Why does compiler need complete type when reference is used?

Question

If I have a header like this:

#include <vector>

class Data;
class A {
  // const Data& getData( int i ) const { return a[i]; }
  std::vector<Data> a;
};

the compiler compiles it normally, because it doesn't need to know a bit about Data type.

Retuning a value by reference does not depend on a class implementation and therefore do not require any knowledge of the class internals. But when I uncomment the accessor compiler starts to complain on invalid use of incomplete type 'class Data'. Why?

Answer 1

So, the C++ Standard is not particularly clear on this point. The problem that you are hitting is whether or not you can instantiate vector<Data> when you Data is not a complete type.

If we look at Section 17.6.4.8 Paragraph 2, we find the following statement:

In particular, the effects are undefined in the following cases:

• ...
• if an incomplete type (3.9) is used as a template argument when instantiating a template component, unless specifically allowed for that component.

std::vector does not explicitly allow incomplete types for the template argument, so this is technically invalid code.

---

This would mean that this is invalid code, even if your compiler accepts it.

class Data;
class A {
    std::vector<Data> a;
};

Answer 2

Thanks to people reaction I understood the problem better. It becomes more clear when we rid from std::vector:

class Data;
class A {
  const Data& getData( int i ) const { return a[i]; }
  Data* a;
};

to return i-th element of array a one needs to know where is it in the memory. For this one needs to know sizeof(Data), which is not possible for incomplete type.

The exactly same problem std::vector should have.