std::allocator() function in C++

In this tutorial we will study about std::allocator() function in C++.

What is Std allocator?

Allocators are objects which are responsible for encapsulating(enclose (something) in or as if in a capsule) for memory management. The default value of parameters that this function uses delete and new for allocating and removing the space.

Why we are using std allocator?
With the new/delete keywords we cannot write C++ STL vectors as we can’t allocate without initializing and we cannot destroy without deallocating. Also, we cannot use malloc as it is ancient and considered bad practice in C++.

Allocators provide a modern solution!

• Splitting up new and delete
  Split new into allocate and construct
  Split delete into destroy and deallocate
• T* allocate(size_t n)
  Allocate memory of size n*sizeof(T) but do not initializes memory.
• void construct(Pointer p, Args&&…args)
  construct an object instance in place at pointer p, pass args to the constructor.
• void destroy(pointer p)
  Destroys the object at pointer p(calls the destructor).
• void deallocate(pointer p, size_t n)
  deallocates block of memory size n*sizeof(T) at pointer p.

Example of std::allocator() function in C++

let’s see an example of this function

//  std  allocator() function 
#include <iostream> 
#include <memory> 
using namespace std; 
int main() 
{ 

  allocator<int> myAllocator; 

  // allocate space for five integers 
  int* array = myAllocator.allocate(5); 

  myAllocator.construct(array, 50); 
  array[3] = 60; 

  cout << array[3] << endl; 
  cout << array[0] << endl; 

  // deallocate function
  myAllocator.deallocate(array, 5); 

  return 0; 
} 

Output

60
50

Where are allocators used?

All STL containers use allocators in one way or another.

• Allow STL containers to work with types that have no default constructors, assignment, operators, copy constructors, etc.
• Node-based containers such as linked list, BST also use allocators for allocating node types.
• allows types like vectors to erase in the middle of an array.

Also read: std::include() function in C++