Unexpected destructor call

[MirzaD]

Hello
Below you will find the problematic program. It is a string wrapper
class with a bare minimum of functionality to keep things simple.

**Code**

//StringClass.h

#include <iostream>
#include <cstring>

using namespace std;

class StringClass{
char* s;
int size;

public:
char* name;
StringClass();
StringClass(cha r* string);

~StringClass();

StringClass operator=(Strin gClass &string);
friend ostream &operator<<(ost ream &output, StringClass &string);
};

StringClass::St ringClass(){
s = new char[0];
size = 0;
}

StringClass::St ringClass(char* string){
size = strlen(string);
s = new char[size + 1];
strcpy(s,string );
}

StringClass::~S tringClass(){
cout << "Destructor : " << name << endl;
delete [] s;
}

StringClass StringClass::op erator=(StringC lass &string){
char* temp;
try{
temp = new char[string.size + 1];
}
catch (bad_alloc ex){
exit(1);
}
strcpy(temp,str ing.s);
delete [] s;
s = temp;
return *this;
}

ostream &operator<<(ost ream &out, StringClass &string){
out << string.s;
return out;
}


//StringClass.cpp

#include "StringClas s.h"

using namespace std;

int main(){
StringClass s("test"),t;
s.name = "s";
t.name = "t";
t = s;
cout << "s = " << s << endl << "t = " << t <<endl;
return 0;
}

**End code**

The above will display the following:
Destructor: t
s = test;
t = $*%
Destructor: t
Destructor: s

Why is the destructor called on t after the assignment takes place?
This would make sense to me if t was a pointer and the destructor was
called on the object it was pointing to (the last reference to the
object was just removed). Any help/clarification is appreciated.
Regards,

Mirza

[Victor Bazarov]

Re: Unexpected destructor call

MirzaD wrote:[color=blue]
> Below you will find the problematic program. It is a string wrapper
> class with a bare minimum of functionality to keep things simple.
>
> **Code**
>
> //StringClass.h
>
> #include <iostream>
> #include <cstring>
>
> using namespace std;
>
> class StringClass{
> char* s;
> int size;
>
> public:
> char* name;
> StringClass();
> StringClass(cha r* string);
>
> ~StringClass();
>
> StringClass operator=(Strin gClass &string);
> friend ostream &operator<<(ost ream &output, StringClass &string);
> };
> [..][/color]

The class violates the Rule of Three.

V

[Alf P. Steinbach]

Re: Unexpected destructor call

* MirzaD:[color=blue]
> Hello
> Below you will find the problematic program. It is a string wrapper
> class with a bare minimum of functionality to keep things simple.
>
> **Code**
>
> //StringClass.h
>
> #include <iostream>[/color]

Preferentially don't include <iostream> in a header file.
Use <iosfwd> if necessary (but I'd avoid even that, in general).
[color=blue]
> #include <cstring>
>
> using namespace std;[/color]

_Never_ put that in a header file.

[color=blue]
> class StringClass{
> char* s;
> int size;
>
> public:
> char* name;[/color]

Don't provide public access to your data members.

[color=blue]
> StringClass();
> StringClass(cha r* string);[/color]

Should be

StringClass( char const* string );

[color=blue]
>
> ~StringClass();[/color]

Look up the FAQ item on The Big Three: you're taking charge of copying,
and so you need a copy constructor.

[color=blue]
>
> StringClass operator=(Strin gClass &string);[/color]

Should be

StringClass& operator=( StringClass const& string );

[color=blue]
> friend ostream &operator<<(ost ream &output, StringClass &string);[/color]

Should be

friend ostream &operator<<(
ostream &output, StringClass const& string
);

[color=blue]
> };
>
> StringClass::St ringClass(){
> s = new char[0];
> size = 0;
> }[/color]

Since this is still in the header file, needs to be declared 'inline'.

[color=blue]
> StringClass::St ringClass(char* string){
> size = strlen(string);
> s = new char[size + 1];
> strcpy(s,string );
> }[/color]

Since this is still in the header file, needs to be declared 'inline'.

[color=blue]
> StringClass::~S tringClass(){
> cout << "Destructor : " << name << endl;
> delete [] s;
> }[/color]

Since this is still in the header file, needs to be declared 'inline'.

[color=blue]
> StringClass StringClass::op erator=(StringC lass &string){
> char* temp;
> try{
> temp = new char[string.size + 1];
> }
> catch (bad_alloc ex){
> exit(1);
> }[/color]

That's a bit draconian. Let the client code decide what to do with an
exception.

[color=blue]
> strcpy(temp,str ing.s);
> delete [] s;
> s = temp;
> return *this;
> }[/color]

Since this is still in the header file, needs to be declared 'inline'.

[color=blue]
> ostream &operator<<(ost ream &out, StringClass &string){
> out << string.s;
> return out;
> }[/color]

Since this is still in the header file, needs to be declared 'inline'.

[color=blue]
> //StringClass.cpp
>
> #include "StringClas s.h"
>
> using namespace std;
>
> int main(){
> StringClass s("test"),t;
> s.name = "s";
> t.name = "t";
> t = s;
> cout << "s = " << s << endl << "t = " << t <<endl;
> return 0;
> }
>
> **End code**
>
> The above will display the following:
> Destructor: t
> s = test;
> t = $*%
> Destructor: t
> Destructor: s
>
> Why is the destructor called on t after the assignment takes place?[/color]

Your operator= returns a copy, and you haven't defined a copy
constructor, so invoking the auto-generated copy constructor.

--
A: Because it messes up the order in which people normally read text.
Q: Why is it such a bad thing?
A: Top-posting.
Q: What is the most annoying thing on usenet and in e-mail?

[MirzaD]

Re: Unexpected destructor call

Adding the copy constructor did solve the problem. Thank you very
much.

Mirza

[Jay Nabonne]

Re: Unexpected destructor call

On Tue, 29 Nov 2005 12:03:15 -0800, MirzaD wrote:
[color=blue]
>
> StringClass::St ringClass(){
> s = new char[0];
> size = 0;
> }[/color]

Unrelated to your question, but this code plus the assignment operator
together cause a bug. The above code does not produce a zero-terminated
string.
[color=blue]
>
> StringClass StringClass::op erator=(StringC lass &string){
> char* temp;
> try{
> temp = new char[string.size + 1];
> }
> catch (bad_alloc ex){
> exit(1);
> }
> strcpy(temp,str ing.s);[/color]

The above line will fail if string is a default-constructed StringClass
object, since there will not be a terminating zero.
[color=blue]
> delete [] s;
> s = temp;
> return *this;
> }
>[/color]

- Jay

[Puppet_Sock]

Re: Unexpected destructor call

Alf P. Steinbach wrote:[color=blue]
> * MirzaD:[/color]
[snips][color=blue][color=green]
> > StringClass::~S tringClass(){
> > cout << "Destructor : " << name << endl;
> > delete [] s;
> > }[/color]
>
> Since this is still in the header file, needs to be declared 'inline'.[/color]

What problems does not putting 'inline' here cause?
Socks

[Victor Bazarov]

Re: Unexpected destructor call

Puppet_Sock wrote:[color=blue]
> Alf P. Steinbach wrote:
>[color=green]
>>* MirzaD:[/color]
>
> [snips]
>[color=green][color=darkred]
>>>StringClass: :~StringClass() {
>>> cout << "Destructor : " << name << endl;
>>> delete [] s;
>>>}[/color]
>>
>>Since this is still in the header file, needs to be declared 'inline'.[/color]
>
>
> What problems does not putting 'inline' here cause?[/color]

A violation of ODR if the header is included in more than one TU.

V