The following article refers to C++ specific programming constructs, and Microsoft Visual C++ specific compiler options.

The __purecall function (C++)

The __purecall function is a special function (defined with C linkage) that is defined in the default run-time library, and may be called at run time if there is an error referencing an object's virtual function table. This will usually happen as a result of a program error. The following code causes this error:

struct A;
void x(A*);
struct A
    virtual void f() = 0;
    A() { x(this); }

struct B : A
    void f() { }

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

void main()
    B aB;    // calls B::B calls A::A calls
}            // x calls A::f (pure virtual function)

Because such errors cannot be caught at compile time, the __purecall function is called, generating the following run-time error:

runtime error R6025
- pure virtual function call

If an application (such as a device driver) must be compiled without using the standard run-time libraries (compiler option /Zl or linker option /NODEFAULTLIB), but requires virtual function calls, you will get an "undefined symbol" linker error on __purecall. You can work around this error in one of two ways:

  • Define your own __purecall function and compile with the /Gd option (specify __cdecl calling convention). The following is a minimal example:

          int __purecall( void )
              /* insert your own error reporting code here */
              return 0;

    If __purecall is redefined in exactly this way, no run time error is generated if the user calls a pure virtual function. The custom __purecall function should contain appropriate error checking and recovery code, (for example, calling _exit or printing a fatal error message) as noted in the sample.

  • Extract the object module that contains __purecall from the LIBC.LIB library, and link the object explicitly. Using this method, all other symbols in the object module will be included. To avoid this potentially undesirable side-effect, the first method is recommended.

Microsoft Knowledge Base Q120919 ('Visual C++ Vers 2.0 README.WRI, Part 3 Compiler')

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