return const reference vs temporary object

Question

I'm wondering why returning const reference of a local object is illegal whereas returning a local object is legal as long as you assign it to a const reference?

vector<int> f_legal() {
    vector<int> tempVec;
    tempVec.push_back(1);
    return tempVec;
}

const vector<int>& f_illegal() {
    vector<int> tempVec;
    tempVec.push_back(1);
    return tempVec;
}

void g() {
    const vector<int>& v1 = f_legal(); // legal
    const vector<int>& v2 = f_illegal(); // illegal
}

Edit: My point is that if assigning a const ref to a returned local variable is legal, then shouldn't assigning a const ref to a returned const ref of a local variable be legal as well?

Answer 1

Even if you assign it to a const reference, the return value is declared as passed by value, that means it'll be copied^[1] to outside as a temporary object, and then binded to the const reference. Binding temporary object to a const reference is fine, the object won't be destroyed until getting out of the lifetime of the const reference.

On the other hand, returning reference of a local variable is illegel. The local variable'll be destroyed when the function returned, that means the outside reference will be dangled.

EDIT

My point is that if assigning a const ref to a returned local variable is legal, then shouldn't assigning a const ref to a returned const ref of a local variable be legal as well?

The point is the 1st case is not assigning a const ref to a returned local variable, it's assigning a const ref to a returned temporary variable. (Which might copied from the local variable.)

---

^{[1] The copy might be omitted according to RVO technically.}

Answer 2

Returning a reference to a local variable is illegal (Undefined Behavior). Period. There is no const or mutable clause.

This is because local function variables have automatic storage duration. They are "destroyed" after the function exits. If the function returns a reference to such a variable, that reference it is said to be dangling: it refers to an object that no longer exists.

The first one is legal because of a special C++ rule: initializing a reference to a prvalue extends the lifetime of that temporary object to the lifetime of the reference.