C++ : program to concanate two string

#include<iostream>
#include<string.h>
#include<conio.h>
using namespace std;

class String
{
private:
    int length;
    char *str;
public:
    String()
    {
        length=0;
        str= new char[length+1];
    }
    String(char *s)
    {

        length=strlen(s);
        str=new char[length+1];
        strcpy(str,s);
    }
    friend String operator +(String &s1, String &s2);
    friend void show(String &s);
};

void show(String &s)
{
    cout<<s.str<<endl;
}

String operator +(String &s1, String &s2)
{
    String temp;
    temp.length=s1.length+s2.length;
    delete temp.str;
    temp.str=new char[temp.length+1];
    strcpy(temp.str,s1.str);
    strcat(temp.str,s2.str);
    return temp;
}

int main()
{
    String s1("Ram"), s2("Kumar");
    String s3;
    s3=s1+s2;
    cout<<"s3 is:";
    show(s3);
    getch();
    return 0;

}