Question

According to the C++03 standard, is it valid to have references to incomplete types? I’m not aware of any implementation that implements references as any other than non-null pointers, so such code ought to work. However, I wonder whether references to incomplete types are standard conforming.

I would appreciate answers with quotes and references to the C++ standard.

Answer 1

The C++ standard doesn’t explicitly say, as far as I know, that you can have a reference to an incomplete type. But the standard does have a section that specifies when a type must be complete:

3.2 One definition rule [basic.def.odr]

4. Exactly one definition of a class is required in a translation
unit if the class is used in a way that requires the class type to be
complete. [Example: the following complete translation unit is
well-formed, even though it never defines X:

    struct X;     // declare X as a struct type
    struct X* x1; // use X in pointer formation
    X* x2;        // use X in pointer formation

—end example] [Note: the rules for declarations and expressions
describe in which contexts complete class types are required. A class
type T must be complete if:

• an object of type T is defined (3.1, 5.3.4), or
• an lvalue-to-rvalue conversion is applied to an lvalue referring to
  an object of type T (4.1), or
• an expression is converted (either implicitly or explicitly) to
  type T (clause 4, 5.2.3, 5.2.7, 5.2.9, 5.4), or
• an expression that is not a null pointer constant, and has type
  other than void * is converted to the type pointer to T or reference
  to T using an implicit conversion (clause 4), a dynamic_cast (5.2.7)
  or a static_cast (5.2.9), or
• a class member access operator is applied to an expression of type
  T (5.2.5), or
• the typeid operator (5.2.8) or the sizeof operator (5.3.3) is
  applied to an operand of type T, or
• a function with a return type or argument type of type T is defined
  (3.1) or called (5.2.2), or
• an lvalue of type T is assigned to (5.17). ]

It appears that in every other case, including declarations of references to incomplete types, the type can be incomplete.