What is the major difference between a vector and a stack?

Question

Both act like a stack. Both have push and pop operations.

Is the difference in some memory layouts?

Answer 1

I'm not aware of all the implementation details, but according to this, stack is a container adaptor. It makes sure the underlying container, which can be a vector, list or deque, works as a stack, i.e. only allows push and pop, and not random access.

So, a vector can work as a stack, but a stack cannot work as a vector, because you cannot insert or get an element at a random position.

Answer 2

stack is a stack. It can only push and pop. A vector can do other things, like insert into the middle. This increases flexibility, but reduces guarantees.

For example, for a stack, if you push A then B onto the back then you are guaranteed that they will be removed in the order B, then A. vector doesn't guarantee that.

Answer 3

Stack is basically a special case of vector. Theoretically speaking vector can grow as you wish. You can remove elements at any index in a vector. However, in case of a stack you can remove elements and insert them only at its top (hence a special case of vector).

In face in many libraries that provide an implementation of a stack, they generally inherit from the vector class/structures. I am not sure, but I think STL (C++) does it.

Answer 4

As cplusplus.com suggests:

Stacks are a type of container adaptor, specifically designed to operate in a LIFO context (last-in first-out), where elements are inserted and extracted only from one end of the container.

The key word here is only, as in elements are only inserted and extracted from one end of the container.

You say both vectors and stacks act like stacks, but this is only partially true. Vectors can act like stacks, but they can also act very much not like stacks, by allowing you to do things such as insert at any index, access any element, iterate over the entire structure, etc.

Stacks take a container (such as, for example, a vector) and only permit stack-like interactions with it. This effectively guarantees that all interactions with the container will obey LIFO: only the most recently added element in the container will be able to be accessed or removed.

If you want a container with stack-like behavior, you should use a stack if it is particularly important to you that it behaves exclusively a stack. You should use a vector if you want stack-like behavior but might also want to do things like iterate over elements or modify elements in arbitrary positions etc.

Answer 5

std::vector has several accessibility and modification operations compared to std::stack. In case of std::stack, you may have to perform operations only in systematic way, where you can push() above the last element or pop() the last element.

std::vector is more flexible in that sense, where it has several operations, where you can insert() in between or erase() in between.

The major point is that, std::stack needs to be provided the underlying container. By default it's std::deque, but it can be std::vector or std::list too.
On other hand, std::vector is guaranteed to be a contiguous array which can be accessed using operator [].

Answer 6

I think the main difference is that vector is a range based container. It can be easily used thanks to its member functions such as begin and end. Vector can be easily initiated with {} form. We can use new features of modern C++ like range-based loops.

vector<int> vec{ 7, 3, 1, 9, 5 };
for ( auto &i : vec ) {
    std::cout << i << std::endl;
}

Whereas it is not possible for std::stack.