not able to modify this pointer

Question

why I am not able to modify this pointer? getting compilation error "error: lvalue required as left operand of assignment". Following is the program

#include<iostream>
using namespace std;

class Test
{
private:
  int x;
public:
  Test(int x = 0) { this->x = x; }
  void change(Test *t) { this = t; }
  void print() { cout << "x = " << x << endl; }
};

int main()
{
  Test obj(5);
  Test *ptr = new Test (10);
  obj.change(ptr);
  obj.print();
  return 0;
}

Because you can't; `this` isn't an lvalue. What are you hoping to accomplish by doing it? — molbdnilo, Jun 05 '15 at 14:51
You can change value of the members (`this->x = t->x`) but not pointer to the object itself. — anxieux, Jun 05 '15 at 14:56
It does not make sense. This is the pointer to the object you are dealing with, changing it would be like 'i'm fed with this object, I'll pass to another one'. — marom, Jun 05 '15 at 14:57
ok , got it , this is part of standard , but is there any reasoning behind it? why cant we change value of this pointer? — pankaj kushwaha, Jun 05 '15 at 14:58
@pankajkushwaha It would just not make any sense. `this` *always* holds the address of the current object. You cannot change said address. Thus, you cannot change `this`. — Baum mit Augen, Jun 05 '15 at 14:59

score 4 · Accepted Answer · answered Jun 05 '15 at 15:03

4

You can't assign to this, you should think of it as conceptually constant pointer Test *const this.

What you actually need to do in change() is just copy the contents of t:

void change(Test *t) {this->x = t->x;}

If you want to be a good C++ citizen you could also make t constant:

void change(const Test *t) {this->x = t->x;}

answered Jun 05 '15 at 15:03

Sean

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thanks sean..i got it... – pankaj kushwaha Jun 05 '15 at 15:07

not able to modify this pointer

1 Answers1