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

Malloc and Constructors: Understanding the Difference

Unlike the new and delete operators, which automatically invoke constructors and destructors, the std::malloc() function simply allocates memory. This raises the question: how can we create an object using std::malloc() while ensuring that its constructor is called?

The most straightforward solution is to use the new operator, as it is specifically designed for object instantiation. However, there are cases where we may prefer to use std::malloc() directly. In such situations, we can explicitly call the constructor using a technique known as "placement new."

Placement New for Explicit Constructor Invocation

Placement new allows us to instantiate an object within memory that we have allocated ourselves using std::malloc(). The syntax for placement new is as follows:

<code class="cpp">new (address) Type();</code>

where address is the memory address where the object should be created and Type is the type of the object.

Example Using Placement New

Consider the following code:

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

a->~A();
free(a);</code>

Here, we first allocate memory for an object of type A using std::malloc(). We then explicitly invoke the constructor of A using placement new. The a pointer now points to a fully constructed A object.

After the object is no longer needed, we manually call its destructor using the a->~A() syntax. Finally, we release the allocated memory using free().

Conclusion

While new and delete remain the preferred operators for object creation and destruction, understanding how to use std::malloc() in conjunction with placement new gives us greater control over memory management and allows us to handle specific scenarios where explicit constructor invocation is necessary.

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