This is an unofficial snapshot of the ISO/IEC JTC1 SC22 WG21 Core Issues List revision 112c. See for the official list.


478. May a function parameter be an array of an abstract class type?

Section:  [dcl.array]     Status: NAD     Submitter: Steve Adamczyk     Date: 28 Sep 2004

Consider the following example:

    struct S {
      virtual void v() = 0;

    void f(S sa[10]);     // permitted? [dcl.array] paragraph 1 says that a declaration like that of sa is ill-formed:

T is called the array element type; this type shall not be a reference type, the (possibly cv-qualified) type void, a function type or an abstract class type.

On the other hand, [dcl.fct] paragraph 3 says that the type of sa is adjusted to S*, which would be permitted:

The type of each parameter is determined from its own decl-specifier-seq and declarator. After determining the type of each parameter, any parameter of type “array of T” or “function returning T” is adjusted to be “pointer to T” or “pointer to function returning T,” respectively.

It is not clear whether the parameter adjustment trumps the prohibition on declaring an array of an abstract class type or not. Implementations differ in this respect: EDG 2.4.2 and MSVC++ 7.1 reject the example, while g++ 3.3.3 and Sun Workshop 8 accept it.

Rationale (April, 2005):

The prohibition in [dcl.array] is absolute and does not allow for exceptions. Even though such a type in a parameter declaration would decay to an allowed type, the prohibition applies to the type before the decay.

This interpretation is consistent with the resolution of issue 337, which causes template type deduction to fail if such types are deduced. It was also observed that pointer arithmetic on pointers to abstract classes is very likely to fail, and the fact that the programmer used array notation to declare the pointer type is a strong indication that he/she expected to use subscripting.