Why does C++ standard queue have back function when stack doesn't have bottom function?

Question

On one of many unofficial C++ reference websites, there are listed member functions front() and back() for std::queue. However, std::stack only has top() function.

It makes sense for the stack to not have a bottom() function because that's the definition of a stack.

What I don't get is why did the C++ standard committee chose not to follow the definition of a queue and provide with back() function for queue and chose to follow the definition of a stack and not provide with bottom() function.

Answer 1

There might be other reasons for back(), but you needed it for a queue because of the idiom from C++03 of cheaply copying an "empty" object into a container and then swapping that new element with a "full" object that would be very expensive to copy. This reason is more or less obsolete in C++11 thanks to move semantics, but of course back() is still needed for compatibility.

You don't need bottom() for a stack for that (or any other) reason.

Answer 2

It actually makes sense in a weird way. In a queue, you push on one side and pop from the other so both sides are very likely to change a lot. With a stack, you push and pop both from the top, and the bottom of the stack very rarely changes. So, it's rarely interesting to query the current value of the bottom of a stack.