Operator "&"

[iceman1992]

I have a question about the "&" operator.
I know that it's an address operator, that &var1 gives the address of var1.
My question is what is the use of "&" after the type name, in a declaration?
I was reading tutorials about SFML and came across this line of code
const sf::Input& Input = App.GetInput();
And I don't know what the "&" does.

[Texan40]

It is declaring a const reference to an instance of sf::Input and assigning it whatever value is returned from App.GetInput(). More simplistic:

class foo
{
  private:
    int x;
  public:
    foo(int i) : x(i) {}
    int getx() { return x; }
};

foo myfoo(12);
const int &myfoox = myfoo.getx();

[zb0S216C]

When an ampersand (&) is used in a declaration, just like Input, it means you're declaring a reference. References are types that behave like synonyms for existing variables/objects. The variable/object that it's bound to is known as a referent, and cannot be redirected once bound.

This reference declaration:

int const &r_(32);

...is known as a R-Value reference, which basically means it's a reference to a variable/object who's state cannot change. However, R-Value reference has become ambiguous since the release of C++11's R-Value reference:

int &&r_(12); // R-Value reference.  

This reference declaration:

int a_(0);
int &r_(a_);

...is known as a L-Value reference, which means it's a reference to a variable/object who's state can change.

For more information, see here: http://en.wikipedia.org/wiki/Reference_(C%2B%2B)

_Wazzak

[iceman1992]

Hmmm, I think I understand..
So essentially it's like a compiler-managed pointer?

Texan40, if I understand correctly, App.GetInput() returns an object, an instance of the class sf::Input?

Framework, what do you mean by "cannot be redirected once bound"?
Does it mean I can't do this?

int a = 5;
int b = 10;
int& c = a;
c = b;

If so, then it's rather different to a pointer, as non-constant pointers can be reassigned.

PS : I received an email notification that someone with the username "Moschops" replied my post, but why doesn't it show up on this thread?

[Moschops]

So essentially it's like a compiler-managed pointer?

Please don't think of it like that. It is true that many compilers deal with it by having a pointer hanging around behind the scenes, but that's just how they happen to implement it. It's not a good way to think about it.

A reference is another name for the exact same object.

int a = 5;
int b = 10;
int& c = a; // c is another name for a
// At this point, the value of c is 5, and the value of a is 5, because
//  they are the exact same object. Not a copy of each other. There is only one object.
c = b; // remember that c is another name for a
// At this point, the value of c is 10, and the value of a is 10
b = 20;
// // At this point, the value of c is 10, and the value of a is 10
// changing the value of b had no effect. 
// c and a are the same object. Changing one is changing the other. 

"cannot be redirected once bound"?

In the above code, c is another name for a. You cannot change that; you cannot come back later and say that it's now going to be another name for b.

[iceman1992]

Okay then another name for the same object, and it can't be changed.

I'll do some more searching for pass by reference, as you told me. I didn't know what it was called.

Thanks very much for the help, everyone.