Is an lvalue reference returned from a function actually an rvalue (from the perspective of the caller)?

Question

How is it possible to have an lvalue reference refer to an lvalue reference returned from a function? Isn't the lvalue reference returned from that function actually an rvalue from the perspective of the caller? For example:

class Obj {};

Obj& refToRef(Obj& o) {
    return o;
}

int main() {
    Obj o;
    Obj& o2 = refToRef(o);
}

How is o2 (an lvalue reference) able to refer to what appears to be an rvalue?

Answer 1

从调用者的角度来看，从该函数返回的左值引用实际上不是右值吗？

不，因为函数调用的结果并不总是右值。

从[expr.call] / 14（重点是我的）：

如果结果类型是左值引用类型或对函数类型的右值引用，则函数调用为左值；如果结果类型是对对象类型的右值引用，则函数调用为xvalue；否则为prvalue。

我敢肯定，这条规则是专门为避免您的问题所提出的问题而存在的。