CODEWITHSUNDEEP
c++alias
subbrl
subbrl

Reputation: 327

An alias for an alias?

Is it ok to create references for reference variables (alias for an alias in itself ) ?

If yes, what is its application ?

Upvotes: 0

Views: 284

Answers (3)

Cybeyond
Cybeyond

Reputation: 1

In Python, like this:

a = 1
b = a

after this processing, the id for "a" and "b" is the same one.

Upvotes: 0

Kerrek SB
Kerrek SB

Reputation: 477030

In C++98, it was illegal to form references to reference types. In C++11, there are new reference collapsing rules, which means in a nutshell that a reference to a reference is still just a reference (but there are subtleties regarding lvalue and rvalue references). Consider this code:

typedef int & ir;
int a;
ir & b = a;

In C++98, the last line is illegal, since ir & is not a valid type (an attempted reference to a reference). In C++11, the references collapse and ir & is the same as int &.

Bear in mind that references are immutable, and once initialized you can never change the target of the reference. In the above code, b will always be an alias of a, and can never be changed into an alias to something else. Thus there is no need for a double indirection, as it wouldn't allow you to do anything more than what you already can do with ordinary references.

For completeness, the reference collapsing rules are as follows. Suppose T is not a reference type. Then conceptually we have:

(T&)& == T&    (T&)&& == T&    (T&&)& == T&    (T&&)&& == T&&

Upvotes: 4

Steve Jessop
Steve Jessop

Reputation: 279255

You can't create a reference to a reference, and C++ has no reference-to-reference types.

If you use a reference to initialize another reference, for example:

int i = 1;
int &a = i;
int &b = a;

Then what you've actually done is bound the referand of a to b. a is a name for the same object that i is a name for, and consequently int &b = a; has exactly the same effect as int &b = i;. So you have two references to the same object, i.

I can't immediately think of a reason to have two references in the same function, but you'd commonly create multiple references if you have a function f that takes a reference parameter, and passes this on to another function g that also takes a reference parameter. Then f and g each has a reference to the same object.

Upvotes: 2

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