const a2), if I recall.