Memory Management with std::allocator in C++

Van Chan Ngo · January 19, 2021

In this post, I will talk about memory management with std::allocator in C++

Introduction

As we know all the STL containers in C++ have an Allocator as a type parameter which is std::allocator by default. Underlying, it uses the new and delete operators to allocate and deallocate memory.

By definition, allocators are objects responsible for encapsulating memory management. It is useful when we want to separate memory allocation and object construction in two steps and separate deallocation and object destruction in two steps as well.

So what does this mean? For example, consider the following code:

int* vPtr = new int[3];

With new operator, it forces us to allocate and construct all objects, 3 integer values, at the same time. Let’s say we only want to allocate (or preserve) 3 integer size memory areas and construct these integer values later, for instance we only want to construct the first integer value. std::allocator can help us to do that.

Allocators

The allocator is responsible for managing raw memory storage and also for constructing and destroying allocated objects in two seperated steps. Its declaration is:

template <class T> class allocator;

The most advantage of using allocator is:

Allocator allow us to have control over which constructors are called so the allocation and constructor are seperated

The most importants methods of std::allocator are:

  • allocate: Used for allocation of memory
  • deallocate: Used for deallocation of memory

Example

#include <memory>
#include <iostream>
#include <string>
 
int main()
{
    std::allocator<int> intAlloc;
    // Allocate memory area for 100 integers
    int* intArray = intAlloc.allocate(100);
    // Construct the 5th element
    intArray[4] = 2011;
    std::cout << "intArray[4]: " << intArray[4] << std::endl;
    // Deallocate the memory area
    intAlloc.deallocate(intArray, 100);
}