PRB: Cause of the R6025 Run-Time Error

• Microsoft Visual C++ for Windows, versions 1.0, 1.5, 1.51, 1.52
• Microsoft Visual C++, 32-bit Edition, versions 1.0, 2.0, 2.1, 4.0, 5.0

SYMPTOMS

The C++ program fails, and reports the following run-time error:

   runtime error R6025
   - pure virtual function call

CAUSE

This error occurs when your application indirectly calls a pure virtual member function in a context where a call to the function is not valid. In most cases, the compiler detects this and reports the error when building the application. But depending on how your code is written, sometimes the problem is detected only at run-time.

The error occurs when you indirectly call a pure virtual function within the context of a call to the constructor or destructor of an abstract base class. This is illustrated in the sample code below, along with some more description.

RESOLUTION

Calling a pure virtual function is a programming error, so you need to find the call to the pure virtual function and rewrite the code so it is not called.

One way to find a call to a pure virtual function is to replace the pure virtual function with an implementation that calls the Windows API function DebugBreak(). When using the debugger, DebugBreak() will cause a hard-coded breakpoint. When the code stops running at this breakpoint, you can view the callstack to see where the function was called.

STATUS

This behavior is by design.

MORE INFORMATION

For more information, refer to the "C Run-Time Errors R6002 through R6025" chapter of the "Build Errors" book included with Visual C++ 32-bit Edition, version 4.0, Books Online or search the Books Online for "R6025."

NOTE: The R6025 error is not documented in the manuals or online help included with Visual C++, versions 2.x or below. However, it is briefly discussed in Part 3 of the README.WRI file shipped with Visual C++ version 2.0.

Sample Code

   /* Compile options needed: none
   */

   class A;

   void fcn( A* );

   class A
   {
   public:
       virtual void f() = 0;
       A() { fcn( this ); }
   };

   class B : A
   {
       void f() { }
   };

   void fcn( A* p )
   {
       p->f();
   }

   // The declaration below invokes class B's constructor, which
   // first calls class A's constructor, which calls fcn. Then
   // fcn calls A::f, which is a pure virtual function, and
   // this causes the run-time error. B has not been constructed
   // at this point, so the B::f cannot be called. You would not
   // want it to be called because it could depend on something
   // in B that has not been initialized yet.

   B b;

   void main()
   {
   }