Re: Multiple inheritance: Wrong constructors being called?
On 11 Mar, 07:56, Adam Nielsen <a.niel...@shikadi.rem.ove.net> wrote:
Hi everyone,
I'm having some trouble getting the correct chain of constructors to be called when creating an object at the bottom of a hierarchy. Have a look at the code below - the inheritance goes like this:
Shape
|
+-- Ellipse
|
+-- Circle
When I create a new Circle, it calls the Ellipse's constructor, which in turn calls the Shape's constructor. The problem is that I'm passing parameters to the constructors, and the Ellipse's constructor passes these parameters on to the Shape's constructor - but these parameters are ignored and the Shape's default constructor is called instead of the one taking parameters!
What am I doing wrong???
----------------------------------------------
#include <iostream>
class Shape
{
public:
Shape(void)
{
std::cout << "Default shape constructor, should be unused" << std::endl;
}
Shape(int width, int height)
{
std::cout << "Creating a new shape with size " << width << "x" << height << std::endl;
}
};
class Ellipse: virtual public Shape
{
public:
Ellipse(int width, int height):
Shape(width, height)
{
std::cout << "In Ellipse constructor, a shape with size should have already been created above" << std::endl;
}
};
class Circle: virtual public Ellipse
{
public:
Circle(int width, int height):
Ellipse(width, height)
{
std::cout << "In Circle constructor, Ellipse should already have been created above" << std::endl;
}
};
int main(void)
{
Circle *c = new Circle(10, 20);
delete c;
return 0;
}
----------------------------------------------
$ g++ -o test test.cpp && ./test
Default shape constructor, should be unused
In Ellipse constructor, a shape with size should have already been created above
In Circle constructor, Ellipse should already have been created above
----------------------------------------------
Thanks,
Adam.
Hiya,
In C++, when you create an instance of a class, virtual base class
constructors are called first. If you don't have an explicit call to
a virtual base class ctor, then the compiler will call it under the
covers *and will use the default [no parameter] constructor*. If you
want it to call a non-default ctor, you must make an explicit call to
the ctor in the initializer list.
So, your Circle has two virtual base classes - Ellipse and Shape. The
standard says that these base classes should be constructed in left-to-
right order depth first search. In your case, that means the compiler
will want to initialize Shape first, then Ellipse. The compiler sees
that you haven't specified an explicit initializer call to a Shape
ctor, so it adds in a call to the default ctor for you.
So, if you add the following to your Circle ctor initializer list:
public:
Circle(int width, int height):
Shape(width, height), <<<
Ellipse(width, height)
{
then you end up with what you wanted.
Hope that helps,
Doug