Global vs Member overloaded operator new

In an earlier post we saw how to overload operator new, which will allow us to have customized memory allocation for an object on heap.

Its possible to place this overloaded operator new function either globally or as member function within a class.

 

// Global overloaded operator new
void *operator new(size_t size)
{
    return malloc(size);
}

 

// Class CTest
class CTest
{
public:

    // Member overloaded operator new
    void *operator new(size_t size)
    {
        return malloc(size);
    }

};

 

Is there any behavioral difference between these two versions of operator new?

Yes, there is.

If we have overloaded new as a member function within a class, it will always have high precedence over the globally defined version.

If we have a globally defined operator new function, the same will be used for all classes which doesn't have overloaded operator new defined within.

If both exist, the member version will be preferred over the global version.

Is there any advantage of this feature?

Yes.

Global version of overloaded operator new will be very much useful when we need to have a common strategy for memory allocation for all the classes in the module.

If we are in a situation where we need to have a specific way for allocation for a particular class which is different from others then we can overload operator new for the class with desired allocation method. A good example would be to allocate and initialize the allocated memory with zeros.

Overloading "new"

In one of the previous article about operator new, we saw that it can always be overloaded.

operator new can be overloaded to define its own way of allocating memory for an object. User should be able to control the way how it allocates the memory for the object.

The global new operator is the one which invokes a call to appropriate version of operator new if its overloaded and then it will call the constructor to complete the object instantiation process.

Below is the pseudo code of what new operator does

new()
{

    // Call correct version of operator new() based on the parameters passed while initializing the object
    operator new(sizeof(CMyClass));

    // Call appropriate version of constructor
    CMyClass::CMyClass();    

}

Function operator new always has size_t as one of its parameter. The functionality of a default operator new will be as shown below,

void* operator new(size_t size)
{
    // allocate and return the pointer
    return malloc(size); 
}

Where it allocates size number of bytes and return the pointer which points to the allocated memory.

If user tries to instantiate an object of type CMyClass dynamically,

CMyClass *pObj = new CMyClass();

This will call the global new operator() function which will first call operator new(sizeof(CMyClass)) passing sizeof(CMyClass) as parameter to allocate required memory, then it will call the constructor CMyClass::CMyClass() to finish the object instantiation process.

In some cases user may want to allocate memory for an object in a predefined memory area. This can be achieved by overloading operator new().

void* operator new(size_t size, void *pMem)
{
    // just return the pre allocated memory pointer
    return pMem; 
}

User can invoke this by doing the following,

char *buf  = new char[500];                //pre-allocated buffer
CMyClass *pObj = new (buf) CMyClass();    //placement new

This will invoke the overloaded version of function operator new() which takes two parameters, first would be the size of the class and the next would be the pointer to pre-allocated buffer.

operator new(sizeof(CMyClass), buff);

And this overloaded version will just return the pointer to new operator() function which will then call the appropriate constructor.

This technic is popularly known as Placement New.

In the same way user should be able to overload operator new with different parameters.

Cheers!

What's "new"?

Today's article talks about the new, which is an essential operator in C++ for dynamically instantiating an object.

Why do we need new operator?
There's a lot of difference between C's malloc and new. It does more than just allocating memory. It also calls the constructor. Which facilitates to initiate the object.

Believe me, operator new and new operator are completely different!
When we instantiate an object
CTest *pObj = new CTest();

This calls global new operator which performs 2 main tasks
1. Call operator new for allocating memory.
2. Call constructor of the particular object.

We can always overload operator new which performs memory allocation operations. But we cannot change the meaning of the global new operator.

Typical example where we overload operator new will be in case of placement new where we change the actual meaning of operator new by passing address of memory where it has to instantiate the object.

Let's talk about placement new in forth coming articles.