Can multiple 'const reference' variables share the same memory?

user_1160124 0
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I'm wondering if there's a memory cost to having multiple "constant reference" variables in the same scope pointing to the same object:

const Animal& animal = getAnimal();
const Dog& dog = static_cast<const Dog&>(animal);

Conceptually, animal and dog are two variables, each of pointer size, and thus would take up 2 registers (or a 2*pointer-size region on the stack).

But (assuming that there's no multiple inheritance etc.), the compiler could know that they both must hold the same pointer value throughout their lifetime.

So, can these two variables share a single register (or a single pointer-sized region on the stack)?
By "can", I mean:

  • Does the C++ standard allow it?
  • Would modern compilers do it?
compiler-optimization memory reference c++
2 Answers

Does the C++ standard allow it?

Sure, why not. There is no way you could tell the difference. The so-called "as-if rule" allows compilers to do any optimization as long the observable behavior is the same as if it didn't do any optimizations (side note: there are exceptions, where optimizations are allowed to change observable behavior).

Conceptually, animal and dog are two variables, each of pointer size, ...

Nope. Conceptually, references are aliases. They need not take any space, because they are just a different name for the actual object. The C++ standard does not specify the size a reference takes or how it is implemented. sizeof a reference gives you the size of the refered to object. The address of a reference is the address of the referd to object. I am not aware of any way to tell the difference if a reference is implemented as pointer or in any other way (and I strongly doubt that there is a portable way).

Would modern compilers do it?

To answer this I suggest you to take some real code and look at the compilers output. This is a nice tool to help with that: https://godbolt.org/.

PS: I sense a small misunderstanding. In fact the const in your example is not that relevant. Having a const reference does not imply that the value will not change. It only means that you are not allowed to change the value through that reference. Maybe best exaplained by a small example:

struct foo {
    const int& ref;
};

int main() {
    int x = 1;
    foo f{x};
    x = 42;
}

Here f holds a const reference to x. That doesn't mean that x will never get modified. It only means that f cannot modify x via ref. This is especially important in a multithreaded environment, where assuming that an object is const just because you have a const reference will cause trouble.

user_41177280

Can multiple 'const reference' variables share the same memory?

Sure, variables can sometimes share memory. In some cases, neither variable might use any memory.

This applies to all trivial types.

Conceptually, animal and dog are two variables, each of pointer size

Conceptually, the standard doesn't specify the size of references. In practice though, if a reference variable needs storage, then it will be the size of a pointer indeed.

Does the C++ standard allow it?

Yes.

Would modern compilers do it?

Depends on context. There are cases where they would, and cases where they practically can't.

user_20793030
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