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1,std::allocator
template<class T> class allocator;
Allocators are classes that define memory models to be used by some parts of the Standard Library, and most specifically, by STL containers.2,
2,std::deque
template<class T,class Allocator=allocator<T>>deque;
std::deque (double-ended queue) is an indexed sequence container that allows fast insertion and deletion at both its beginning and its end. In addition, insertion and deletion at either end of a deque never invalidates pointers or references to the rest of the elements.
As opposed to std::vector, the elements of a deque are not stored contiguously: typical implementations use a sequence of individually allocated fixed-size arrays, with additional bookkeeping, which means indexed access to deque must perform two pointer dereferences, compared to vector's indexed access which performs only one.
The storage of a deque is automatically expanded and contracted as needed. Expansion of a deque is cheaper than the expansion of a std::vector because it does not involve copying of the existing elements to a new memory location. On the other hand, deques typically have large minimal memory cost;
The complexity (efficiency) of common operations on deques is as follows:
Random access - constant O(1)
Insertion or removal of elements at the end or beginning - constant O(1)
Insertion or removal of elements - linear O(n)
3,std::stack
template<class T,class Container = deque<T>>stack;
Stacks are a type of container adaptor, specifically designed to operate in a LIFO context (last-in first-out), where elements are inserted and extracted only from one end of the container.
stacks are implemented as containers adaptors, which are classes that use an encapsulated object of a specific container class as its underlying container, providing a specific set of member functions to access its elements. Elements are pushed/popped from the "back" of the specific container, which is known as the top of the stack.
The underlying container may be any of the standard container class
templates or some other specifically designed container class. The
container shall support the following operations: