Return const reference to const reference argument [duplicate]

Question

Can I return a const reference argument from a function in C++? I.e., does this code lead to undefined behavior?

template<typename T>
class Option {
public:
    ...

    const T & getOrElse(const T &x) const
    {
        return val == nullptr ? x : *val;
    }

private:
    T *val = nullptr;
}

Option<Foo> foo;
foo.getOrElse(Foo());  // <- using a temporary `Foo`

Answer 1

It's fine to return a reference as long as the object referred to is not destroyed before control leaves the function (i.e., an automatic local variable or parameter, or a temporary created inside the function).

In this case, you are potentially returning a reference to the temporary Foo() in the calling context, which is fine because that temporary is guaranteed to survive until the end of the full-expression containing the call. However, the reference would become dangling after the full-expression, i.e., the lifetime of the Foo() temporary is not extended, so care must be taken not to access it again after that point.