Why are references not reseatable in C++

Question

C++ references have two properties:

• They always point to the same object.
• They can not be 0.

Pointers are the opposite:

• They can point to different objects.
• They can be 0.

Why is there no "non-nullable, reseatable reference or pointer" in C++? I can't think of a good reason why references shouldn't be reseatable.

Edit: The question comes up often because I usually use references when I want to make sure that an "association" (I'm avoiding the words "reference" or "pointer" here) is never invalid.

I don't think I ever thought "great that this ref always refers to the same object". If references were reseatable, one could still get the current behavior like this:

int i = 3; int& const j = i;

This is already legal C++, but meaningless.

I restate my question like this: "What was the rationale behind the 'a reference is the object' design? Why was it considered useful to have references always be the same object, instead of only when declared as const?"

Cheers, Felix

Answer 1

The reason that C++ does not allow you to rebind references is given in Stroustrup's "Design and Evolution of C++" :

It is not possible to change what a reference refers to after initialization. That is, once a C++ reference is initialized it cannot be made to refer to a different object later; it cannot be re-bound. I had in the past been bitten by Algol68 references where r1=r2 can either assign through r1 to the object referred to or assign a new reference value to r1 (re-binding r1) depending on the type of r2. I wanted to avoid such problems in C++.

Answer 2

In C++, it is often said that "the reference is the object". In one sense, it is true: though references are handled as pointers when the source code is compiled, the reference is intended to signify an object that is not copied when a function is called. Since references are not directly addressable (for example, references have no address, & returns the address of the object), it would not semantically make sense to reassign them. Moreover, C++ already has pointers, which handles the semantics of re-setting.