Operator Overloading problem

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3 total messages Started by Jason Morris Wed, 16 Dec 1992 19:27

Author: Jason Morris
Date: Wed, 16 Dec 1992 19:27

27 lines
1751 bytes

  I have a class, Matrix, and I want to overload the operators +,-,* etc.
Matrix has a private member Data, which is a pointer to some dynamically
allocated storage. Matrix() allocates this storage, and ~Matrix() frees it.
Fine so far.
  Now I want to overload, say, operator+. The problem is that I want to take 
lhs and rhs Matrix arguements, and return the answer in a third Matrix. I
can't pass in a reference to the third Matrix, as operator+ has to take only
two arguements. I can't just pass back a reference to a Matrix declared within
the operator+() function, as it goes out of scope when the function exits.
  So I ended up with:

    friend Matrix operator+(Matrix& lhs,Matrix& rhs);

  And I just do "return <local Matrix>"

  The problem now is that although I'm getting back a copy of the local
variable, not the original (scope is okay), when the operator+ function
returns the destructor is called for the local Matrix, thus deleting the free
store that the data is held in. As my copy has the same value for the pointer
Data, it is now pointing to a useless storage area.

  Question is: How should I be doing this? Surely I don't have to use a
function add(Matrix lhs,Matrix rhs,Matrix answer)? That seems to render the
operator overloading pretty useless.

    Jas.

Re: Operator Overloading problem

Author: Paul Johns
Date: Thu, 17 Dec 1992 21:40

33 lines
1485 bytes

Any time you have a class which contains a pointer to 
dynamically-allocated memory (as your class does), you
WILL NEED an appropriate:

	Assignment operator
	Copy constructor
	Destructor

You will likely also need a default constructor.

Your symptoms are consistent with forgetting one
or both of the assignment operator or copy constructor,
depending on how the addition operator was used. 
(Forgetting the destructor results in a memory leak.)

If you forget any of these, you'll end up with the
kinds of problems you're seeing.

Note that these problems can be silent:  since the
compiler provides you with these functions if you
don't provide them, you won't get any sort of
compiler error.

What I think is happening in your case is that the
compiler supplied assignment operator or copy 
constructor is just doing a memberwise copy of
your object.  Your addition function is creating
the temporary and allocating memory for it.  The
pointer to this memory is copied from the temporary
to the object to which you're assigning/copying
rather than copying the memory.

// Paul Johns

Re: Operator Overloading problem

Author: John MAX Skaller
Date: Sat, 19 Dec 1992 18:26

41 lines
2213 bytes

In article <1992Dec16.192756.13271@dcs.warwick.ac.uk> jas@dcs.warwick.ac.uk (Jason Morris) writes:
>
>  I have a class, Matrix, and I want to overload the operators +,-,* etc.
>Matrix has a private member Data, which is a pointer to some dynamically
>allocated storage. Matrix() allocates this storage, and ~Matrix() frees it.
>Fine so far.

	No. No good. You MUST also have a copy constructor and assignment
operator that copy the contents of Data, and delete and copy its contents,
respectively.

	At least initially, forget about the inefficieny, fix that later.
This technique is called the 'Orthodox Idiom' by Coplien, it 
ensures the objects are first class 'just like int'. Coplien
provides the full recipe, follow it slavishly.

>
>    friend Matrix operator+(Matrix& lhs,Matrix& rhs);

	friend Matrix operator+(const Matrix& lhs, const Matrix& rhs);
	//                      ^^^^^
	//   if you want to pass it expressions
>
>  And I just do "return <local Matrix>"
>

	And now it works. To make it more efficient, there are
lots of methods, Coplien is filled with them. Some matrix 
packages go so far as to just build a 'parse tree' of the
calculation rather than doing it straight away, to optimise
speed and memory use [defered binding]

	However, you should be able to make your package
more efficient without having to rewrite existing working
client code (although you might have to recompile).

-- 
;----------------------------------------------------------------------
        JOHN (MAX) SKALLER,         maxtal@extro.ucc.su.oz.au
	Maxtal Pty Ltd, 6 MacKay St ASHFIELD, NSW 2131, AUSTRALIA
;--------------- SCIENTIFIC AND ENGINEERING SOFTWARE ------------------

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