C++: Vector of objects vs. vector of pointers to new objects?

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

Since you are explicitly stating you want to improve your C++, I am going to recommend you start using Boost. This can help you with your problem in three different ways:

Using a shared_ptr

Using a shared_ptr could declare your vector like this:

std::vector< boost::shared_ptr< ObjectBase > > object_list; 

And use it like this:

typedef std::vector< boost::shared_ptr< ObjectBase > >::iterator ObjectIterator;  for ( ObjectIterator it = object_list.begin(); it != object_list.end(); it++ )     (*it)->draw(view); 

This would give you polymorphism and would be used just like it was a normal vector of pointers, but the shared_ptr would do the memory-management for you, destroying the object when the last shared_ptr referencing it is destroyed.

Note about C++11: In C++11 shared_ptr became part of the standard as std::shared_ptr, so Boost is no longer required for this approach. However, unless you really need shared ownership, it is recommended you use std::unique_ptr, which was newly introduced in C++11.

Using a ptr_vector

Using a ptr_vector you would do it like this:

boost::ptr_vector< ObjectBase > object_list; 

And use it like this:

typedef boost::ptr_vector< ObjectBase >::iterator ObjectIterator;  for ( ObjectIterator it = object_list.begin(); it != object_list.end(); it++ )     (*it)->draw(view); 

This would again be used like a normal vector of pointers, but this time the ptr_vector manages the lifetime of your objects. The difference to the first approach is, that here your objects get destroyed when the vector gets destroyed, whereas above they may live longer than the container, if other shared_ptrs referencing them exist.

Using a reference_wrapper

Using a reference_wrapper you would declare it like this:

std::vector< boost::reference_wrapper< ObjectBase > > object_list; 

And then use it like this:

typedef std::vector< boost::reference_wrapper< ObjectBase > >::iterator      ObjectIterator;  for ( ObjectIterator it = object_list.begin(); it != object_list.end(); it++ )     it->draw(view); 

Notice that you do not have to dereference the iterator first as in the above approaches. This does however only work if the lifetime of your objects is managed elsewhere and is guaranteed to be longer than that of the vector.

Note about C++11: reference_wrapper has also been standardized in C++11 and is now usable as std::reference_wrapper without Boost.

As pointed out in Maciej Hs answer, your first approach results in object slicing. In general you may want to look into iterators when using containers.