Calling parent constructor from child constructor

[pantalaimon]

I'm trying to write a GUI for a game I'm making. Till now I've always
done this: ChildClass(int x,int y) : ParentClass(x,y) whenever my
compiler complains about "no default constructor found". But in one of
my classes I need to do some calculations first and THEN call the
parent's constructor. Here is an example:

class ParentClass
{
public:
Parentclass(int sum);
};

class ChildClass : public ParentClass
{
public:
ChildClass(int x,int y)
{
int sum = x + y;
ParentClass(sum);
}

Problem is I don't know how to do it (if it is even possible). The
above code doesn't work. The compiler thinks I want to instantiate an
instance of the ParentClass.
Thanks in advance!

[Alf P. Steinbach]

* pantalaimon:

I'm trying to write a GUI for a game I'm making. Till now I've always
done this: ChildClass(int x,int y) : ParentClass(x,y) whenever my
compiler complains about "no default constructor found". But in one of
my classes I need to do some calculations first and THEN call the
parent's constructor. Here is an example:

class ParentClass
{
public:
Parentclass(int sum);
};

class ChildClass : public ParentClass
{
public:
ChildClass(int x,int y)
{
int sum = x + y;
ParentClass(sum);
}

Problem is I don't know how to do it (if it is even possible). The
above code doesn't work. The compiler thinks I want to instantiate an
instance of the ParentClass.

ChildClass( int x, int y ): ParentClass( x + y ) {}

--
A: Because it messes up the order in which people normally read text.
Q: Why is it such a bad thing?
A: Top-posting.
Q: What is the most annoying thing on usenet and in e-mail?

[Nicolas Pavlidis]

mo*******@hotmail.com (pantalaimon) writes:

I'm trying to write a GUI for a game I'm making. Till now I've always
done this: ChildClass(int x,int y) : ParentClass(x,y) whenever my
compiler complains about "no default constructor found". But in one of
my classes I need to do some calculations first and THEN call the
parent's constructor. Here is an example:

class ParentClass
{
public:
Parentclass(int sum);
};

class ChildClass : public ParentClass
{
public:
ChildClass(int x,int y)
{
int sum = x + y;
ParentClass(sum);
}

This doesn't work because the constructor of ParentClass is called
automatically before processing the constructor of ChildClass You can not work
around this, because the code is inserted by the compiler, but you can
do the following:

ChildClass(int x, int y) : ParentClass(x+y)
{
}

By doing this the compiler shouldbe satisfied and should do the right
thing.

BTW You can make the scenario I described "visible" by trying this:

#include <iostream>
using namespace std;

class Base
{
public:
Base()
{
cout << "Base::Base() called" << endl;
}
~Base()
{
cout << "Base::~Base() called()" << endl;
}

};

class Derived : public Base
{
public:
Derived()
{
cout << "Derived::Derived() called" << endl;
}
~Derived()
{
cout << "Derived::~Derived() called" << endl;
}
};

int main()
{
Derived object;
return(0);
}

The output should be:

Base::Base() called
Derived::Derived() called
Derived::~Dervied() called
Base::~Base() called

HTH && Kind regards,
Nicolas
--
| Nicolas Pavlidis | Elvis Presly: |\ |__ |
| Student of SE & KM | "Into the goto" | \|__| |
| pa****@sbox.tugraz.at | ICQ #320057056 | |
|-------------------University of Technology, Graz----------------|

[John Harrison]

"pantalaimon" <mo*******@hotmail.com> wrote in message
news:fe************************@posting.google.com ...

I'm trying to write a GUI for a game I'm making. Till now I've always
done this: ChildClass(int x,int y) : ParentClass(x,y) whenever my
compiler complains about "no default constructor found". But in one of
my classes I need to do some calculations first and THEN call the
parent's constructor. Here is an example:

class ParentClass
{
public:
Parentclass(int sum);
};

class ChildClass : public ParentClass
{
public:
ChildClass(int x,int y)
{
int sum = x + y;
ParentClass(sum);
}

Problem is I don't know how to do it (if it is even possible). The
above code doesn't work. The compiler thinks I want to instantiate an
instance of the ParentClass.
Thanks in advance!

If your calculation is as simple as x + y then do as Alf and Nicolas have
already indicated. If your calculation is a bit more complex then a static
function sometimes works well.

class ParentClass
{
public:
ParentClass(int sum);
};

class ChildClass : public ParentClass
{
public:
ChildClass(int x,int y) : ParentClass(complex_calculation(x, y))
{
}
private:
static int complex_calculation(int x, int y)
{
...
}
};

john