防止别名重新定义C ++

Question

I want hide library implementation from user and provide public API with aliasing of implementation, to reduce code duplication. So if I have two logically different API structs, but they implementation is equal, implementation would be aliased as shown in code snippet.

// library implementation
namespace impl {
    struct A {};
}

// library public API
using A1 = impl::A;
using A2 = impl::A;

// library usage
int main(int argc, char* argv[]) {
    using type = A1;
    // do some stuff
    using type = A2; // prevent this redefinition with reporting warning or error
    // do some more stuff
    return 0;
}  

But it may lead to hidden logical errors if user decide to redefine alias to library public API as in code snippet, because it compiles just fine. So the question is:

Is it possible to prevent alias to alias redefinition if redefined aliases are referenced to the same types?

Answer 1

If you point to the same type, the compiler is totally fine with that, and I am afraid you cannot do anything against that.

From the typedef keyword which, in your case, is essentially equivalent to the using keyword:

https://en.cppreference.com/w/cpp/language/typedef

Typedef cannot be used to change the meaning of an existing type name (including a typedef-name). Once declared, a typedef-name may only be redeclared to refer to the same type again.

So, it is an error to re-declare a typedef-name with a different type-id . It does nothing to re-declare a typedef-name with the same type-id .

Answer 2

The easiest fix might be inheritance:

struct A1 : public impl::A
{
  using A::A;
};
struct A2 : public impl::A
{
  using A::A;
};

This defines new types, but reuses the implementation. You could even use private inheritance but that will require more using declarations for other members.