STL Sample for the new operator Function

ID: Q156808

The information in this article applies to:

The Standard C++ Library, included with:
- Microsoft Visual C++, 32-bit Editions, version 4.2

SUMMARY

The sample code below illustrates how to use the new operator from the Standard Library in Visual C++.

MORE INFORMATION

The new operator will return NULL or throw an exception on failure.

Required Header


   <new>

Prototype


   void *operator new(size_t n)
   void *operator new(size_t n, const nothrow&)
   void *operator new[](size_t n);

NOTE: The class/parameter names in the prototype may not match the version in the header file. Some have been modified to improve readability.

Description

The first operator new will attempt to allocate memory and if it fails, will throw an exception.

The second operator new accepts a second parameter of type nothrow. This parameter indicates that if the allocation fails, it should return NULL and not throw an exception.

The third operator new will allocate memory for an array of that type and if it fails, will throw an exception.

Sample Code


////////////////////////////////////////////////////////////////////// 
// 
// Compile options needed: /GX
// 
// <filename> :  newop.cpp
// 
// Functions:
// 
//    void *operator new(size_t n)
// 
//    void *operator new(size_t n, const nothrow&)
// 
//    void *operator new[](size_t n);
// 
// Written by Linda Koontz
// of Microsoft Product Support Services,
// Copyright (c) 1996 Microsoft Corporation. All rights reserved.
////////////////////////////////////////////////////////////////////// 

/* Compile options needed: /GX
*/ 

#include <new>
#include <iostream>

class BigClass {
public:
    BigClass() {};
    ~BigClass(){}
        double BigArray[99999999];
};

void main()
{
    try {
    BigClass * p = new BigClass;
    }
    catch( bad_alloc a) {
        const char * temp = a.what();
        cout << temp << endl;
        cout << "Threw a bad_alloc exception" << endl;
    }
    BigClass * q = new(nothrow) BigClass;
    if ( q == NULL )
        cout << "Returned a NULL pointer" << endl;

    try {
    BigClass * r = new BigClass[3];
    }
    catch( bad_alloc a) {
        const char * temp = a.what();
        cout << temp << endl;
        cout << "Threw a bad_alloc exception" << endl;
    }
}

Program Output is:


bad allocation
Threw a bad_alloc exception
Returned a NULL pointer
bad allocation
Threw a bad_alloc exception

REFERENCES

Visual C++ Books On Line: Visual C++ Books:C/C++:Standard C++ Library Reference. Query on [ASCII 147]operator new,[ASCII 148] or look in Help for the <new> header file.

NOTE: The Online Help for the new header file lists the following prototypes for the new operators demonstrated in this article:


    void *operator new(size_t n) throw(bad_alloc);

    void *operator new(size_t n, const nothrow&) throw();

    void *operator new[](size_t n) throw(bad_alloc);

The throw() following the prototype is an exception specification that is not implemented in Visual C++ version 4.2. This is noted in the Online Help:


   Microsoft C++ does not support the function exception
   specification mechanism, as described in section 15.4 of the
   ANSI C++ draft.

An exception-specification specifies the type of exceptions a function can throw. For example,


   void Func() throw (ProblemOne, ProblemTwo) {}

is equivalent to:


    void Func() {
    {
    try {}
    catch (ProblemOne) {}
    catch (ProblemTwo) {}
    catch (...) { unexpected(); }
    }

Additional query words: STL STLSample new


Keywords          : kbcode STLIss 
Version           : WINNT:4.2;
Platform          : NT WINDOWS 
Issue type        : kbhowto

Last Reviewed: July 29, 1999