Question

I am seeking to improve my C++ skills by writing a sample software renderer. It takes objects consisting of points in a 3d space and maps them to a 2d viewport and draws circles of varying size for each point in view. Which is better:

class World{
    vector<ObjectBaseClass> object_list;
public:
    void generate(){
        object_list.clear();
        object_list.push_back(DerivedClass1());
        object_list.push_back(DerivedClass2());

or…

class World{
    vector<ObjectBaseClass*> object_list;
public:
    void generate(){
        object_list.clear();
        object_list.push_back(new DerivedClass1());
        object_list.push_back(new DerivedClass2());

?? Would be using pointers in the 2nd example to create new objects defeat the point of using vectors, because vectors automatically call the DerivedClass destructors in the first example but not in the 2nd? Are pointers to new objects necessary when using vectors because they handle memory management themselves as long as you use their access methods? Now let’s say I have another method in world:

void drawfrom(Viewport& view){
    for (unsigned int i=0;i<object_list.size();++i){
        object_list.at(i).draw(view);
    }
}

When called this will run the draw method for every object in the world list. Let’s say I want derived classes to be able to have their own versions of draw(). Would the list need to be of pointers then in order to use the method selector (->) ?

Answer 1

You wont get what You want with this code

class World{
    vector<ObjectBaseClass> object_list;
public:
    void generate(){
        object_list.clear();
        object_list.push_back(DerivedClass1());
        object_list.push_back(DerivedClass2());

What is going to happen is called object slicing. You will get a vector of ObjectBaseClass.

To make polymorphism work You have to use some kind of pointers. There are probably some smart pointers or references in boost or other libraries that can be used and make the code much safer than the second proposed solution.