The String Class

Hello. I have been trying to create a program to concatenate two strings using operator overloading for a class STRING. The output I get consists of random characters. Please tell me why this should happen.

Code:

---

/*
Program to create a class string, which has a provision to concatenate two strings.
*/
#include<iostream.h>
#include<conio.h>
#include<string.h>
#include<stdio.h>

void nullify(char *,int);

class STRING
{
        private:
                char *p;
                int len,allocated;
        public:
                STRING();
                STRING(char *);
                friend STRING operator+(STRING,STRING);
                void getstring();
                void showstring();
                ~STRING();
};

STRING::STRING()
{
        len=0;
        p=new char[1]; nullify(p,1);
        allocated=1;
}

STRING::STRING(char *str)
{
        if(allocated==1)
                delete p;
        len=strlen(str);
        p=new char[len+1]; nullify(p,len+1); allocated=1;
        strcpy(p,str);
}

STRING operator+(STRING s1,STRING s2)
{
        STRING sum;
        char *temp=new char[s1.len+s2.len+1]; nullify(temp,s1.len+s2.len+1);
        strcpy(temp,s1.p); strcat(temp,s2.p);
        sum=STRING(temp);
        delete temp;
        return(sum);
}

void STRING::getstring()
{
        if(allocated==1)
                delete p;
        p=new char[256]; nullify(p,256);
        gets(p);
}

void STRING::showstring()
{
        cout<<p;
}

STRING::~STRING()
{
        if(allocated==1)
                delete p;
}

int main()
{
        clrscr();
        STRING s1,s2,s3;
        cout<<"Enter the first string:"; s1.getstring();
        cout<<"Enter the second string:"; s2.getstring();
        s3=s1+s2;
        cout<<"The concatenated string is:"; s3.showstring();
        cout<<endl<<endl<<"PRESS A KEY..."; getch();
        return(0);
}

void nullify(char *str,int limit)
{
        int ctr;
        for(ctr=0; ctr<limit; ++ctr)
                str[ctr]=char(0);
}

---

Compiler : Borland C++ 5.5

You haven't defined operator= for this class. When you try to use it you get a shallow copy of the members. Since you return a local variable, the memory for it is released and s3's p points to released memory.

In other words, don't make it a friend function; make it a member function of STRING.

Also, make nullify a method of STRING.

You use the wrong delete.
new <-> delete
new[] <-> delete []

You also need a (user defined) copy-constructor.