Operator Overloading (+) - problem in assignment

[Sourabh Verma]

Here's my code:

#include <iostream.h>#include <conio.h>
#include <string.h>


class string {
    char *str;
    int len;
public:
    string(){}
    string(char *a) {
        len = strlen(a);
        str = new char[len+1];
        strcpy(str, a);
    }
    string (string &a) {
        len = a.len;
        str = new char[len+1];
        strcpy(str, a.str);
    }
    ~string()
    { delete str; }


    string operator+(string);


    void display()
    { cout << str; }
};


int main() {
    string s1("Hello, "), s2("how are you?");
    clrscr();
    cout << "\n\nName: Sourabh Verma | Roll Number: 2K11-MRCE-CSE-116\n\n";
    string s3 = s1 + s2; // Use Reference Here !!!!!
    return 0;
}


string string :: operator+(string a) {
    string temp;
    temp.len = a.len + len + 1;
    strcpy(temp.str, str);
    strcat(temp.str, a.str);
    return temp;
}

I get this runtime error: "Null Pointer Assignmenw"
Notice the 'w' at the end ... weird, I know !!

How can I do the assigning at line marked with comment with reference ?

[Elysia]

You are using an ancient compiler. First, you have to update. Get Visual Studio, gcc or clang.
This is to make sure you can written standards compliant code.

[whiteflags]

I would create another constructor that can make a string of specific length. Then you might succeed.

class string {
   public:
      friend string operator + (const string &a, const string &b);
      // whatever else
};

string operator + (const string &a, const string &b) {
   string temp(a.length + b.length + 1);
   strcpy(temp.str, a.str);
   strcat(temp.str, b.str);
   return temp;
}

[Sourabh Verma]

You are using an ancient compiler. First, you have to update. Get Visual Studio, gcc or clang.
This is to make sure you can written standards compliant code.

I know I am using an outdated complier but my school wants the c++ code in that outdated form only. So i have to work on it. I do learn new standards simultaneously too.

[hk_mp5kpdw]

string string :: operator+(string a) {
    string temp;
    temp.len = a.len + len + 1;
    strcpy(temp.str, str);
    strcat(temp.str, a.str);
    return temp;
}

When temp is created, it uses the default constructor which does nothing according to your code - I would at the very least like to see the len data member initialized to 0 and the pointer initialized to null. That aside, please note that temp's str pointer is uninitialized and contains a random value (address). Then you attempt to strcpy/strcat to that uninitialized/random pointer. I hope you understand that this is bad, writing to a random address in memory.