CODEWITHSUNDEEP
c++reference
user997112
user997112

Reputation: 30615

Set reference to second variable

I was expecting the value of b to be 100 below, but I get 12. Why is this the case? Clearly my c = b line is not assigning c as an alias for b?

#include "stdafx.h"
#include <iostream>

using namespace std;

int _tmain(int argc, _TCHAR* argv[])
{
    int a = 4;
    int b = 12;
    int& c = a;

    c = 8;

    cout << a << endl;
    c = b;
    c = 100;

    cout << b << endl;


    int bb;
    cin >> bb;
    return 0;
}

Upvotes: 1

Views: 943

Answers (3)

Oliver Charlesworth
Oliver Charlesworth

Reputation: 272467

Clearly my c = b line is not assigning c as an alias for b?

Exactly, references cannot be reseated in C++. c will always be a reference to a.

c = b is just a straightforward assignment.

Upvotes: 7

leftaroundabout
leftaroundabout

Reputation: 120711

c = b in fact does the following:

  1. c is a reference to a, so whatever operation we request on c will actually work on a.
  2. We're assigning to c – that means, by 1., we must in fact assign to a.
  3. a is an lvalue of type int, so assigning b to it changes the value of a to whatever value b has at that moment. (For class-type objects, this would call the copy assignment operator.)
  4. b has the value 12, so we give this value to a.

And that's it. Nothing there changes anything about what c refers to: it was a previously, and it is a afterwards, only a now has a different value.

Apparently you want to reseat c so that it then refers to b. Well, you can't do that with C++ references, and IMO this is a good thing. But you can do it with pointers:

#include <iostream>

int main() {
    int a = 4;
    int b = 12;
    int* c = &a;

    *c = 8;

    std::cout << a << std::endl;
    c = &b;
    *c = 100;

    std::cout << b << std::endl;

    return 0;
}

will output what you expected from your program.

Upvotes: 2

Stuart Golodetz
Stuart Golodetz

Reputation: 20616

You can't reseat references - your c = b; line just assigns to c (and thereby also to a) the current value of b.

When you then assign 100 to c, b therefore doesn't change, so it still has its original value of 12 when you print it out.

See this FAQ question:

http://www.parashift.com/c++-faq/reseating-refs.html

Upvotes: 6

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