Thread: Need help with homemade String

I need help with this homemade String. I can't get it work. I am told not to do anything with the main.cpp, only String.cpp and String.h am I allowed to do what ever I want. This is what I have done so far.

////////////////////////////////////////////////////////////////
// String.h
#ifndef _STRING_H_
#define _STRING_H_

#include <iostream>

using std:stream;

class String
{
private:
char * str;
int len;
public:
String();
String(String const &s);
String(char const* s);
~String();
String &operator=(String const &s);
friend ostream &operator<<(ostream &o, String const &s);
String operator+(String const &s) const;
};

#endif

/////////////////////////////////////////////////////////
// String.cpp
#include <iostream>
#include "String.h"

String::String()
{
str = new char[1];
str[0] = '\0';
len = 1;
}

String::String(String const &s)
{
str = new char[0];
*this = s;
}

String::String(const char* s)
{
len = strlen(s);
str = new char[len + 1];
strcpy(str, s);
}

String & String:perator=(String const &s)
{
if (this != &s)
{
delete [] str;
str = new char[s.len + 1];
for (int i = 0; i <= s.len; i += 1)
str[i] = s.str[i];
len = s.len;
}
return *this;
}

String String:perator +(String const &s) const
{
char *temp = new char[len + s.len + 1];
for (int i = 0; i < len; i += 1)
temp[i] = str[i];
for (i = 0; i <= s.len; i += 1)
temp[i + len] = s.str[i];
String res(temp);
return res;
}

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

ostream &operator<<(ostream &o, String const &s)
{
o << s.str;
return o;
}

///////////////////////////////////////////////////////////////
// main.cpp
#include "String.h"
#include <iostream>

using std::cout;
using std::endl;

int main()
{
String t1("Donald");
String t2;
String t3(t1);
t2 = "Here comes " + t1; // I get an error message
t3 = t2 + " Duck";
cout << t1 << endl;
cout << t2 << endl;
cout << t3 << endl;

return 0;
}

// Error message in MS Visual C++ 6.0 sp5, Windows 2000
// binary '+': no global operator defined which takes
// type 'class String' for there is no acceptable conversion


Thanks.
Mikael Raja
[email protected]

you got to write a global function like this..

String& operator+(char * charstr,String& strOB){return strOB;}

should be a 'friend' if it accesses any of private members.

Ok that fixed the error but end result is.
//////////////////////////////////////////////
Donald
Donald
Donald Duck
/////////////////////////////////////////////

I want the end result to be
/////////////////////////////////////////////
Donald
Here comes Donald
Here comes Donald Duck
/////////////////////////////////////////////

How do I do that, because I don't seem to understand where things go wrong this time.

Thanks.
Mikael Raja
[email protected]

I gave you that as an example..

String& operator+(char * charstr,String& strOB)
{
// Please write some code here to add
// 'charstr' and 'strOB.str' and copy it
// to 'strOB.str' ....!!

return strOB;}

I think this will help you..

Ok now my project look like this

////////////////////////////////////////
// String.h

#ifndef _STRING_H_
#define _STRING_H_

#include <iostream>

using std:stream;

class String
{
private:
char * str;
int len;
public:
String();
String(String const &s);
String(char const* s);
~String();
String &operator=(String const &s);
friend ostream &operator<<(ostream &o, String const &s);
String operator+(String const &s) const;
friend String operator+(char * charstr, String& strOB);
};

#endif

////////////////////////////////////////////
// String.cpp

#include <iostream>
#include "String.h"
#include <cstring>

String::String()
{
str = new char[1];
str[0] = '\0';
len = 1;
}

String::String(String const &s)
{
str = new char[0];
*this = s;
}

String::String(const char* s)
{
len = strlen(s);
str = new char[len + 1];
strcpy(str, s);
}

String & String:perator=(String const &s)
{
if (this != &s)
{
delete [] str;
str = new char[s.len + 1];
for (int i = 0; i <= s.len; i += 1)
str[i] = s.str[i];
len = s.len;
}
return *this;
}

String String:perator +(String const &s) const
{
char *temp = new char[len + s.len + 1];
for (int i = 0; i < len; i += 1)
temp[i] = str[i];
for (i = 0; i <= s.len; i += 1)
temp[i + len] = s.str[i];
String res(temp);
return res;
}

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

ostream &operator<<(ostream &o, String const &s)
{
o << s.str;
return o;
}

String operator+(char * charstr, String& strOB)
{
int len = 1000;
char *temp = new char[len + strOB.len + 1];
for (int i = 0; i < len; i += 1)
temp[i] = charstr[i];
String test(temp);

return test;
}

//////////////////////////////////////////
// main.cpp

#include "String.h"
#include <iostream>

using std::cout;
using std::endl;

int main()
{
String t1("Donald");
String t2;
String t3(t1);
t2 = "Here comes " + t1;
t3 = t2 + " Duck";

cout << t1 << endl;
cout << t2 << endl;
cout << t3 << endl;

return 0;
}

////////////////////////////////////////////
// End result

Donald
Here comes
Here comes Duck

////////////////////////////////////////////
// The end result I want

Donald
Here comes Donald
Here comes Donald Duck

///////////////////////////////////////////////////

I must have missed something, but I can't see it.

Thanks
Mikael Raja
[email protected]

I think your current problem is with this line:

t2 = "Here comes " + t1;

The way I think this will be interpretted by the compiler is to add t1 to "Here comes " and then assign the result to t2. The problem is "Here comes " is a constant string literal and cannot be modified. Therefore I would try this:

t2 = "Here comes ";
t2 = t2 + t1;

#include<string.h>

String operator+(char *str,String strOB)
{
char * cptr=new char[strlen(str)+strlen(strOB.str)+1];
strcpy(cptr,str);
strcat(cptr,strOB.str);
return String(cptr);
}

Thanks. Problem solved.