C++ copy constructor called at return
error: use of deleted function 'A::A(const A&)'
return tmp;
^~~
Why is the copy constructor called only when there is a virtual destructor in A? How to avoid this?
struct B {};
struct A{
std::unique_ptr<B> x;
virtual ~A() = default;
};
A f() {
A tmp;
return tmp;
}
c++
asked Mar 21 at 20:23
SobuchSobuch
886
add a comment |
error: use of deleted function 'A::A(const A&)'
return tmp;
^~~
Why is the copy constructor called only when there is a virtual destructor in A? How to avoid this?
struct B {};
struct A{
std::unique_ptr<B> x;
virtual ~A() = default;
};
A f() {
A tmp;
return tmp;
}
c++
asked Mar 21 at 20:23
SobuchSobuch
886
1
see: In which situations is the C++ copy constructor called?
– kmdreko
Mar 21 at 20:29
5
C++ handles objects different than C#/Java. When an instance goes out of scope (tmphere) its destructor must be called. Therefore, when youreturn tmpthen you're asking it to make a copy oftmpto be return to whomever calls the function. Once copied,tmpwill be destroyed and its copy will be available for use.
– Everyone
Mar 21 at 20:29
3
@Everyone except that it is usually a move rather than a copy, which is what the question is about.
– Quentin
Mar 22 at 9:24
add a comment |
error: use of deleted function 'A::A(const A&)'
return tmp;
^~~
Why is the copy constructor called only when there is a virtual destructor in A? How to avoid this?
struct B {};
struct A{
std::unique_ptr<B> x;
virtual ~A() = default;
};
A f() {
A tmp;
return tmp;
}
c++
asked Mar 21 at 20:23
SobuchSobuch
886
error: use of deleted function 'A::A(const A&)'
return tmp;
^~~
Why is the copy constructor called only when there is a virtual destructor in A? How to avoid this?
struct B {};
struct A{
std::unique_ptr<B> x;
virtual ~A() = default;
};
A f() {
A tmp;
return tmp;
}
c++
c++
asked Mar 21 at 20:23
SobuchSobuch
886
asked Mar 21 at 20:23
SobuchSobuch
886
edited Mar 21 at 21:18
Sobuch
asked Mar 21 at 20:23
SobuchSobuch
886
asked Mar 21 at 20:23
SobuchSobuch
886
asked Mar 21 at 20:23
SobuchSobuch
886
886
1
see: In which situations is the C++ copy constructor called?
– kmdreko
Mar 21 at 20:29
5
C++ handles objects different than C#/Java. When an instance goes out of scope (tmphere) its destructor must be called. Therefore, when youreturn tmpthen you're asking it to make a copy oftmpto be return to whomever calls the function. Once copied,tmpwill be destroyed and its copy will be available for use.
– Everyone
Mar 21 at 20:29
3
@Everyone except that it is usually a move rather than a copy, which is what the question is about.
– Quentin
Mar 22 at 9:24
add a comment |
1
see: In which situations is the C++ copy constructor called?
– kmdreko
Mar 21 at 20:29
5
C++ handles objects different than C#/Java. When an instance goes out of scope (tmphere) its destructor must be called. Therefore, when youreturn tmpthen you're asking it to make a copy oftmpto be return to whomever calls the function. Once copied,tmpwill be destroyed and its copy will be available for use.
– Everyone
Mar 21 at 20:29
3
@Everyone except that it is usually a move rather than a copy, which is what the question is about.
– Quentin
Mar 22 at 9:24
1
1
see: In which situations is the C++ copy constructor called?
– kmdreko
Mar 21 at 20:29
see: In which situations is the C++ copy constructor called?
– kmdreko
Mar 21 at 20:29
5
5
C++ handles objects different than C#/Java. When an instance goes out of scope (
tmp here) its destructor must be called. Therefore, when you return tmp then you're asking it to make a copy of tmp to be return to whomever calls the function. Once copied, tmp will be destroyed and its copy will be available for use.– Everyone
Mar 21 at 20:29
C++ handles objects different than C#/Java. When an instance goes out of scope (
tmp here) its destructor must be called. Therefore, when you return tmp then you're asking it to make a copy of tmp to be return to whomever calls the function. Once copied, tmp will be destroyed and its copy will be available for use.– Everyone
Mar 21 at 20:29
3
3
@Everyone except that it is usually a move rather than a copy, which is what the question is about.
