Malloc and Constructors: An Explorative Guide

In the realm of memory allocation, the standard library provides both std::malloc and new expressions. While new conveniently initializes objects through constructors, std::malloc does not. This raises the question: how can we create an object and ensure its constructor invocation when using std::malloc?

One straightforward approach is to simply employ the new expression, as it serves the intended purpose. However, if you prefer to stick with std::malloc, there's an alternative method: explicitly calling the constructor using a technique known as "placement new."

Using Placement New

Placement new allows us to explicitly create an object at a memory location specified by us. To achieve this:

1. Use std::malloc to allocate memory for the object.
2. Use new (pointer) to initialize the object at that location.

The syntax for placement new looks like this:

pointer = (type*)malloc(sizeof(type));
new (pointer) type();

After creating the object, don't forget to destruct it using the explicit ~type() syntax and free the memory with free.

Here's a code snippet demonstrating placement new:

A* a = (A*)malloc(sizeof(A));
new (a) A();

a->~A();
free(a);

By utilizing placement new, you can create objects with std::malloc while still invoking constructors.