This is a technical C++ post regarding the
Microsoft runtime error R6025 Pure Virtual Function Call
that sometime occurs in programs compiled with
Microsoft Visual C++ 6.0.
Please consider the following simple illegal C++ program:
class Listener {
public:
virtual void onEvent(int n) = 0;
};
class A : public Listener {
public:
Listener * listener;
int val;
explicit A(int n) {listener = this; val = n;}
virtual ~A() {}
virtual void run() {listener->onEvent(val);}
};
int main() {
A(42).run();
return 0;
}
This program is illegal because in the main() function
there is a call to A constructor but A cannot be constructed
because it does not declare the onEvent function.
When you try to compile it using g++
you correctly obtaing the following error:
virtual.cpp: In function `int main ()':
virtual.cpp:14: cannot allocate an object of type `A'
virtual.cpp:14: since the following virtual functions are abstract:
virtual.cpp:3: void Listener::onEvent (int)
But, surprise!, when you try to compile it using
the Microsoft C++ 6.0 compiler you have no error.
Only when you execute it you will obtain the infamous
Microsoft runtime error R6025 Pure Virtual Function Call.
If your application sometime suffers of R6025 error
there are two manners to discover the problem:
<1> Try to compile the program using an
alternative correct c++ compiler (like g++),
-or-
<2> Make a code inspection on the program
and try to figure out all code containg the pattern:
className(someParam).methodInvocation(otherParam)
and substitute it with:
className xxx(someParam);
xxx.methodInvocation(otherParam)
If you try <2> on the previous simple illegal C++ program
you will obtain a main() like this:
int main() {
A xxx(42);
xxx.run();
return 0;
}
and now, when you compile it with using the Microsoft C++ compiler
you will correctly obtain the error:
error C2259: 'A' : cannot instantiate abstract class ...
Short Story
===========
IMHO a C++ program error is masked by a bug
of the Microsoft C++ 6.0 compiler.
Accurate code inspection -or- using an alternative
correct C++ compiler (like g++)
may help in finding the error.
- Dario
