Changing the underlying container of std::stack

By default, std::stack uses std::deque as its underlying container. However, you can change this to other container types that meet certain requirements. There are a few potential reasons to do this:

1. Memory Usage: If memory is a constraint and you have a large number of small stacks, using std::vector as the underlying container can be more memory-efficient. This is because std::deque allocates memory in chunks, which can lead to some wasted space, while std::vector uses contiguous memory.
2. Consistency: If your code extensively uses a particular container type, using the same type for your stacks can make the code more consistent and potentially easier to understand and maintain.
3. Specific Operations: Although rare, if you need to perform operations on the stack that the underlying container doesn't support efficiently, changing the container can be beneficial. For example, if you frequently need to splice elements from one stack into another, using std::list would make this operation efficient.

Here's an example of declaring a stack with a different underlying container:

1#include <stack>
2#include <vector>
3
4int main() {
5  std::stack<int, std::vector<int>> s;
6  // operations on s will use std::vector
7}

However, it's important to note that the underlying container must support certain operations (back(), push_back(), pop_back(), and emplace_back()) for std::stack to work correctly. Most standard containers like std::vector, std::list, and std::deque support these.

Also, changing the underlying container is an optimization that should be done judiciously based on profiling and performance requirements. For most use cases, the default std::deque is efficient and flexible enough.