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

ID: Q131104

The information in this article applies to:

Microsoft OLE Libraries for Windows and Win32s, version 2.03
Microsoft OLE Libraries, used with:
- Microsoft Windows NT 3.5
- Microsoft Windows 95
Microsoft Visual C++ for Windows, 16-bit edition, version 1.52
on the following platforms: WINDOWS
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.

Additional query words: 2.03 1.52 4.00 3.50 2.10

Keywords : kbole kbprg kbNTOS350 kbOLE200 kbVC152 kbVC210 kbWinOS95 kbprb
Version : WINDOWS:1.52,2.03,95; :2.1
Platform : WINDOWS
Issue type :