Pointer to vector vs vector of pointers vs pointer to vector of pointers

Question

Just wondering what you think is the best practice regarding vectors in C++.

If I have a class containing a vector member variable. When should this vector be declared a:

1. "Whole-object" vector member varaiable containing values, i.e. vector<MyClass> my_vector;
2. Pointer to a vector, i.e vector<MyClass>* my_vector;
3. Vector of pointers, i.e. vector<MyClass*> my_vector;
4. Pointer to vector of pointers, i.e. vector<MyClass*>* my_vector;

I have a specific example in one of my classes where I have currently declared a vector as case 4, i.e. vector<AnotherClass*>* my_vector; where AnotherClass is another of the classes I have created.

Then, in the initialization list of my constructor, I create the vector using new:

MyClass::MyClass()
: my_vector(new vector<AnotherClass*>())
{}

In my destructor I do the following:

MyClass::~MyClass()
{
  for (int i=my_vector->size(); i>0; i--)
  {
    delete my_vector->at(i-1);
  }
  delete my_vector;
}

The elements of the vectors are added in one of the methods of my class. I cannot know how many objects will be added to my vector in advance. That is decided when the code executes, based on parsing an xml-file.

Is this good practice? Or should the vector instead be declared as one of the other cases 1, 2 or 3 ?

When to use which case?

I know the elements of a vector should be pointers if they are subclasses of another class (polymorphism). But should pointers be used in any other cases ?

Thank you very much!!

Answer 1

Usually solution 1 is what you want since it’s the simplest in C++: you don’t have to take care of managing the memory, C++ does all that for you (for example you wouldn’t need to provide any destructor then).

There are specific cases where this doesn’t work (most notably when working with polymorphous objects) but in general this is the only good way.

Even when working with polymorphous objects or when you need heap allocated objects (for whatever reason) raw pointers are almost never a good idea. Instead, use a smart pointer or container of smart pointers. Modern C++ compilers provide shared_ptr from the upcoming C++ standard. If you’re using a compiler that doesn’t yet have that, you can use the implementation from Boost.