PRB: C2517, C2653, or C2600 If Local Struct Contains Class. (129596)

SYMPTOMS

In a C++ program, when you define a local struct, class, or union in which one or more of the fields contains a struct, class, or union type that defines member functions, the compiler generates these compiler errors:

In Visual C++ versions 1.5x:

   Error C2517 : 'S' : right of '::' is undefined
   Error C2270 : 'S' : modifiers not allowed on nonmember functions
   Error C2561 : 'S' : function must return a value
   Error C2510 : 'S' : left of '::' must be a class/struct/union
   Error C2084 : function 'S' already has a body

In Visual C++ versions 2.x:

   Error C2653 : 'S' : is not a class name
   Error C2561 : 'S' : function must return a value
   Error C2084 : function 'int _cdecl S(void)' already has a body

In Visual C++ version 4.0:

   error C2600: 'a' : uses 'A' in local class 'S' causing compiler-
   generated special member functions
   error C2601: functions cannot be defined in local classes

Using any MFC classes in the local structure will also cause this problem.

CAUSE

Function definitions are found in a local struct, class, or union definition. Support for member functions of local classes is part of the current draft of the C++ Working Paper, but they are not currently supported by Microsoft Visual C++. They will be supported in a future release of Microsoft Visual C++.

RESOLUTION

To work around the problem, choose one of the following two methods:

Declare the structure in a file scope instead of a unit scope. -or-
If a local structure is needed for a class member function, define it inside the class definition instead of within the function body as in this example:
```
   class B;
   class A
   {
     struct S
     {
       int a;
       B   b;
     };
   };
						
```

MORE INFORMATION

In the sample code below, even though the local struct S does not, at first glance, appear to have any member functions, the compiler attempts to generate a non-trival default constructor and destructor for the local struct S. Because the compiler is not designed to support member functions of local classes, this hidden attempt to generate two such functions causes the compiler to produce the error messages descibed in the SYMPTOMS section of this article.

Sample Code to Reproduce Behavior

   /* Compile options needed: none
   */ 
   struct A
   {
      A() {};
      ~A() {};
   };
   void main()
   {
      struct S
      {
         A a;
      };
      S s_test;
   }