Re: Instantiate Class with/without operator new?

From:

David Wilkinson <no-reply@effisols.com>

Newsgroups:

microsoft.public.vc.language

Date:

Sat, 21 Jul 2007 18:29:41 -0400

Message-ID:

<#VtGSa#yHHA.4004@TK2MSFTNGP05.phx.gbl>

Gerry Hickman wrote:

Hi,

I'm trying to understand the difference between instantiating a generic
C++ class, A) using operator new, B) not using operator new. I have a
simple class called Numbers, that can add or multiply two ints supplied
to the constructor. It seems to work regardless of whether I use new or
not. I'm interested to know when I should use new for Class
instantiation and when not to use it. I tried to find the answer by
searching, but 'new' is not a very common word.

Here's how my calling code looks in _tmain(), note one instance uses
pointers, the other does not

    int a,b;

    Numbers * n1 = new Numbers(4,5);
    a = n1->Add();
    wcout << a << endl; // prints 9
    b = n1->Multiply();
    wcout << b << endl; // prints 20
    delete n1;

    Numbers n2 = Numbers(5,6);
    a = n2.Add();
    wcout << a << endl; // prints 11
    b = n2.Multiply();
    wcout << b << endl; // prints 30

Gerry:

The second way (on the stack) is the C++ way. It will not leak memory
even if the code throws an exception (or returns) at some intermediate
point. If the class Numbers itself allocates memory or other resources
then those resources will be freed automatically in the Numbers
destructor. This is the RAII principle (look it up on Google, or read
Stroudstrup's book, if you do not know what this is).

A simple, but important, example is the difference between allocating an
array with new[] and using std::vector. Using std::vector is always
better because it frees its own memory. Another example is the
difference between using FILE* and std::fstream (the latter always
closes the file handle).

More often than not, it is a mistake to use new or new[] in your own code.

--
David Wilkinson
Visual C++ MVP