Can 'this' be assigned a pointer to it?

[fl]

Hi,

I read C++ for some time, but I still has problem in understand it.
Below is from web tutorial. It requires to debug it, but I cannot figure out
what is wrong with the void change function.

The error message says that this must be a lvalue. Does that mean 'this'
cannot be assigned a value? Then the change function is invalid?
What it wants me to do?

Please at least give a little hint on it.


thanks,






#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;
}

[Norman J. Goldstein]

There are a couple of things, here, that are confusing:
1. Overusing the variable name, 'x'. Change the declaration
of Test to be Test( int x0 = 0 );
2. The rules don't allow you to assign to "this". However, you
can pass around &obj and ptr inside the main function to change
which Test object you are working with.

[Robert Wessel]

On Tue, 10 Mar 2015 22:29:48 -0700 (PDT), fl <rxj...@gmail.com>
wrote:

"9.3.2 The this pointer
In the body of a nonstatic (9.3) member function, the keyword this is
a nonlvalue expression whose value is the address of the object for
which the function is called."

this is not an lvalue, IOW, you can't modify it.

[JiiPee]

a good answer, +1 :)

[Lőrinczy Zsigmond]

This code wouldn't do any good, even if worked:

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

But this version would do something mode-or-less useful:
void change(Test *t) { *this = *t; }

[Martijn Lievaart]

On Tue, 10 Mar 2015 22:29:48 -0700, fl wrote:

> Hi,
>
> I read C++ for some time, but I still has problem in understand it.
> Below is from web tutorial. It requires to debug it, but I cannot figure
> out what is wrong with the void change function.
>

Others pointed out the problem itself, just wanted to point out that a
tutorial that makes such basic errors should be ditched immediately.

M4

[Melzzzzz]

`this` is just syntactic sugar for first parameter of function.
say func(Test* this) ....
Even if you assign value to it nothing will change.

[Haddock]

Assigning a different reference to the this pointer is like, say, assigning 5 to 3. Then everywhere in your system every 5 is now a 3. This is something that shouldn't be done. Needless to say that the application would also shut down immediately if every 5 would be a 3 from now on. I don't know whether this sample is meaningful, but I think you know what I mean.

[Greg Martin]

The tutorial was suggesting the user should debug the program so it
would make sense there should be an error for the reader to fix.

--
http://www.softsprocket.com

[Martijn Lievaart]

On Wed, 11 Mar 2015 16:48:51 +0000, Greg Martin wrote:

> The tutorial was suggesting the user should debug the program so it
> would make sense there should be an error for the reader to fix.

Fair enough, might have the subgoal of reiterating that this is const.

M4

[Öö Tiib]

The keyword 'this' is actually a reference with pointer semantics (it can't
be modified and it can't be 'nullptr'). The sole reason why it is not
a keyword returning reference was that the C++ language did not have
references when the 'this' keyword was added.