NetNews Usenet Archive 1992 #31/NN_1992_31.iso/spool/comp/lang/cplus/18369

Text File | 1992-12-23 | 2.9 KB | 75 lines

Newsgroups: comp.lang.c++
Path: sparky!uunet!microsoft!wingnut!pauljo
From: [e-mail] (Paul Johns)
Subject: Re: Can't I initialize a dynamically allocated object in its constructor?
Message-ID: <[e-mail]>
Date: 23 Dec 92 20:27:51 GMT
Organization: Microsoft Corp.:  Redmond, Washington, USA
References: <[e-mail]> <[e-mail]>
Lines: 64

I forgot to change the distribution to world in my last post, so it
never got to the person who was asking.  Mea culpa.

I'm enclosing it verbatim below.  Mea culpa for the repeat.

I like the points the others have made, too!

// Paul Johns

In article <[e-mail]> [e-mail] wrote:
> You've made common beginning mistake #1:  not defining
> your own copy constructor when one is needed.
>
> Your code WILL NOT WORK correctly until you provide a proper
> copy constructor.  The fact that it seems to work in one case
> is not the whole story:  it is actually failing in subtle
> ways you're not looking at.  And the initialization you ask
> about is not the issue here.
>
> What's going on is that since you didn't provide a copy
> constructor, the compiler provides one for you which simply
> copies the members of your class one member at a time.
>
> But the data in the vector doesn't get copied--only the pointer.
> And you end up deleting data twice since two different objects
> point to it.
>
> This is really bad--it tends to mess up the heap something awful.
>
> There is at least one C++ book whose author tells nothing
> of copy constructors.  I hope you didn't use that one!!!
>
> Good texts (Lippman, Stroustrup, various Microsoft Press books,
> etc.) do a good job of explaining copy constructors and why
> they're needed.  Check out the dynamic string classes, which
> are very similar to your vector class.
>
> If your class needs an assignment operator, it will almost
> certainly need a copy constructor and destructor as well.
>
> Two other points I noticed in your code:
>
> 1.  You assignment operator doesn't check for doing "a = a;"  Also,
>     it DOES make sense to assign vectors of different sizes,
>     you may want to allow that.  Check how a good dynamic
>     string class works for ideas.
>
> 2.  operator + is usually implemented as a file-scope (global)
>     function so that both operands can be converted, not
>     just the RHS operand.  In your particular case, you
>     don't have any useful (meaning you'd want to write
>     something like "vecx = 5 + vecy;") conversions, so
>     this isn't an issue.  You may want to rethink the
>     int constructor since it provides an implicit conversion
>     from int to vector, meaning that an int or any expression
>     that can be converted with a standard conversion to an
>     int can be used any place a vector can.  (So "vec2 + 5"
>     **IS** legal for your class, although meaningless.)
>
> Good luck!
>
> // Paul Johns