Why we use & for gettop function ?

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C++

Hello all

I was studying C++ , ADT Stack , and there are two functions which I confused about.
I wrote the codes above , my question is:
we use & while using bool pop function b/c we change the stack but
why we use & for getTop function ? we just retrieve the top of a stack, we don't change it right ?

What I have tried:

bool pop(StackItemType& stackTop);

// Retrieves and removes the top of a stack

// Precondition: None.

// Postcondition: If the stack is not empty,
// stackTop contains the item that was added
// most recently and the item is removed.
// However, if the stack is empty, deletion
// is impossible and stackTop is unchanged.

----------------------------------------------------

bool getTop(StackItemType& stackTop) const;

// Retrieves the top of a stack.

// Precondition: None.

// Postcondition: If the stack is not empty,
// stackTop contains the item that was added
// most recently. However, if the stack is
// empty, the operation fails and stackTop
// is unchanged. The stack is unchanged.