PRB: Compiler Doesn't Lay Out Overloaded Functions in Order

• Microsoft OLE Libraries for Windows and Win32s, version 2.03
• Microsoft OLE libraries included with:

      - Microsoft Windows NT version 3.5
      - Microsoft Windows 95
  

• Microsoft Visual C++ for Windows, version 1.52
• Microsoft Visual C++, 32-Bit Edition, version 2.1

SYMPTOMS

The Microsoft Visual C++ compiler does not lay out overloaded functions in the vtable in the order in which they are declared. This does not adhere to the COM binary standard. (See the OLE Specification section 3.1.1.1 for a discussion of the COM binary standard.) If there are no overloaded functions, the COM binary standard is followed.

In general, it is not a good idea to use overloaded methods in a COM interface because this precludes use of the interface from languages like C that do not support overloading.

CAUSE

All the overloads of a function are laid out together in successive entries in the reverse order of their declaration, overall in the order of the first overload of each name. Therefore given this declaration:

class A {

   virtual void a();
   virtual void b();
   virtual void a(int);
   virtual void b(int);

Entries in the vtable are laid out in this order: A::a(int), A::a(), A::b(int), A::b(). The COM binary standard requires that functions be laid out in the order in which they are declared.

STATUS

This behavior is built into the design. It cannot be modified because of legacy code that depends on this behavior.