why does this result in a runtime error?

[Dylan]

thanks for any comments. Here's the code

//------------------------------------------------------------------

#include <iostream>
class Base
{
protected:

virtual int GetValue()=0;

public:

Base(){PrintValue();}


void PrintValue(){std::cout << GetValue();}
};


class Child : public Base
{
public:

int GetValue(){return 2;}
};


int main()
{

Child c;

return 0;
}

 

[Karl Heinz Buchegger]

thanks for any comments. Here's the code

//------------------------------------------------------------------

#include <iostream>
class Base
{
protected:

virtual int GetValue()=0;

public:

Base(){PrintValue();}

void PrintValue(){std::cout << GetValue();}
};

class Child : public Base
{
public:

int GetValue(){return 2;}
};

int main()
{

Child c;

return 0;
}

Because of order of construction:
When Child gets constructed, first the Base part
of it gets constructed. That is: when the constructor
of Base runs, the Child object is not yet in existence
and thus the call to GetValue can only be resolved by
using the Base::GetValue. Unfortunately this function
is not implemented and thus the runtime error.

A simple rule to remember: During construction (and
destruction) of objects, the virtual mechanism is
not working.

 

[Dylan]

Thanks Karl

Because of order of construction:
When Child gets constructed, first the Base part
of it gets constructed. That is: when the constructor
of Base runs, the Child object is not yet in existence
and thus the call to GetValue can only be resolved by
using the Base::GetValue. Unfortunately this function
is not implemented and thus the runtime error.

A simple rule to remember: During construction (and
destruction) of objects, the virtual mechanism is
not working.