Virtual function declaration and use

Figure 1 shows a program that, as of z/OS® V1R6 C/C++ compiler, would generate an exception under the IBM object model because the call to a member function version() on the object _b occurs before the declaration of _b.

Figure 1. Example that highlights sequence of statements to declare and call a virtual function

#include 

class A {
    public:
      A(int i) : v(i) {}
      virtual int version() {return 0;}  1 ;
      private: int v; 
};

class B:virtual public A {
   public:
   B(int i) : A(i) {}
};

extern B _b;                            2 
static int ver = _b.version();          3 
B _b(1);                                4  

int main() {
   printf("version: %d\n", ver);
   return 0;
}

Notes:

The virtual keyword tells the compiler that the function is virtual and it can be overloaded by any derived class of A.
A reference to externally defined _b of type B.
The value of static global variable ver is initialized with the value returned by member function version() called by object _b. An exception will be raised because the object _b is not fully constructed at the time of the call to the member function version().
The declaration of the polymorphic object _b occurs after its use on the previous line. This line should precede the definition of ver to ensure that the virtual function version() is found at run time.