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| Categories: containers, adaptors | Component type: type |
Description
A stack is an adaptor that provides a restricted subset of Container functionality: it provides insertion, removal, and inspection of the element at the top of the stack. Stack is a "last in first out" (LIFO) data structure: the element at the top of a stack is the one that was most recently added. [1] Stack does not allow iteration through its elements. [2]
Stack is a container adaptor, meaning that it is implemented on top of some underlying container type. By default that underlying type is Deque, but a different type may be selected explicitly.
Example
int main() {
stack<int> S;
S.push(8);
S.push(7);
S.push(4);
assert(S.size() == 3);
assert(S.top() == 4);
S.pop();
assert(S.top() == 7);
S.pop();
assert(S.top() == 8);
S.pop();
assert(S.empty());
}
Definition
Defined in the standard header stack, and in the nonstandard backward-compatibility header stack.h.
Template parameters
| Parameter | Description | Default |
T | The type of object stored in the stack. | |
Sequence | The type of the underlying container used to implement the stack. | Deque<T> |
Model of
Assignable, DefaultConstructible
Type requirements
Public base classes
None.
Members
| Member | Where defined | Description |
value_type | stack | See below. |
size_type | stack | See below. |
stack() | DefaultConstructible | The default constructor. Creates an empty stack. |
stack(const stack&) | Assignable | The copy constructor. |
stack& operator=(const stack&) | Assignable | The assignment operator. |
bool empty() const | stack | See below. |
size_type size() const | stack | See below. |
value_type& top() | stack | See below. |
const value_type& top() const | stack | See below. |
void push(const value_type&) | stack | See below. |
void pop() [3] | stack | See below. |
bool operator==(const stack&, const stack&) | stack | See below. |
bool operator<(const stack&, const stack&) | stack | See below. |
New members
These members are not defined in the Assignable and DefaultConstructible requirements, but are specific to stack.
| Member | Description |
value_type | The type of object stored in the stack. This is the same as T and Sequence::value_type. |
size_type | An unsigned integral type. This is the same as Sequence::size_type. |
bool empty() const | Returns true if the stack contains no elements, and false otherwise. S.empty() is equivalent to S.size() == 0. |
size_type size() const | Returns the number of elements contained in the stack. |
value_type& top() | Returns a mutable reference to the element at the top of the stack. Precondition: empty() is false. |
const value_type& top() const | Returns a const reference to the element at the top of the stack. Precondition: empty() is false. |
void push(const value_type& x) | Inserts x at the top of the stack. Postconditions: size() will be incremented by 1, and top() will be equal to x. |
void pop() | Removes the element at the top of the stack. [3] Precondition: empty() is false. Postcondition: size() will be decremented by 1. |
bool operator==(const stack&, const stack&) | Compares two stacks for equality. Two stacks are equal if they contain the same number of elements and if they are equal element-by-element. This is a global function, not a member function. |
bool operator<(const stack&, const stack&) | Lexicographical ordering of two stacks. This is a global function, not a member function. |
Notes
[1] Stacks are a standard data structure, and are discussed in all algorithm books. See, for example, section 2.2.1 of Knuth. (D. E. Knuth, The Art of Computer Programming. Volume 1: Fundamental Algorithms, second edition. Addison-Wesley, 1973.)
[2] This restriction is the only reason for stack to exist at all. Note that any FrontInsertionSequence or BackInsertionSequence can be used as a stack; in the case of Vector, for example, the stack operations are the member functions back, push_back, and pop_back. The only reason to use the container adaptor stack instead is to make it clear that you are performing only stack operations, and no other operations.
[3] One might wonder why pop() returns void, instead of value_type. That is, why must one use top() and pop() to examine and remove the top element, instead of combining the two in a single member function? In fact, there is a good reason for this design. If pop() returned the top element, it would have to return by value rather than by reference: return by reference would create a dangling pointer. Return by value, however, is inefficient: it involves at least one redundant copy constructor call. Since it is impossible for pop() to return a value in such a way as to be both efficient and correct, it is more sensible for it to return no value at all and to require clients to use top() to inspect the value at the top of the stack.
See also
queue, priority_queue, Container, Sequence
