RVALUE reference variables and rvalue reference variables returned by the function

Question

void fcn(int&& a) { }

int&& reti() {
    return 3;
}

int main() {
    int&& a = 3;
    fcn(a);    //error
    fcn(reti()); //right
    return 0;
}

why does the first call generate an error, but the second call does not? they are all the int&& type.

`a` in `fcn(a)` is an lvalue expression. There are plenty of dupes for this. — Jason, Jul 14 '22 at 08:39
Another [dupe](https://stackoverflow.com/questions/30339922/c-rvalue-references-in-function-parameters) — Jason, Jul 14 '22 at 08:46

songyuanyao · Answer 1 · 2022-07-14T08:36:46.417

1

Types and value categories are different things. Even a's type is rvalue-reference but as the name of variable a is an lvalue expression itself.

(emphasis mine)

The following expressions are lvalue expressions:

the name of a variable, a function, a template parameter object (since C++20), or a data member, regardless of type, such as std::cin or std::endl. Even if the variable's type is rvalue reference, the expression consisting of its name is an lvalue expression;

You can use std::move to convert it to rvalue as:

fcn(std::move(a));

On the other hand, reti returns rvalue-reference and reti() is an rvalue expression. (PS: reti() always returns a dangling reference.)

The following expressions are xvalue expressions:

a function call or an overloaded operator expression, whose return type is rvalue reference to object, such as std::move(x);

edited Jul 14 '22 at 08:36

answered Jul 14 '22 at 08:29

songyuanyao

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Doesn't reti() also a variable of type int&&? – air Jul 14 '22 at 08:36
Isn't also `a` in function `main` dangling? Or does bounding (temporary) `3` to a r-value reference extend its lifetime? – paolo Jul 14 '22 at 08:37
@air No, it's an xvalue (rvalue) expression. – songyuanyao Jul 14 '22 at 08:38
@paolo Yes, lifetime gets extended to the lifetime of `a`, i.e. in the scope of `main()`. – songyuanyao Jul 14 '22 at 08:39
@songyuanyao Thanks. I knew this is the case with l-value refernces to `const`; I wasn't aware it also applies to r-value references. – paolo Jul 14 '22 at 08:42
@paolo Yes, same life-extension rule as lvalue-reference to `const`. – songyuanyao Jul 14 '22 at 08:42
I would also distinguish between types of `a` the variable and `a` the expression. Expressions can't have reference types. – HolyBlackCat Jul 14 '22 at 11:40

RVALUE reference variables and rvalue reference variables returned by the function

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