Is it safe to access a default argument by reference?

Question

I have a function that looks like this:

class SomeClass {
    // ...
};

void some_function(const SomeClass& arg = SomeClass());

The function some_function accesses its argument by reference and has a default value. Is it safe to do this, or will the reference be invalid when I call the function without an argument?

Answer 1

Yes, it's safe. A const reference bound to a temporary extends the life of that temporary to the lifetime of the reference. The same is true of rvalue references.

Answer 2

It will be valid. The lifetime of the temporary used as a default value is a superset of the lifetime of the function call. This is also no different than if you had passed in a temporary explicitly (default arguments are basically syntactic sugar, saving you from typing, but behave more or less identically to arguments passed explicitly.