– Quentin
Mar 22 at 9:24
@Everyone except that it is usually a move rather than a copy, which is what the question is about.
– Quentin
Mar 22 at 9:24
add a comment |
1 Answer
1
active
oldest
votes
virtual ~A() = default; is a user declared destructor. Because of that, A no longer has a move constructor. That means return tmp; can't move tmp and since tmp is not copyable, you get a compiler error.
There are two ways you can fix this. You can add a move constructor like
struct A{
std::unique_ptr<B> x;
A() = default; // you have to add this since the move constructor was added
A(A&&) = default; // defaulted move
virtual ~A() = default;
};
or you can create a base class that has the virtual destructor and inherit from that like
struct C {
virtual ~C() = default;
};
struct A : C {
std::unique_ptr<B> x;
};
This works because A no longer has a user declared destructor (Yes, C does but we only care about A) so it will still generate a move constructor in A. The important part of this is that C doesn't have a deleted move constructor, it just doesn't have one period, so trying to move it will cause a copy. That means
C's copy constructor is called in A's implicitly generated move constructor since C(std::move(A_obj_to_move_from)) will copy as long as it doesn't have a deleted move constructor.
answered Mar 21 at 20:32
NathanOliverNathanOliver
97.1k16137214
10
Better to follow the Rule of Zero/Five. Either add all of (copy ctor, move ctor, copy assignment, move assignment, destructor) or add none of them. In this example, none of them are necessary.
– 0x5453
Mar 21 at 20:33
3
@0x5453 Unless this is a parent class and the OP wants the derived classes to get destroyed properly. You need a virtual destructor if you have polymorphism.
– NathanOliver
Mar 21 at 20:34
3
@Tzalumen no delete is required (because that's what the unique pointer does for you), but a virtual destructor is required so that the unique pointer won't have UB.
– eerorika
Mar 21 at 20:40
3
@Tzalumen If youdeletean object of classXthrough a pointer to a base class ofXand that base class doesn't have a virtual dtor, it's Undefined Behaviour. Regardless of what the destructor does.
– Angew
Mar 21 at 20:41
3
@Tzalumen If you have polymorphism, you must have a virtual destructor. If you don't the destructor for the derived class won't be called and you have UB.
– NathanOliver
Mar 21 at 20:41
|
show 24 more comments
Your Answer
StackExchange.ifUsing("editor", function () {
StackExchange.using("externalEditor", function () {
StackExchange.using("snippets", function () {
StackExchange.snippets.init();
});
});
}, "code-snippets");
StackExchange.ready(function() {
var channelOptions = {
tags: "".split(" "),
id: "1"
};
initTagRenderer("".split(" "), "".split(" "), channelOptions);
StackExchange.using("externalEditor", function() {
// Have to fire editor after snippets, if snippets enabled
if (StackExchange.settings.snippets.snippetsEnabled) {
StackExchange.using("snippets", function() {
createEditor();
});
}
else {
createEditor();
}
});
function createEditor() {
StackExchange.prepareEditor({
heartbeatType: 'answer',
autoActivateHeartbeat: false,
convertImagesToLinks: true,
noModals: true,
showLowRepImageUploadWarning: true,
reputationToPostImages: 10,
bindNavPrevention: true,
postfix: "",
imageUploader: {
brandingHtml: "Powered by u003ca class="icon-imgur-white" href="https://imgur.com/"u003eu003c/au003e",
contentPolicyHtml: "User contributions licensed under u003ca href="https://creativecommons.org/licenses/by-sa/3.0/"u003ecc by-sa 3.0 with attribution requiredu003c/au003e u003ca href="https://stackoverflow.com/legal/content-policy"u003e(content policy)u003c/au003e",
allowUrls: true
},
onDemand: true,
discardSelector: ".discard-answer"
,immediatelyShowMarkdownHelp:true
});
}
});
Sign up or log in
StackExchange.ready(function () {
StackExchange.helpers.onClickDraftSave('#login-link');
});
Sign up using Google
Sign up using Facebook
Sign up using Email and Password
Post as a guest
Required, but never shown
StackExchange.ready(
function () {
StackExchange.openid.initPostLogin('.new-post-login', 'https%3a%2f%2fstackoverflow.com%2fquestions%2f55288708%2fc-copy-constructor-called-at-return%23new-answer', 'question_page');
}
);
Post as a guest
Required, but never shown
1 Answer
1
active
oldest
votes
1 Answer
1
active
oldest
votes
active
oldest
votes
active
oldest
votes
virtual ~A() = default; is a user declared destructor. Because of that, A no longer has a move constructor. That means return tmp; can't move tmp and since tmp is not copyable, you get a compiler error.
