I have learned that evaluating an uninitialized variable is undefined behavior. In particular, int i = i; is undefined behavior. I have read What's the behavior of an uninitialized variable used as its own initializer?
But, is using a reference variable to initialize itself also undefined behavior? In particular, is int &ref = ref; also UB, as per the C++ standard?
int &ref = ref; // Is this well-formed or ill-formed or UB
All compilers compile the above program (with clang giving a warning). Is this because it is undefined behavior so anything is allowed to happen, or is the program well-formed?
Additionally, if I were to assign some value to ref, will the behavior of the program change from the previous case?
int &ref = ref;
int main()
{
ref = 1; //does this change the behavior of the program from previous case
}
I noticed that for this 2nd snippet, we get a segfault.
Some references that I've read are:
What's the behavior of an uninitialized variable used as its own initializer?
It is not defined behaviour.
[...] A reference shall be initialized to refer to a valid object or function.
And there is no int object to refer to.
You could also say that the reference is being used outside of its lifetime.
The lifetime of a reference begins when its initialization is complete. The lifetime of a reference ends as if it were a scalar object requiring storage.
The properties ascribed to objects and references throughout this document apply for a given object or reference only during its lifetime.
So, the reference cannot be "used" to initialize itself before it is itself initialized, which is how Clang complains about this (https://godbolt.org/z/Ea4qPoWbs):
note: use of reference outside its lifetime is not allowed in a constant expression 2 | int& ref = ref; ^