DaniWeb Forum Index > Software Development > C++

C++ RSS

Constrcutors and Destructors

Please support our C++ advertiser: Intel Parallel Studio Home

Thread Solved

Last »

•

Join Date: May 2005

Posts: 86

Reputation: vicky_dev is an unknown quantity at this point

Solved Threads: 2

vicky_dev

Offline

Junior Poster in Training

Constrcutors and Destructors

Jul 18th, 2006

Hi...

I am trying to write a string class...this part is supposed to concatenate two strings, but I am having trouble with constructors/destructors.:rolleyes:

Please explain the exact sequence of calls and the correct way to code.

 Help with Code Tags 
 C++ Syntax (Toggle Plain Text) 
 
#include <iostream>
#include <cstring>
#include <conio.h>
using namespace std;
 
class STRING
{
      char *str;
      int len;
 
 public:
 
       STRING( ) {   cout<<"*** Default Constructor called. ***"<<endl;   str = NULL, len = 0; }
       STRING( const char *s );
       STRING( const STRING& );
       ~STRING( );
 
       friend ostream& operator<<( ostream &os, STRING  s );
       STRING operator+( STRING &s );
};
 
 
STRING::STRING( const char *s )
{
     cout<<"*** String constructor called. ***\n";
     len = strlen( s );
     str = new char[len+1];
     strcpy( str, s );
}
 
STRING::STRING( const STRING& s )
{
     cout<<"*** Copy constructor called. ***\n";
     len = s.len;
     str = new char[len+1];
     strcpy( str, s.str );
}
 
STRING::~STRING( )
{
     cout<<"*** Destuctor called. ***"<<endl;
     delete[] str;
     len = 0;
}
 
ostream& operator<<( ostream &os, STRING s )
{
 	if( s.len == 0 )
 	    cout<<"(Empty)\n";
	else
    	os<<s.str<<endl;
    return os;
}
 
STRING STRING::operator+( STRING &s )
{
    STRING res;
 
    res.len = len + s.len;
    res.str = new char[res.len+1];
 
    strcpy( res.str, str );
    strcat( res.str, s.str );
 
    return res;
}
 
int main( )
{
    STRING s1("String1");
    STRING s2("String2");
    STRING s3;
 
    s3 = s1+s2;
 
    cout<<s1<<s2<<s3;
 
    getch( );
    return 0;
}
 
#include <iostream>
#include <cstring>
#include <conio.h>
using namespace std;

class STRING
{
      char *str;
      int len;
      
 public:

       STRING( ) {   cout<<"*** Default Constructor called. ***"<<endl;   str = NULL, len = 0; }
       STRING( const char *s );
       STRING( const STRING& );
       ~STRING( );

       friend ostream& operator<<( ostream &os, STRING  s );
       STRING operator+( STRING &s );
};


STRING::STRING( const char *s )
{
     cout<<"*** String constructor called. ***\n";
     len = strlen( s );
     str = new char[len+1];
     strcpy( str, s );
}

STRING::STRING( const STRING& s )
{
     cout<<"*** Copy constructor called. ***\n";
     len = s.len;
     str = new char[len+1];
     strcpy( str, s.str );
}

STRING::~STRING( )
{
     cout<<"*** Destuctor called. ***"<<endl;
     delete[] str;
     len = 0;
}

ostream& operator<<( ostream &os, STRING s )
{
 	if( s.len == 0 )
 	    cout<<"(Empty)\n";
	else
    	os<<s.str<<endl;
    return os;
}

STRING STRING::operator+( STRING &s )
{
    STRING res;
    
    res.len = len + s.len;
    res.str = new char[res.len+1];
    
    strcpy( res.str, str );
    strcat( res.str, s.str );
    
    return res;
}

int main( )
{
    STRING s1("String1");
    STRING s2("String2");
    STRING s3;
    
    s3 = s1+s2;
    
    cout<<s1<<s2<<s3;
    
    getch( );
    return 0;
}

•

Join Date: Jul 2005

Posts: 1,688

Reputation: Lerner is a name known to all

Solved Threads: 265

Lerner

Offline

Posting Virtuoso

Re: Constrcutors and Destructors

Jul 18th, 2006

Here's what I think happens.

 Help with Code Tags 
 C++ Syntax (Toggle Plain Text) 
 
STRING s1("String1");  //string constructor called
STRING s2("String2");  //string constructor called
STRING s3;                 //default constructor called
 
    s3 = s1+s2;           //default constructor called within the + operator
                                //copy constructor called by compiler to create temporary object from res returned from + operator
                                //destructor called to remove res
                                //assignment operator called using default assignment constructor provided by compiler
                                //destructor called to remove temporary object
 
    cout<<s1<<s2<<s3;
 
    getch( );
    return 0;    //destructor called to destroy s1, s2, and s3.
STRING s1("String1");  //string constructor called
STRING s2("String2");  //string constructor called
STRING s3;                 //default constructor called
    
    s3 = s1+s2;           //default constructor called within the + operator
                                //copy constructor called by compiler to create temporary object from res returned from + operator
                                //destructor called to remove res
                                //assignment operator called using default assignment constructor provided by compiler
                                //destructor called to remove temporary object
    
    cout<<s1<<s2<<s3;
    
    getch( );
    return 0;    //destructor called to destroy s1, s2, and s3.

