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How Can I Construct Objects Using `std::malloc`?

Mary-Kate Olsen
Release: 2024-10-28 08:19:30
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 How Can I Construct Objects Using `std::malloc`?

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:

<code class="c++">pointer = (type*)malloc(sizeof(type));
new (pointer) type();</code>
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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:

<code class="c++">A* a = (A*)malloc(sizeof(A));
new (a) A();

a->~A();
free(a);</code>
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By utilizing placement new, you can create objects with std::malloc while still invoking constructors.

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