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.