Copy constructor not being invoked
 

Hi i have written a code for concatenation of two strings by overloading the string operator. The program works fine but i don't understand the sequence of steps happening.

[Use language = C++]

#include <iostream>

using namespace std;



class String

{

    int len;

    char *p;



    public:

        String ()

        {

            cout << "default constructor " << this << endl;

            len = 0;

            p = NULL;

        }

        String (const char *s)

        {

            cout << "String constructor called " << s << endl;

            len = strlen(s);

            p = new char[len + 1];

            strcpy(p, s);

        }

        String (const String &s)

        {

            cout << "Copy constructor invoked " << s.p << endl;;

            len = s.len;

            p = new char[len + 1];

            strcpy(p, s.p);

        }

        ~String ()

        {

             cout << "destructor invoked\n";

            delete p;

        }

        void display(void)

        {

            cout << p << "\t address of p = " << &p << "\tobject this = " <<  this << endl;

        }

        void operator = (const String &s)

        {

            cout << "assignment operator invoked\n";

            len = s.len;

            p = new char[len + 1];

            strcpy(p, s.p);

        }

        friend String operator + (const String &s, const String &t);

};



String operator + (const String &s, const String &t)

{

    String temp;



    temp.len = s.len + t.len;

    temp.p = new char[temp.len + 1];

    strcpy(temp.p, s.p);

    strcat(temp.p, t.p);



    cout << "temp = "; temp.display();

    return temp;

}



main()

{

    String s1("abcdefg"), s2("hijklmn ");



    String s3 = s2 + s1 ;



    cout << "S1 = "; s1.display();

    cout << "S2 = "; s2.display();

    cout << "S3 = "; s3.display();



    return 0;

}

[/Use]


The output of the above program is as follows.

String constructor called abcdefg
String constructor called hijklmn
default constructor 0xbff1e840
temp = hijklmn adcdefg address of p = 0xbff1e844 object this = 0xbff1e840
S1 = abcdefg address of p = 0xbff1e864 object this = 0xbff1e860
S2 = hijklmn address of p = 0xbff1e854 object this = 0xbff1e850
S3 = hijklmn abcdefg address of p = 0xbff1e844 object this = 0xbff1e840
destructor invoked
destructor invoked
destructor invoked

why is the copy constuctor not called for S3 and also why is the destructor not called for temp?

Re: Copy constructor not being invoked
 

The C++ standard explicitly allows compilers to eliminate temporary objects if the only way of detecting if those temporary objects exist is to track constructor and destructor calls.

A special case of this is the Return Value Optimisation (RVO), which your compiler is implementing. This optimisation basically means the compiler can avoid multiple copies if a variable within your function (eg temp within operator+()) will only be copied into a variable in the caller (eg S3 in main()).

Re: Copy constructor not being invoked
 

in the call to the operator+ you use [/code]

String temp;

[/code]
which invokes the default constructor.

Thanx,
Sean

Re: Copy constructor not being invoked
 

Oh...

Right the compiler can avoid making the copy on return. What he said. Sorry, waas a bit distracted there...

Sean