Some operators in Class String

Please tell me what's wrong with that code.
I have problems with "operator+=" and because of it with "operator+" also.
Besides, please take a look at "operator<<" and "operator>>" and correct me if i have wrote in wrong.
I also have a problem with "operator[]" when i use that operator in the left side. Why is that?
Thank you for your very usefull help.

Header file:

Code:

---

#ifndef STRING_H
#define STRING_H

#include <iostream>

using namespace std;


class String
{
        char* value;
private:
        virtual void show(ostream&)const;
        void CopyValue(const String& copied);
public:
        String(void);
        String(const char*);
        String(const String&);
        ~String(void);
        String& operator= (const String&);
        size_t length() const;
        //String& trim (void);
        char& operator[] (int);
        char operator[] (int) const;
        //String& operator+= (const String&);
        //friend String operator+ (const String&) const;
        friend istream& operator>> (istream&, const String&);
        friend ostream& operator<< (ostream&, const String&);
        friend bool operator== (const String&, const String&);
        friend bool operator!= (const String&, const String&);
};

#endif

---

Cpp file:

Code:

---

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

using namespace std;

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

String::String(const char* data): value(NULL)
{
        if (data)
        {
                value = new char [strlen(data) + 1];
                strcpy(value,data);
        }
        else
        {
                value = new char[1];
                value[0] = '\0';
        }
}

String::~String(void)
{
        delete [] value;
}

String::String(const String& copied): value(NULL)
{
        //value = new char [strlen(copied.value) + 1];
        //strcpy(value, copied.value);
        this -> CopyValue(copied);
}

String& String:: operator= (const String& copied)
{
        //delete [] value;
        //value = new char [strlen(copied.value) + 1];
        //strcpy(value, copied.value);
        if (&copied != this)
                this -> CopyValue(copied);
        return *this;
}

void String:: CopyValue(const String& copied)
{
        delete [] value;
        value = NULL;
        if (copied.value)
        {
                value = new char [strlen(copied.value) + 1];
                strcpy(value, copied.value);
        }
}

void String:: show(ostream& out)const
{
        if (value)
                out << value << endl;
}

bool operator== (const String& first, const String& second )
{
        if (strcmp(first.value, second.value) == 0)
                return true;
        return false;
}

bool operator!= (const String& first, const String& second)
{
        if (first == second)
                return false;
        return true;
}

ostream& operator<< (ostream& out, const String& obj)
{
        obj.show(out);
        return out;
}

istream& operator>> (istream& in, const String& obj)
{
        in >> obj.value;
        return in;
}

size_t String:: length() const
{
        return strlen(value) + (size_t)1;
}

/*String& String:: operator+= (const String& rhs)
{
        if (this != &rhs)
                strcat(value, rhs.value);
        return *this;
}

String operator+ (const String& rhs) const
{
        return String(*this) += rhs;
}*/

char& String:: operator[] (int place)
{
        return value [place];
}

char String:: operator[] (int place) const
{
        return value [place];
}

---

main:

Code:

---

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

using namespace std;

int main (void)
{
        String a;
        const String b("LINDA IS MY DOG");
        String c(b);
        cout << b;
        //b.show();  BEFORE I HAVE WROTE COUT
        cout << c;
        //c.show();  BEFORE I HAVE WROTE COUT
        cout << a;
        //a.show();  BEFORE I HAVE WROTE COUT
        if (a == b)
                cout << "A and B are the same" << endl;
        if (a != b)
                cout << "A and B are absolutly not the same" << endl;
        a = b;
        cout << a;
        //a.show();  BEFORE I HAVE WROTE COUT
        if (a == b)
                cout << "A and B are the same" << endl;
        if (a != b)
                cout << "A and B are absolutly not the same" << endl;
        cout << b.length() << endl;
        //b [6] = 'k';  ERROR!!
        //b += c;  ERROR!!
        cout << b;
        cout << b [6] << endl;
        String d("Meyers is my friend");
        cin >> d;
        cout << d;

        return 0;
}

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Well...just out of curiosity...why are you implementing yet another string class?

operator +=
----------------
think why you can't use 'strcat' the way you've
implemented operator +=
...

use the condition:
if(*this != rhs)
and not:
if(this != &rhs)

------------------------
change the prototype of the const version operator[] to :
const char& String:: operator[] (int place) const

This looks like a school assignment, otherwise std::string already does this work for you.

Another thing implement != in terms of ==.

Code:

---

bool operator== (const String& first, const String& second )
{
  if (strcmp(first.value, second.value) == 0)
    return true;
  return false;
}

bool operator!= (const String& first, const String& second)
{
  return !(first == second);
}

---

There is no need to write more code for !=.

Regards,

Paul McKenzie

Quote:

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Originally posted by Begins
/*String& String:: operator+= (const String& rhs)
{
if (this != &rhs)
strcat(value, rhs.value);
return *this;
}
[/CODE]

---

You're not allocating enough space to concatenate the string, so you're writing pass the array boundary.

The >> operator cannot take a const argument, you are modifiying it.