How can I create an object using malloc and ensure constructor invocation?

Constructor Invocation with Malloc: A Contrast to New and Delete

In contrast to the ubiquitous new and delete expressions, std::malloc presents a unique behavior: it does not automatically invoke an object's constructor upon memory allocation. This raises the question: how can we create an object using malloc while ensuring constructor invocation?

Answer:

The recommended and intuitive approach is to utilize the new operator, which inherently handles both memory allocation and constructor invocation. Alternatively, we can manually invoke the constructor through "placement new," a technique that employs explicit constructor calls.

Custom Handling via Placement New

The following code snippet illustrates the use of placement new:

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

Here, the pointer a points to allocated memory of size sizeof(A), and the placement new expression new (a) A() subsequently constructs an A object at that memory location.

When the object is no longer needed, we manually call the destructor and free the allocated memory:

<code class="cpp">a->~A();
free(a);</code>

It is important to note that placement new is typically not employed unless there are specific circumstances that prohibit the use of new and delete.

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