Re: Anyone could help me with this memeory problem...

[Larry I Smith]

Zhou Lei wrote:
> I'm a newbie to use C++, and I wrote a little program it can create a
> binary tree. But when the copy constructor or the assignment (overloaded
> one) occurs, the program doesn't run as I expect. The copy construtor is
> like this:
>
> ptree::ptree(const ptree& pt){
>       weight=pt.weight;
>       c=pt.c;
>       lchild=pt.lchild->ntree(); // Copy left sub tree recursively using
>                                     // ntree()
>       rchild=pt.rchild->ntree(); // Copy right sub tree.
> }
>
> That function invokes this function to complete the copy tree jobs
> recursively.
>
> ptree* ptree::ntree() const{
>       ptree* p;
>
>       if (!this)
>            return 0;
>
>       p = new ptree;
>       p->c = c;
>       p->weight = weight;
>       p->lchild = lchild->ntree(); // Copy the left sub tree recursively.
>       p->rchild = rchild->ntree(); // Copy right sub tree.
>       return p;
> }
>
> On my computer the tree was not built correctly, and I traced into the
> functions and I found that the "segment fault" occurs when I step into
> "new" keyword (functions of system library may be invoked). My machine
> is running Debian Linux (kernel 2.4.18) and the program is compiled with
> g++ (version 3.4). But everything's okay on a UNIX machine (Solaris on
> SPARC). I'm not sure whether it's a bug or not, and the source code is
> attached, could anyone give me some suggestions?


You don't check for NULL lchild/rchild in ntree().  That could be a
problem.  Add the checks, something like this:

  p->lchild = (lchild ? lchild->ntree() : 0);
  p->rchild = (rchild ? rchild->ntree() : 0);

Also, what if 'p = new ptree;' fails?  You're not catching the
exception.  Since about 1994 the C++ Std has said that 'new'
throws a 'std:bad_alloc' exception on error; 'new' does not return
NULL on error UNLESS the 'nothrow' syntax is used:

  #include <new>
    .
    .
    .
  p = new (nothrow) ptree;

Ref:

  http://www.informit.com/guides/content.asp?g=cplusplus&seqNum=37

Regards,
Larry

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