•

Join Date: May 2005

Posts: 86

Reputation: vicky_dev is an unknown quantity at this point

Solved Threads: 2

vicky_dev

Offline

Junior Poster in Training

Re: Constrcutors and Destructors

-1

Jul 18th, 2006

If that is how it happens, how come I dont get correct answer for s3 ( the result is shown blank, it must be String1String2 )

•

Join Date: Aug 2005

Posts: 5,266

Reputation: iamthwee is a splendid one to behold

Solved Threads: 377

iamthwee

Offline

Posting Expert

Re: Constrcutors and Destructors

Jul 19th, 2006

Hi I just made one now, this appears to work although it's more of a trial and error thing.

I've highlighted the part that may be suspect in blue. Someone who knows what they're doing can tell you if my coding is logical and safe.

Help with Code Tags

(Toggle Plain Text)

#include <iostream>
#include <iomanip>
#include <cstring>


class String 
{
private:
       char *mem;
       int size;
public:
       String ( const char *p );
       ~String();

  
  friend std::ostream& operator<< ( std::ostream& os, const String& s );
  String operator+(String); //Overloaded + operator
};

String::String ( const char *p )
{
  size = strlen ( p );
  mem = new char[size + 1];
  strcpy ( mem, p );
}

String::~String() 
{ 
    delete [] mem; 
}



std::ostream& operator<< ( std::ostream& os, const String& s )
{
  return os << s.mem;
}

String String::operator+(String c) 
{ 
    char *crap;
       crap = new char[size + c.size+1];
    
    strcpy(crap,mem);
     
    strcat(crap,c.mem);
    String temp = crap;
    
    return temp;
}

int main()
{
  String a = "My cool string";
  String b = " is so crappy";
  String c = " ya know";
  
  String d = a  + b + c;
  
  std::cout << d << '\n';
  
  
  std::cin.get();
  return 0;
  
}

*Voted best profile in the world*

•

Join Date: May 2005

Posts: 86

Reputation: vicky_dev is an unknown quantity at this point

Solved Threads: 2

vicky_dev

Offline

Junior Poster in Training

Re: Constrcutors and Destructors

Jul 19th, 2006

I dont think the code will work if you write it as: ( in main )

Help with Code Tags

(Toggle Plain Text)

String a = "My cool string";
  String b = " is so crappy";
  String c = " ya know";

String d; 

d = a + b + c ;

This makes all the difference.

Last edited by vicky_dev; Jul 19th, 2006 at 11:08 am.

•

Join Date: Aug 2005

Posts: 5,266

Reputation: iamthwee is a splendid one to behold

Solved Threads: 377

iamthwee

Offline

Posting Expert

Re: Constrcutors and Destructors

Jul 19th, 2006

•

Originally Posted by vicky_dev

I dont think the code will work if you write it as: ( in main )
Help with Code Tags

(Toggle Plain Text)
String a = "My cool string";
  String b = " is so crappy";
  String c = " ya know";

String d; 

d = a + b + c ;    
String a = "My cool string"; String b = " is so crappy"; String c = " ya know"; String d; d = a + b + c ;
This makes all the difference.

Couldn't you just overload the equals operator then? I don't know, it's got to be something simple right?

Last edited by iamthwee; Jul 19th, 2006 at 11:32 am.

*Voted best profile in the world*

•

Join Date: Jul 2005

Posts: 1,688

Reputation: Lerner is a name known to all

Solved Threads: 265

Lerner

Offline

Posting Virtuoso

Re: Constrcutors and Destructors

Jul 19th, 2006

Run time debugging is cumbersome. Here's how I do it; lots of output statements with pauses to see that status is as I want it to be. I suspect everybody does it differently.

 Help with Code Tags 
 C++ Syntax (Toggle Plain Text) 
#include <iostream>
#include <cstring>
#include <conio.h>
using namespace std;
 
class STRING
{
      char *str;
      int len;
 
 public:
 
       STRING( ) {   cout<<"*** Default Constructor called. ***"<<endl;   str = NULL, len = 0; }
       STRING( const char *s );
       STRING( const STRING& );
       ~STRING( );
 
       friend ostream& operator<<( ostream &os, const STRING &  s );  //slight change to syntax here
       STRING operator+( STRING &s );
};
 
 
STRING::STRING( const char *s )
{
     cout<<"*** String constructor called. ***\n";
     len = strlen( s );
     str = new char[len+1];
     strcpy( str, s );
}
 
STRING::STRING( const STRING& s )
{
     cout<<"*** Copy constructor called. ***\n";
     len = s.len;
     str = new char[len+1];
     strcpy( str, s.str );
}
 