There are two ways you can fix this. You can add a move constructor like
struct A{
std::unique_ptr<B> x;
A() = default; // you have to add this since the move constructor was added
A(A&&) = default; // defaulted move
virtual ~A() = default;
};
or you can create a base class that has the virtual destructor and inherit from that like
struct C {
virtual ~C() = default;
};
struct A : C {
std::unique_ptr<B> x;
};
This works because A no longer has a user declared destructor (Yes, C does but we only care about A) so it will still generate a move constructor in A. The important part of this is that C doesn't have a deleted move constructor, it just doesn't have one period, so trying to move it will cause a copy. That means
C's copy constructor is called in A's implicitly generated move constructor since C(std::move(A_obj_to_move_from)) will copy as long as it doesn't have a deleted move constructor.
answered Mar 21 at 20:32
NathanOliverNathanOliver
97.1k16137214
10
Better to follow the Rule of Zero/Five. Either add all of (copy ctor, move ctor, copy assignment, move assignment, destructor) or add none of them. In this example, none of them are necessary.
– 0x5453
Mar 21 at 20:33
3
@0x5453 Unless this is a parent class and the OP wants the derived classes to get destroyed properly. You need a virtual destructor if you have polymorphism.
– NathanOliver
Mar 21 at 20:34
3
@Tzalumen no delete is required (because that's what the unique pointer does for you), but a virtual destructor is required so that the unique pointer won't have UB.
– eerorika
Mar 21 at 20:40
3
@Tzalumen If youdeletean object of classXthrough a pointer to a base class ofXand that base class doesn't have a virtual dtor, it's Undefined Behaviour. Regardless of what the destructor does.
– Angew
Mar 21 at 20:41
3
@Tzalumen If you have polymorphism, you must have a virtual destructor. If you don't the destructor for the derived class won't be called and you have UB.
– NathanOliver
Mar 21 at 20:41
|
show 24 more comments
virtual ~A() = default; is a user declared destructor. Because of that, A no longer has a move constructor. That means return tmp; can't move tmp and since tmp is not copyable, you get a compiler error.
There are two ways you can fix this. You can add a move constructor like
struct A{
std::unique_ptr<B> x;
A() = default; // you have to add this since the move constructor was added
A(A&&) = default; // defaulted move
virtual ~A() = default;
};
or you can create a base class that has the virtual destructor and inherit from that like
struct C {
virtual ~C() = default;
};
struct A : C {
std::unique_ptr<B> x;
};
This works because A no longer has a user declared destructor (Yes, C does but we only care about A) so it will still generate a move constructor in A. The important part of this is that C doesn't have a deleted move constructor, it just doesn't have one period, so trying to move it will cause a copy. That means
C's copy constructor is called in A's implicitly generated move constructor since C(std::move(A_obj_to_move_from)) will copy as long as it doesn't have a deleted move constructor.
answered Mar 21 at 20:32
NathanOliverNathanOliver
97.1k16137214
10
Better to follow the Rule of Zero/Five. Either add all of (copy ctor, move ctor, copy assignment, move assignment, destructor) or add none of them. In this example, none of them are necessary.
– 0x5453
Mar 21 at 20:33
3
@0x5453 Unless this is a parent class and the OP wants the derived classes to get destroyed properly. You need a virtual destructor if you have polymorphism.
– NathanOliver
Mar 21 at 20:34
3
@Tzalumen no delete is required (because that's what the unique pointer does for you), but a virtual destructor is required so that the unique pointer won't have UB.
