vector has capacity and it has size. The capacity is the number of elements for which memory has been allocated. Size is the number of elements which are actually in the vector. A
vector is empty when its size is 0. So,
size() returns 0 and
true. That says nothing about the capacity of the
vector at that point (that would depend on things like the number of insertions and erasures that have been done to the
vector since it was created).
capacity() will tell you the current capacity - that is the number of elements that the
vector can hold before it will have to reallocate its internal storage in order to hold more.
So, when you construct a
vector, it has a certain size and a certain capacity. A default-constructed
vector will have a size of zero and an implementation-defined capacity. You can insert elements into the
vector freely without worrying about whether the
vector is large enough - up to
max_size() being the maximum capacity/size that a
vector can have on that system (typically large enough not to worry about). Each time that you insert an item into the
vector, if it has sufficient capacity, then no memory-allocation is going to be allocated to the
vector. However, if inserting that element would exceed the capacity of the
vector, then the
vector's memory is internally re-allocated so that it has enough capacity to hold the new element as well as an implementation-defined number of new elements (typically, the
vector will probably double in capacity) and that element is inserted into the vector. This happens without you having to worry about increasing the
vector's capacity. And it happens in constant amortized time, so you don't generally need to worry about it being a performance problem.
If you do find that you're adding to a
vector often enough that many reallocations occur, and it's a performance problem, then you can call
reserve() which will set the capacity to at least the given value. Typically, you'd do this when you have a very good idea of how many elements your
vector is likely to hold. However, unless you know that it's going to a performance issue, then it's probably a bad idea. It's just going to complicate your code. And constant amortized time will generally be good enough to avoid performance issues.
You can also construct a
vector with a given number of default-constructed elements as you mentioned, but unless you really want those elements, then that would be a bad idea.
vector is supposed to make it so that you don't have to worry about reallocating the container when you insert elements into it (like you would have to with an array), and default-constructing elements in it for the purposes of allocating memory is defeating that. If you really want to do that, use
reserve(). But again, don't bother with
reserve() unless you're certain that it's going to improve performance. And as was pointed out in another answer, if you're inserting elements into the
vector based on user input, then odds are that the time cost of the I/O will far exceed the time cost in reallocating memory for the
vector on those relatively rare occasions when it runs out of capacity.
capacity() // Returns the number of elements that the vector can hold
reserve() // Sets the minimum capacity of the vector.
clear() // Removes all elements from the vector.
empty() // Returns true if the vector has no elements.
resize() // Changes the size of the vector.
size() // Returns the number of items in the vector.