Question

I have some questions about vector in STL to clarify…..

1. Where are the objects in vector allocated? heap?

2. does vector have boundary check? If the index out of the boundary, what error will happen?

3. Why array is faster than vector?

4. Is there any case in which vector is not applicable but array is a must?

Answer 1

1. In a contiguous memory block on the heap. A vector<int> allocates memory the same way new int[x] would.
2. Only if you use the at method. It throws an std::out_of_range exception if the boundary check fails. The operator[] doesn’t perform bounds checking.
3. Because an array gives direct access to memory, while accessing a vector element most likely involves a method call. The difference can be ridiculously small though, especially if your compiler decides to inline the calls.
4. In general, you’ll use a vector if you want your container to have a dynamic size, and a simple array if a known fixed size is enough. Be sure to check out the other containers, like deque and list, to be sure you pick the most appropriate one. Otherwise, if you need to deal with non-C++ APIs, you’ll obviously need to have access to a regular array. (edit) @BillyONeal says you should use &vector[0] to get the address of the underlying array, but use it with care since it can change if the vector’s capacity changes.