– eerorika
Mar 21 at 20:40
3
@Tzalumen If youdeletean object of classXthrough a pointer to a base class ofXand that base class doesn't have a virtual dtor, it's Undefined Behaviour. Regardless of what the destructor does.
– Angew
Mar 21 at 20:41
3
@Tzalumen If you have polymorphism, you must have a virtual destructor. If you don't the destructor for the derived class won't be called and you have UB.
– NathanOliver
Mar 21 at 20:41
|
show 24 more comments
virtual ~A() = default; is a user declared destructor. Because of that, A no longer has a move constructor. That means return tmp; can't move tmp and since tmp is not copyable, you get a compiler error.
There are two ways you can fix this. You can add a move constructor like
struct A{
std::unique_ptr<B> x;
A() = default; // you have to add this since the move constructor was added
A(A&&) = default; // defaulted move
virtual ~A() = default;
};
or you can create a base class that has the virtual destructor and inherit from that like
struct C {
virtual ~C() = default;
};
struct A : C {
std::unique_ptr<B> x;
};
This works because A no longer has a user declared destructor (Yes, C does but we only care about A) so it will still generate a move constructor in A. The important part of this is that C doesn't have a deleted move constructor, it just doesn't have one period, so trying to move it will cause a copy. That means
C's copy constructor is called in A's implicitly generated move constructor since C(std::move(A_obj_to_move_from)) will copy as long as it doesn't have a deleted move constructor.
answered Mar 21 at 20:32
NathanOliverNathanOliver
97.1k16137214
virtual ~A() = default; is a user declared destructor. Because of that, A no longer has a move constructor. That means return tmp; can't move tmp and since tmp is not copyable, you get a compiler error.
There are two ways you can fix this. You can add a move constructor like
struct A{
std::unique_ptr<B> x;
A() = default; // you have to add this since the move constructor was added
A(A&&) = default; // defaulted move
virtual ~A() = default;
};
or you can create a base class that has the virtual destructor and inherit from that like
struct C {
virtual ~C() = default;
};
struct A : C {
std::unique_ptr<B> x;
};
This works because A no longer has a user declared destructor (Yes, C does but we only care about A) so it will still generate a move constructor in A. The important part of this is that C doesn't have a deleted move constructor, it just doesn't have one period, so trying to move it will cause a copy. That means
C's copy constructor is called in A's implicitly generated move constructor since C(std::move(A_obj_to_move_from)) will copy as long as it doesn't have a deleted move constructor.
answered Mar 21 at 20:32
NathanOliverNathanOliver
97.1k16137214
edited Mar 22 at 12:44
answered Mar 21 at 20:32
NathanOliverNathanOliver
97.1k16137214
answered Mar 21 at 20:32
NathanOliverNathanOliver
97.1k16137214
answered Mar 21 at 20:32
NathanOliverNathanOliver
97.1k16137214
97.1k16137214
10
Better to follow the Rule of Zero/Five. Either add all of (copy ctor, move ctor, copy assignment, move assignment, destructor) or add none of them. In this example, none of them are necessary.
– 0x5453
Mar 21 at 20:33
3
@0x5453 Unless this is a parent class and the OP wants the derived classes to get destroyed properly. You need a virtual destructor if you have polymorphism.
– NathanOliver
Mar 21 at 20:34
3
@Tzalumen no delete is required (because that's what the unique pointer does for you), but a virtual destructor is required so that the unique pointer won't have UB.
– eerorika
Mar 21 at 20:40
3
@Tzalumen If youdeletean object of classXthrough a pointer to a base class ofXand that base class doesn't have a virtual dtor, it's Undefined Behaviour. Regardless of what the destructor does.
– Angew
Mar 21 at 20:41
3
@Tzalumen If you have polymorphism, you must have a virtual destructor. If you don't the destructor for the derived class won't be called and you have UB.
– NathanOliver
Mar 21 at 20:41
|
show 24 more comments
10
Better to follow the Rule of Zero/Five. Either add all of (copy ctor, move ctor, copy assignment, move assignment, destructor) or add none of them. In this example, none of them are necessary.
