C++ vector of objects vs. vector of pointers to objects

If you are allocating memory for the objects using new, you are allocating it on the heap. In this case, you should use pointers. However, in C++, the convention is generally to create all objects on the stack and pass copies of those objects around instead of passing pointers to objects on the heap.

Why is this better? It is because C++ does not have garbage collection, so memory for objects on the heap will not be reclaimed unless you specifically delete the object. However, objects on the stack are always destroyed when they leave scope. If you create objects on the stack instead of the heap, you minimize your risk of memory leaks.

If you do use the stack instead of the heap, you will need to write good copy constructors and destructors. Badly written copy constructors or destructors can lead to either memory leaks or double frees.

If your objects are too large to be efficiently copied, then it is acceptable to use pointers. However, you should use reference-counting smart pointers (either the C++0x auto_ptr or one the Boost library pointers) to avoid memory leaks.

My question is simple: why did using a vector of pointers work, and when would you create a vector of objects versus a vector of pointers to those objects?

std::vector is like a raw array allocated with new and reallocated when you try to push in more elements than its current size.

So, if it contains A pointers, it's like if you were manipulating an array of A*. When it needs to resize (you push_back() an element while it's already filled to its current capacity), it will create another A* array and copy in the array of A* from the previous vector.

If it contains A objects, then it's like you were manipulating an array of A, so A should be default-constructible if there are automatic reallocations occuring. In this case, the whole A objects get copied too in another array.

See the difference? The A objects in std::vector<A> can change address if you do some manipulations that requires the resizing of the internal array. That's where most problems with containing objects in std::vector comes from.

A way to use std::vector without having such problems is to allocate a large enough array from the start. The keyword here is "capacity". The std::vector capacity is the real size of the memory buffer in which it will put the objects. So, to setup the capacity, you have two choices:

1) size your std::vector on construction to build all the object from the start , with maximum number of objects - that will call constructors of each objects.

2) once the std::vector is constructed (but has nothing in it), use its reserve() function : the vector will then allocate a large enough buffer (you provide the maximum size of the vector). The vector will set the capacity. If you push_back() objects in this vector or resize() under the limit of the size you've provided in the reserve() call, it will never reallocate the internal buffer and your objects will not change location in memory, making pointers to those objects always valid (some assertions to check that change of capacity never occurs is an excellent practice).

What you have to know about vectors in c++ is that they have to use the copy operator of the class of your objects to be able to enter them into the vector. If you had memory allocation in these objects that was automatically deallocated when the destructor was called, that could explain your problems: your object was copied into the vector then destroyed.

If you have, in your object class, a pointer that points towards a buffer allocated, a copy of this object will point towards the same buffer (if you use the default copy operator). If the destructor deallocates the buffer, when the copy destructor will be called, the original buffer will be deallocated, therefore your data won't be available anymore.

This problem doesn't happen if you use pointers, because you control the life of your elements via new/destroy, and the vector functions only copy pointer towards your elements.