The operator new Function

When a statement such as the following is encountered in a program, it translates into a call to the function operator new:

char *pch = new char[BUFFER_SIZE];

If the request is for zero bytes of storage, operator new returns a pointer to a distinct object (that is, repeated calls to operator new return different pointers). If there is insufficient memory for the allocation request, by default operator new returns NULL. You can change this default behavior by writing a custom exception-handling routine and calling the _set_new_handler run-time library function with your function name as its argument. For more details on the recovery scheme, see the following section, Handling Insufficient Memory Conditions.

The two scopes for operator new functions are described in Table 11.4.

Table 11.4   Scope for operator new Functions

The first argument to operator new must be of type size_t (a type defined in STDDEF.H), and the return type is always void *.

The global operator new function is called when the new operator is used to allocate objects of built-in types, objects of class type that do not contain user-defined operator new functions, and arrays of any type. When the new operator is used to allocate objects of a class type where an operator new is defined, that class’s operator new is called.

An operator new function defined for a class is a static member function (which cannot, therefore, be virtual) that hides the global operator new function for objects of that class type. Consider the case where new is used to allocate and set memory to a given value:

#include <malloc.h>
#include <memory.h>

class Blanks
{
public:
    Blanks(){}
    void *operator new( size_t stAllocateBlock, char chInit );
};
void *Blanks::operator new( size_t stAllocateBlock, char chInit )
{
    void *pvTemp = malloc( stAllocateBlock );
    if( pvTemp != 0 )
        memset( pvTemp, chInit, stAllocateBlock );
    return pvTemp;
}

For discrete objects of type Blanks, the global operator new function is hidden. Therefore, the following code allocates an object of type Blanks and initializes it to 0xa5:

int main()
{
    Blanks *a5 = new( 0xa5 ) Blanks;

    return a5 != 0;
}

The argument supplied in parentheses to new is passed to Blanks::operator new as the chInit argument. However, the global operator new function is hidden, causing code such as the following to generate an error:

Blanks *SomeBlanks = new Blanks;

For previous versions of the compiler, nonclass types and all arrays (regardless of whether they were of class type) allocated using the new operator always used the global operator new function.

Beginning with Visual C++ 5.0, the compiler supports member array new and delete operators in a class declaration. For example:

class X {
public:
   void*   operator new[] (size_t);
   void      operator delete[] (void*);
};

void f() {
   X *pX = new X[5];
   delete [] pX;
}