STRING::~STRING( )
{
     cout<<"*** Destuctor called. ***"<<endl;
     delete[] str;
     len = 0;
}
 
ostream& operator<<( ostream &os, const STRING & s ) //slight change here
{
 	if( s.len == 0 )
 	    os<<"(Empty)";  //slight change here
	else
    	   os<<s.str;           //slight change here
    return os;
}
 
STRING STRING::operator+( STRING &s )
{
    STRING res;
    char ch;
 
    cout << "concatenation opereator called" << endl;
 
    res.len = len + s.len;
    res.str = new char[res.len+1];
 
    strcpy( res.str, str );
 
    cout << "res.str now is" << res.str << endl;
    cin >> ch;
 
    strcat( res.str, s.str );
 
    cout << "res.str now is " << res.str << endl;
    cin >> ch;
 
    cout << "end concatenation operator" << endl;
 
    return res;
}
 
int main( )
{
    STRING s1("String1");
    STRING s2("String2");
    STRING s3;
 
    //confirm expected values
    cout << s1 << endl;
    cout << s2 << endl;
    cout << s3 << endl;
 
    s3 = s1+s2;
 
    //determine if assignment operator worked.
    cout << s3;
    cout << endl;
 
    cout << s1 << endl << s2 << endl << s3;  //slight change in syntax
 
    getch( );
    return 0;
}
#include <iostream>
#include <cstring>
#include <conio.h>
using namespace std;

class STRING
{
      char *str;
      int len;
      
 public:

       STRING( ) {   cout<<"*** Default Constructor called. ***"<<endl;   str = NULL, len = 0; }
       STRING( const char *s );
       STRING( const STRING& );
       ~STRING( );

       friend ostream& operator<<( ostream &os, const STRING &  s );  //slight change to syntax here
       STRING operator+( STRING &s );
};


STRING::STRING( const char *s )
{
     cout<<"*** String constructor called. ***\n";
     len = strlen( s );
     str = new char[len+1];
     strcpy( str, s );
}

STRING::STRING( const STRING& s )
{
     cout<<"*** Copy constructor called. ***\n";
     len = s.len;
     str = new char[len+1];
     strcpy( str, s.str );
}

STRING::~STRING( )
{
     cout<<"*** Destuctor called. ***"<<endl;
     delete[] str;
     len = 0;
}

ostream& operator<<( ostream &os, const STRING & s ) //slight change here
{
 	if( s.len == 0 )
 	    os<<"(Empty)";  //slight change here
	else
    	   os<<s.str;           //slight change here
    return os;
}

STRING STRING::operator+( STRING &s )
{
    STRING res;
    char ch;

    cout << "concatenation opereator called" << endl;

    res.len = len + s.len;
    res.str = new char[res.len+1];
    
    strcpy( res.str, str );

    cout << "res.str now is" << res.str << endl;
    cin >> ch;

    strcat( res.str, s.str );
    
    cout << "res.str now is " << res.str << endl;
    cin >> ch;

    cout << "end concatenation operator" << endl;

    return res;
}

int main( )
{
    STRING s1("String1");
    STRING s2("String2");
    STRING s3;
    
    //confirm expected values
    cout << s1 << endl;
    cout << s2 << endl;
    cout << s3 << endl;
    
    s3 = s1+s2;
    
    //determine if assignment operator worked.
    cout << s3;
    cout << endl;
    
    cout << s1 << endl << s2 << endl << s3;  //slight change in syntax

    getch( );
    return 0;
}

•

Join Date: May 2005

Posts: 86

Reputation: vicky_dev is an unknown quantity at this point

Solved Threads: 2

vicky_dev

Offline

Junior Poster in Training

Re: Constrcutors and Destructors

Jul 19th, 2006

•

Originally Posted by iamthwee

Couldn't you just overload the equals operator then? I don't know, it's got to be something simple right?

Overloading the = operator is same as writing a copy constructor, the same thing will be called when

s3 = s1 + s2;

is executed.

•

Join Date: Jul 2005

Posts: 1,688

Reputation: Lerner is a name known to all

Solved Threads: 265

Lerner

Offline

Posting Virtuoso

Re: Constrcutors and Destructors

Jul 19th, 2006

•

Overloading the = operator is same as writing a copy constructor

The implementation of copy constructors and assignment operators is frequently similar, but they are not the same.

•

Join Date: Aug 2005

Posts: 5,266

Reputation: iamthwee is a splendid one to behold

Solved Threads: 377

iamthwee

Offline

Posting Expert

Re: Constrcutors and Destructors

#10

Jul 19th, 2006

•

Originally Posted by vicky_dev

Overloading the = operator is same as writing a copy constructor, the same thing will be called when

s3 = s1 + s2;

is executed.

hmm, yeah I was just taking a stab at what might work. Probably overloading the equals operator won't make much difference if any?

Question: Has lerner's program helped in bringing you closer to a solution that you want?

To change :

Help with Code Tags

C++ Syntax (Toggle Plain Text)