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[BUG] Cannot return object of type by value #400
Description
Cpp1 error when trying to return object (by value?) from local variable
Code example
S : type = {
public x: int = 42;
}
baz: () -> _ = {
return S(); //ok cpp1: return S();
}
foo: () -> _ = {
//foo: () -> move _ = { //same behavior
s: S = ();
return s; //cpp1 error: see below
}
main: () = {
std::cout << baz().x;
std::cout << foo().x;
}
Actual result - cpp1 error
error: call to deleted constructor of 'typename std::remove_reference<S &>::type' (aka 'S')
return std::move(s);
note: 'S' has been explicitly marked deleted here
public: S(S const&) = delete;
Additional info
it is possible to return just constructed object like in baz()
Probably this is desired, but I dont see much difference why local cannot be returned here, and constructed object can.
Another note, it is working if type declared with @struct meta
Activity
[-][BUG] Cannot return type by value[/-][+][BUG] Cannot return object of type by value[/+]JohelEGP commentedon May 1, 2023
The error is clear.
See 10241cd:
So a
@structworks because it is copyable.JohelEGP commentedon May 1, 2023
See also https://wg21.link/P2573.
Right now,
public: S(S const&) = delete;appears in the error message,so a short comment can be appended as a stand-in for a proper error message.
Recently, Clang or GCC started trimming source lines too long in error messages,
so the comment should be short yet helpful.
I suggest
public: S(S const&) = delete; // No `that` constructor, suppressing.realgdman commentedon May 1, 2023
Should baz() still work?
JohelEGP commentedon May 1, 2023
Yes. That looks OK.
realgdman commentedon May 1, 2023
Well I still not fully understand. Isn't foo() supposed to not copy, but move by cpp2 default? Error rises inside std::remove_reference which for me looks like some inner workings, not what I am expressing in cpp2.
JohelEGP commentedon May 1, 2023
Copy and move are an overload set.
Sdoesn't implement a move constructor,so the
deleted copy constructor is used.realgdman commentedon May 1, 2023
I think
baz()works in cpp1 because of guaranteed copy elision in cpp++17? It's still returning S which is not copyable.On other hand, in
foo()cpp2 wraps return instd::move()behind scenes.Does that mean that I forced to write move constructor? Even if I don't want to change
Stype definition?Is there or should be a way to suppress move? I have tried declaring
foo: () -> copy _ = {...}which givesAlso I tried
return copy sorreturn (copy s)which is syntax error too.JohelEGP commentedon May 1, 2023
That's right. Returning a prvalue doesn't require copy/move.
Or an
in thatoperator=(copy constructor).That's subject of #231.
Using
x&*rather thanxworks: https://cpp2.godbolt.org/z/aY13c3GG6.Also using a function call like
as-lvalue.See also #396.
hsutter commentedon May 5, 2023
Thanks for the usability report, and I like the error message suggestion. Applied!
Add comment documenting why copy is =delete'd, closes hsutter#400
Add comment documenting why copy is =delete'd, closes hsutter#400