
Categories: containers, adaptors  Component type: type 
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.
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()); }
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> 
Member  Where defined  Description 

value_type  stack  See below. 
size_type  stack  See below. 
stack()  Default Constructible  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. 
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 elementbyelement. 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. 
[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. AddisonWesley, 1973.)
[2] This restriction is the only reason for stack to exist at all. Note that any Front Insertion Sequence or Back Insertion Sequence 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.
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