P1155R3
More implicit moves

1. Changelog

• R3: Eliminate discussion and move to CWG wording review.

• R2: Added discussion of return ++x.

• R1: Added discussion of return x += y.

2. Background

Each version of C++ has improved the efficiency of returning objects by value. By the middle of the last decade, copy elision was reliable (if not technically guaranteed) in situations like this:

Widget one() {
    return Widget();  // copy elision
}
Widget two() {
    Widget result;
    return result;  // copy elision
}

In C++11, a completely new feature was added: a change to overload resolution which I will call implicit move. Even when copy elision is impossible, the compiler is sometimes required to implicitly move the return statement’s operand into the result object:

std::shared_ptr<Base> three() {
    std::shared_ptr<Base> result;
    return result;  // copy elision
}
std::shared_ptr<Base> four() {
    std::shared_ptr<Derived> result;
    return result;  // no copy elision, but implicitly moved (not copied)
}

The wording for this optimization was amended by [CWG1579]. N4762’s wording in [class.copy.elision]/3 says:

In the following copy-initialization contexts, a move operation might be used instead of a copy operation:

• If the expression in a return statement is a (possibly parenthesized) id-expression that names an object with automatic storage duration declared in the body or parameter-declaration-clause of the innermost enclosing function or lambda-expression, or

• if the operand of a throw-expression is the name of a non-volatile automatic object (other than a function or catch-clause parameter) whose scope does not extend beyond the end of the innermost enclosing try-block (if there is one),

overload resolution to select the constructor for the copy is first performed as if the object were designated by an rvalue. If the first overload resolution fails or was not performed, or if the type of the first parameter of the selected constructor is not an rvalue reference to the object’s type (possibly cv-qualified), overload resolution is performed again, considering the object as an lvalue.

Note: Post-Kona draft N4810 extends this wording to cover co_return statements too. I don’t understand coroutines very well, but I don’t know of any reason why P1155’s changes should not apply to co_return just as well as to return and throw. The proposed wording below reflects my beliefs.

The highlighted phrases above indicate places where the wording diverges from a naïve programmer’s intuition. Consider the following examples...

2.1. Throwing is pessimized

Throwing is pessimized because of the highlighted word function [parameter].

void five() {
    Widget w;
    throw w;  // non-guaranteed copy elision, but implicitly moved (never copied)
}
Widget six(Widget w) {
    return w;  // no copy elision, but implicitly moved (never copied)
}
void seven(Widget w) {
    throw w;  // no copy elision, and no implicit move (the object is copied)
}

Note: The comment in seven matches the current Standard wording, and matches the behavior of GCC. Most compilers (Clang 4.0.1+, MSVC 2015+, ICC 16.0.3+) already do this implicit move.

2.2. Non-constructor conversion is pessimized

Non-constructor conversion is pessimized because of the highlighted word constructor .

struct From {
    From(Widget const &);
    From(Widget&&);
};

struct To {
    operator Widget() const &;
    operator Widget() &&;
};

From eight() {
    Widget w;
    return w;  // no copy elision, but implicitly moved (never copied)
}
Widget nine() {
    To t;
    return t;  // no copy elision, and no implicit move (the object is copied)
}

2.3. By-value sinks are pessimized

By-value sinks are pessimized because of the highlighted phrase rvalue reference .

struct Fish {
    Fish(Widget const &);
    Fish(Widget&&);
};

struct Fowl {
    Fowl(Widget);
};

Fish ten() {
    Widget w;
    return w;  // no copy elision, but implicitly moved (never copied)
}
Fowl eleven() {
    Widget w;
    return w;  // no copy elision, and no implicit move (the Widget object is copied)
}

Note: The comment in eleven matches the current Standard wording, and matches the behavior of Clang, ICC, and MSVC. One compiler (GCC 5.1+) already does this implicit move.

2.4. Slicing is pessimized

Slicing is pessimized because of the highlighted phrase the object’s .

std::shared_ptr<Base> twelve() {
    std::shared_ptr<Derived> result;
    return result;  // no copy elision, but implicitly moved (never copied)
}
Base thirteen() {
    Derived result;
    return result;  // no copy elision, and no implicit move (the object is copied)
}

Note: The comment in thirteen matches the current Standard wording, and matches the behavior of Clang and MSVC. Some compilers (GCC 8.1+, ICC 18.0.0+) already do this implicit move.

We propose to remove all four of these unnecessary limitations.

3. Proposed wording relative to N4810

Modify [class.copy.elision]/3 as follows:

In the following copy-initialization contexts, a move operation might be used instead of a copy operation:

• If the expression in a return or co_return statement is a (possibly parenthesized) id-expression that names an object with automatic storage duration declared in the body or parameter-declaration-clause of the innermost enclosing function or lambda-expression, or

• if the operand of a throw-expression is the name of a non-volatile automatic object (other than a function or catch-clause parameter) whose scope does not extend beyond the end of the innermost enclosing try-block (if there is one),

overload resolution to select the constructor for the copy or the return_value overload to call is first performed as if the object were designated by an rvalue. If the first overload resolution fails or was not performed, or if the type of the first parameter of the selected constructor or the return_value overload is not an rvalue reference to the object’s type (possibly cv-qualified), overload resolution is performed again, considering the object as an lvalue. [Note: This two-stage overload resolution must be performed regardless of whether copy elision will occur. It determines the constructor or the return_value overload to be called if elision is not performed, and the selected constructor or the return_value overload must be accessible even if the call is elided. —end note]

4. Acknowledgments

• Thanks to Lukas Bergdoll for his copious feedback.

• Thanks to David Stone for [P0527], and for offering to shepherd P1155R0 at the San Diego WG21 meeting (November 2018).

• Thanks to Barry Revzin (see [Revzin]) for pointing out the "By-value sinks" case.