CODEWITHSUNDEEP
c++reference
ipkiss
ipkiss

Reputation: 13651

Return the current object (*this) in C++?

I have the following code:

Code 1

class Student {
     int no;
     char grade[M+1];
 public:
     Student() {
         no = 0;
         grade[0] = '\0';
     }
     void set(int n, const char* g) {
         no = n;
         strcpy(grade, g);

     }
     const Student getObject() {
         return *this;
     }
     void display() const {
         cout << no << ", " << grade << endl;
     }
 };

Code 2:

// no change from code 1
const Student& getObject() {
         return *this;
     }
// no change from code 1

As the book I am reading explains the difference in the getObject() of the code 1 and 2 is that the getObject() of code 2 returns a reference to the current object, instead of a copy (for efficiency reasons).

However, I have tested (code 2) as follows:

Tested code:

Student harry, harry1;
    harry.set(123, "ABCD");

    harry1 = harry.getObject();
    harry1.set(1111,"MMMMMM");
    harry.display(); // Line 1 => displayed: 123, ABCD
    harry1.display(); / Line 2 => displayed: 1111, MMMMMM

I dont get it. If the getObject() returns a reference, then Line 1 in the tested code should also display 111, MMMMMM? Because I thought that harry1 should contain the address of harry object??? Or am I misunderstanding something?

Upvotes: 3

Views: 12520

Answers (5)

Lightness Races in Orbit
Lightness Races in Orbit

Reputation: 385224

Although harry.getObject() is a reference to the original object, you then ruin it with the assignment:

harry1 = harry.getObject();

which performs a copy.

Instead:

Student const& harry1 = harry.getObject();

Upvotes: 6

Bruce
Bruce

Reputation: 7132

Common advise : define a copy constructor, private and not implemented:

private:
  Student(Student& xOther);

This way, if you happen to have unwanted copy, compiler will detect them (your harry1 is actually a copy). And if you expect to have copies, you can properly control how they are done, thus avoiding to mess with potential pointers or encapsulated objects.

Upvotes: 1

Simon Steele
Simon Steele

Reputation: 11608

harry1 in your test code is an instance of Student, not a reference. Change it to this:

Student harry;
harry.set(123, "ABCD");

Student& harry1 = harry.getObject();
harry1.set(1111,"MMMMMM");
harry.display(); // Line 1 => displayed: 123, ABCD
harry1.display(); / Line 2 => displayed: 1111, MMMMMM

Upvotes: 1

user2100815
user2100815

Reputation:

You are using actual objects, not pointers or references. When you say:

harry1 = harry.getObject()

the contents of harry are copied by the default assignment operator into harry1.

Upvotes: 2

Alexander Gessler
Alexander Gessler

Reputation: 46627

You are assigning the reference returned by getObject to a Student, so it gets copied just a few steps later as in Code 1 (where it would be copied directly upon returning from getObject). Use 

Student& harry1 = harry.GetObject();

to obtain a reference to harry. Note that you must declare and initialize references in one step.

Upvotes: 4

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