How is vector implemented in C++

I am thinking of how I can implement std::vector from the ground up.

How does it resize the vector?

realloc only seems to work for plain old stucts, or am I wrong?

it is a simple templated class which wraps a native array. It does not use malloc/realloc. Instead, it uses the passed allocator (which by default is std::allocator).

Resizing is done by allocating a new array and copy constructing each element in the new array from the old one (this way it is safe for non-POD objects). To avoid frequent allocations, often they follow a non-linear growth pattern.

UPDATE: in C++11, the elements will be moved instead of copy constructed if it is possible for the stored type.

In addition to this, it will need to store the current "size" and "capacity". Size is how many elements are actually in the vector. Capacity is how many could be in the vector.

So as a starting point a vector will need to look somewhat like this:

template <class T, class A = std::allocator<T> >
class vector {
public:
    // public member functions
private:
    T*                    data_;
    typename A::size_type capacity_;
    typename A::size_type size_;
    A                     allocator_;
};

The other common implementation is to store pointers to the different parts of the array. This cheapens the cost of end() (which no longer needs an addition) ever so slightly at the expense of a marginally more expensive size() call (which now needs a subtraction). In which case it could look like this:

template <class T, class A = std::allocator<T> >
class vector {
public:
    // public member functions
private:
    T* data_;         // points to first element
    T* end_capacity_; // points to one past internal storage
    T* end_;          // points to one past last element
    A  allocator_;
};

I believe gcc's libstdc++ uses the latter approach, but both approaches are equally valid and conforming.

NOTE: This is ignoring a common optimization where the empty base class optimization is used for the allocator. I think that is a quality of implementation detail, and not a matter of correctness.

Resizing the vector requires allocating a new chunk of space, and copying the existing data to the new space (thus, the requirement that items placed into a vector can be copied).

Note that it does not use new [] either -- it uses the allocator that's passed, but that's required to allocate raw memory, not an array of objects like new [] does. You then need to use placement new to construct objects in place. [Edit: well, you could technically use new char[size], and use that as raw memory, but I can't quite imagine anybody writing an allocator like that.]

When the current allocation is exhausted and a new block of memory needs to be allocated, the size must be increased by a constant factor compared to the old size to meet the requirement for amortized constant complexity for push_back. Though many web sites (and such) call this doubling the size, a factor around 1.5 to 1.6 usually works better. In particular, this generally improves chances of re-using freed blocks for future allocations.