Assignment Operator for Char String

Question

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

class String 
{
    private:
        enum { SZ=80 }; 
        char str[SZ]; 
    public:
        String(){
            strcpy(str, "");
        }
        String (const char s[]){
            strcpy(str,s);
        }
        String operator = (String obj){
            String newObj;
            strcpy(newObj.str,obj.str);
            return newObj;
        }
        void display(){
            cout << str;
        }

};
int main()
{
    String s1("ABC"); 
    String s3;
    s3 = s1;
    s3.display();
return 0;
}

I'm trying to copy one Char string Object to second one using the above code (assignment operator) operator=, Why it's not working? I tried my best but still failed.

Why do you think you "failed"? What happened? (I guess you need a copy constructor?) Also are you aware that `strcpy` is massively unsafe? Why are you using 1800s technology? — Lightness Races in Orbit, May 30 '15 at 19:30
I'm student of SE and reading about this... can you provide me detail how it's unsafe? and about this question. s3 is not showing anything.. therefore I'm thinking i failed. — Asif Mushtaq, May 30 '15 at 19:32
@LightnessRacesinOrbit I would be thankful if you can provide me latest method's c++ book to learn. — Asif Mushtaq, May 30 '15 at 19:33
Unless this is homework that prohibits it, start with `#include ` and then use `std::string`. — Jerry Coffin, May 30 '15 at 19:46
Got the solution..!! I should use this for operator definition void operator = (String obj){ strcpy(str,obj.str); } Because in assignment operator one is calling object second one as parameter..!! — Asif Mushtaq, May 30 '15 at 19:51

score 0 · Answer 1 · answered May 31 '15 at 03:05

Your assignment operator is not assigning to the String itself, but to a temporary var. This would be the correct way to do it:

String& operator = (const String& obj) {
   strcpy(str, obj.str);  // in this line, obj is copied to THIS object
   return *this;          // and a reference to THIS object is returned
}

The assignment operator always has to change the object itself, to which something shall be assigned. Typically also a reference to the object itself is returned.

There are some other problems in your code:

strcpy() should not be used, because it has no range check (size of buffer) and though is unsafe.

And when using strings as a parameter (or any bigger objects like classes with a lot of members, vectors etc), you should use const references like in my example (the "const" and "&"). This is safer (because you can not accidently change the content of the parameter) and MUCH faster, since only a (very small) reference has to be copied, and not the whole content of the parameter.

Assignment Operator for Char String

1 Answers1