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Overload Journal #5 - Sep 1994 + Programming Topics + Letters to the Editor   Author: Mike Toms

Dear Mike,

Congratulations on the first four issues of Overload, I would like to add a couple of comments to your answers in the last two issues (in a spirit of .pedantry only). In Issue 3 P34 Q4 you appear to have overcomplicated the solution with conditional compilation!

Simply declare the const array as extern, declare the class using the const array name as the default argument and define the const array in the source file as for any initialised data:

hello.hpp:

extern const char name[];
void print( const char str[] = name );

hello.cpp:

#include "extconst.hpp"
#include <iostream.h)
const char name[] = {"Hello World"};
void print( const char str[] )
{
 cout << str;
}

void main()
{
 print();
}

In issue 4 p44 Q10 you say “… if polymorphic member function is called, only the base member function will be called”. I’m sure it was just a slip of the tongue but in fact it will be the constructors own version of the function that is called, whether it happens to be the base class or not. Interestingly, although the virtual calling mechanism is supposed to be turned off, the virtual method tables are still used to call virtual functions.

The way it works is that as each base object is constructed, the virtual method pointer is set to the table for that class, immediately prior to the body of the constructor being executed. This means that the method table will contain the addresses of the functions that would apply to an instance of the class of the constructor in use. This means that it isn’t possible to subvert this behaviour - even calling the functions via a base pointer in another function will only get the same method table. Interestingly it’s possible to call a pure virtual function this way without the compiler ever realising it - you’ll find out at run-time of course!

Adrian Fagg

Thank’s for the corrections and information.

Mike Toms.

Overload Journal #5 - Sep 1994 + Programming Topics + Letters to the Editor