Re: why does this call the destructor?

From:
"Ben Schumeth" <bens@enfocus.be>
Newsgroups:
comp.lang.c++
Date:
Fri, 04 May 2007 09:42:15 +0200
Message-ID:
<op.trsk0pb8qquevh@g5ben>
On Fri, 04 May 2007 08:14:57 +0200, michael <spam@begone.net> wrote:

Hi All,

I have written the following to illustrate a problem.
I know I have some magic numbers etc please ignore them.
What I do not follow is why the line marked results in a call to the
destructor for the object.

Can someone please explain it for me?

#include <iostream>
#include <fstream>

using std::ostream;

class someClass {
    private:
    char *str;
    public:
    someClass();
    ~someClass();
    friend ostream& operator <<(ostream& lhs, someClass rhs);
};

someClass::someClass(){
    str = new char[10];
    strcpy(str, "something");
}

someClass::~someClass(){
    std::cout << "\nIn someClass destructor...\n";
    delete str;
}


use delete[]

ostream& operator <<(ostream& lhs, someClass rhs){
    lhs << rhs.str; // This results in a call to the destructor=

  =

for
the someClass object....why?
    return lhs;
}


The destructor is not called where you think it is, but when control =

reaches the end of the function. This is indeed the destructor of the =

copy, not of the original object. Your problem here however is that the=
  =

destructor deletes the memory pointed to by str. This is *the same =

memory* as in the original object. To create the copy, the compiler had=
  =

to create a copy constructor. However this copy constructor only copied=
  =

the pointer, not its contents. Conceptually, this is the same as if you=
  =

had written:

class someClass()
{
    // same as what you wrote, but with this addition:
    someClass( const someClass& rhs )
    {
       str = rhs.str; // only copies the pointer, not the memory it po=
ints =

to
    }
};

Solve this by adding a copy constructor and assignment operator. And of=
  =

course you should pass arguments by const reference here - although in =

this case that would have meant you wouldn't have found the problem with=
  =

your code.

Ben

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