Memory Management in C++

Allocating Memory

The C++ way of allocating memory is by using the new operator. new will allocate memory, initialize it, and return its address (which can be assigned to a pointer).

Examples of using the new operator to initialize a pointer:

int *a = new int;       // Just allocate memory
int *b = new int(12);   // Allocate and initialize memory

The operator can also be used to allocate blocks of memory:

int *numbers = new int[15];  // Allocate memory for an array with 15 elements

When allocating blocks of memory, the new operator returns a pointer to the first element.

If there is not enough heap memory available, then new throws an exception of type std::bad_alloc. If nothrow is used, then it returns a NULL porinter instead.

Deallocating Memory

To deallocate memory, use the delete operator:

int *a = new int(12);
delete a;

For blocks/arrays allocated using new, use the following form for delete:

int *numbers = new int[15];
delete[] numbers;

A Word on malloc

In C++ is it considered best practice to use malloc as little as possible. Instead, programmers should make use of the new and delete operators. There are a few reasons for this:

  • malloc is not type-safe, while new and delete are

  • malloc only allocates chunks of raw memory, while new calls class constructors

There are situations where it makes sense to use malloc, such as when you need a realloc-like operation performed. It should also be noted that you cannot mix malloc/free and new/delete.

Allocators

An allocator is an object used to encapsulate memory management. Allocators are useful when you want to separate allocation and construction. It is also useful for separating deallocation and destruction.

By default, all STL containers use std::allocator. This uses the new and delete operators to allocate and free memory from the heap.

A example of an allocator declaration:

template <class T>
class allocator;

One use case for allocators is control over construction. The new operator does not allow you to control which constructors are called, but an allocator can.

For more information, see this article.

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