Getting around base class constructor initialization, a bad habit? - c++

Question

I have a base class in an OpenGL project which represents 3DModel in general. Now I want to create a more specialized class that will inherits from 3DModel. My problem is the mandatory base class constructor call in the initialization list. Is there a proper way to delay this call until I've done some computation in the derived constructor?
Here's the important parts of the code:
class 3DModel {
public:
3DModel(std::vector<...> vertices){ ... };
[...]
private:
std::vector<...> vertices;
[...]
};
class Cylinder : public 3DModel {
public:
Cylinder(float top_bottom_ratio, float base_diameter);
[...]
};
//.cpp
Cylinder(float top_bottom_ratio, float base_width)
:3DModel(...) //<---- Mandatory
{
//I would like to calculate the cylinder vertices here
//and then feed them to the 3DModel constructor
}
Right now, I'm thinking of creating a dummy 3DModel() constructor and then call methods inside the derived constructor to modify the attributes of the base class. But this sounds really weird and it'll create a danger zone in the constructor where the object will be invalid for a few moment.
Another solution would be to thrash this class and simply do the computation inside the main program and use 3DModel constructor. But this is a sad solution and ruins the black-box approach.
Do you have any insights?

Answer 1

You can put the calculation into a helper function. Ideally, make it static, so you can't accidentally access unititialized base class values.
class Cylinder : public 3DModel {
public:
Cylinder(float top_bottom_ratio, float base_diameter);
[...]
private:
static calculateVertices(std::vector<...> vertices);
};
//.cpp
Cylinder(float top_bottom_ration, float base_width)
:3DModel(calculateVertices(top_bottom_ratio, base_width))
{
}
std::vector<...> Cylinder::calculateVertices(float top_bottom_ratio, float base_width) {
// calculate and return vertices here
}
You could also opt for composition instead of inheritance, where Cylindar has a 3DModel instead of being a 3DModel. (It would probably need to be something else, e.g. a Renderable that has a render() method.)

Answer 2

This is an example of the classic question of base-subclass vs composite. While the answer does give an example in the form of "base class - subclass", you really have to ask if this can't just be a class 'Cylinder' that contains the '3DModel' class. Unless your Cylinder subclass (and any other subclass) really adds more functionality to the 3DModel class, you really should make Cylinder a composite of the 3DModel class.