Constructing Objects with malloc: How to Invoke Constructors When Using `malloc` in C ?

Constructor Invocation in Memory Allocation: malloc vs. new

In the realm of C , memory allocation and object creation often go hand in hand. While the new and delete expressions provide a convenient and simplified mechanism for both, the std::malloc function introduces a different approach.

Unlike new, std::malloc does not automatically invoke the constructor associated with the object being allocated. This raises the question: how can we create an object with malloc while ensuring constructor invocation?

One solution is to explicitly call the constructor. This method, known as "placement new," involves two steps:

1. Allocate memory for the object using std::malloc.
2. Call the constructor directly on the allocated memory address.

Placement New Example:

<code class="cpp">A* a = (A*)malloc(sizeof(A));
new (a) A();</code>

Here, a pointer to an object of type A is created, followed by a call to the A constructor. This ensures that the constructor is invoked and the object is properly initialized.

However, in most cases, using the new and delete expressions is the preferred approach. The new expression allocates memory and calls the constructor in a single step, providing a more concise and error-resistant solution.

Normal Memory Allocation:

<code class="cpp">A* a = new A();</code>

The complement to new is the delete expression, which calls the destructor when an object is no longer needed.

Normal Deallocation:

<code class="cpp">delete a;</code>

By understanding the difference in constructor invocation between malloc and new, you can effectively manage memory allocation and object creation in your C programs.

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