• Iliya Peregoudov <iliyap mail.ru> Sep 23 2002
• "Walter" <walter digitalmars.com> Sep 23 2002

Sample code:

/* subcast.cpp */
#include <memory.h>
#include <iostream.h>

class Matrix {
protected:
    int v[3][3];
public:
    Matrix() { cerr << "Matrix::ctor()" << endl; };
    Matrix(const Matrix &m) {
       cerr << "Matrix::ctor(const Matrix &)" << endl;
       memcpy(v, m.v, sizeof(v));
    };
};

class SquareMatrix: public Matrix {
private:
    int det;
public:
    SquareMatrix(): Matrix() {
       cerr << "SquareMatrix::ctor()" << endl;
       det = 0;
    };
    SquareMatrix(const SquareMatrix &m): Matrix(m) {
       cerr << "SquareMatrix::ctor(const SquareMatrix &)" << endl;
       det = m.det;
    };
};

int main()
{
    Matrix m1;
    SquareMatrix m2((SquareMatrix)m1);

    return 0;
}


This compiles ok and runs ok giving on stderr:

Matrix::ctor()
Matrix::ctor(const Matrix &)
SquareMatrix::ctor(const SquareMatrix &)

This mean that only two objects constructed: m1 of type Matrix (first
output line) and m2 of type SquareMatrix (second and third output
lines).  So the question:  How the compiler converts object of class
Matrix to objects of its subclass SquareMatrix without creating any
objects?

This sample code gives compile-time error about unability to convert
from Matrix to SquareMatrix while compiled by g++ and KAI C++.

That's a bug. I'll fix it in the next patch.