– 0x5453
Mar 21 at 20:33
3
@0x5453 Unless this is a parent class and the OP wants the derived classes to get destroyed properly. You need a virtual destructor if you have polymorphism.
– NathanOliver
Mar 21 at 20:34
3
@Tzalumen no delete is required (because that's what the unique pointer does for you), but a virtual destructor is required so that the unique pointer won't have UB.
– eerorika
Mar 21 at 20:40
3
@Tzalumen If youdeletean object of classXthrough a pointer to a base class ofXand that base class doesn't have a virtual dtor, it's Undefined Behaviour. Regardless of what the destructor does.
– Angew
Mar 21 at 20:41
3
@Tzalumen If you have polymorphism, you must have a virtual destructor. If you don't the destructor for the derived class won't be called and you have UB.
– NathanOliver
Mar 21 at 20:41
10
10
Better to follow the Rule of Zero/Five. Either add all of (copy ctor, move ctor, copy assignment, move assignment, destructor) or add none of them. In this example, none of them are necessary.
– 0x5453
Mar 21 at 20:33
Better to follow the Rule of Zero/Five. Either add all of (copy ctor, move ctor, copy assignment, move assignment, destructor) or add none of them. In this example, none of them are necessary.
– 0x5453
Mar 21 at 20:33
3
3
@0x5453 Unless this is a parent class and the OP wants the derived classes to get destroyed properly. You need a virtual destructor if you have polymorphism.
– NathanOliver
Mar 21 at 20:34
@0x5453 Unless this is a parent class and the OP wants the derived classes to get destroyed properly. You need a virtual destructor if you have polymorphism.
– NathanOliver
Mar 21 at 20:34
3
3
@Tzalumen no delete is required (because that's what the unique pointer does for you), but a virtual destructor is required so that the unique pointer won't have UB.
– eerorika
Mar 21 at 20:40
@Tzalumen no delete is required (because that's what the unique pointer does for you), but a virtual destructor is required so that the unique pointer won't have UB.
– eerorika
Mar 21 at 20:40
3
3
@Tzalumen If you
delete an object of class X through a pointer to a base class of X and that base class doesn't have a virtual dtor, it's Undefined Behaviour. Regardless of what the destructor does.– Angew
Mar 21 at 20:41
@Tzalumen If you
delete an object of class X through a pointer to a base class of X and that base class doesn't have a virtual dtor, it's Undefined Behaviour. Regardless of what the destructor does.– Angew
Mar 21 at 20:41
3
3
@Tzalumen If you have polymorphism, you must have a virtual destructor. If you don't the destructor for the derived class won't be called and you have UB.
– NathanOliver
Mar 21 at 20:41
@Tzalumen If you have polymorphism, you must have a virtual destructor. If you don't the destructor for the derived class won't be called and you have UB.
– NathanOliver
Mar 21 at 20:41
|
show 24 more comments
Thanks for contributing an answer to Stack Overflow!
- Please be sure to answer the question. Provide details and share your research!
But avoid …
- Asking for help, clarification, or responding to other answers.
- Making statements based on opinion; back them up with references or personal experience.
To learn more, see our tips on writing great answers.
Sign up or log in
StackExchange.ready(function () {
StackExchange.helpers.onClickDraftSave('#login-link');
});
Sign up using Google
Sign up using Facebook
Sign up using Email and Password
Post as a guest
Required, but never shown
StackExchange.ready(
function () {
StackExchange.openid.initPostLogin('.new-post-login', 'https%3a%2f%2fstackoverflow.com%2fquestions%2f55288708%2fc-copy-constructor-called-at-return%23new-answer', 'question_page');
}
);
Post as a guest
Required, but never shown
Sign up or log in
StackExchange.ready(function () {
StackExchange.helpers.onClickDraftSave('#login-link');
});
Sign up using Google
Sign up using Facebook
Sign up using Email and Password
Post as a guest
Required, but never shown
Sign up or log in
StackExchange.ready(function () {
StackExchange.helpers.onClickDraftSave('#login-link');
});
Sign up using Google
Sign up using Facebook
Sign up using Email and Password
Post as a guest
Required, but never shown
Sign up or log in
StackExchange.ready(function () {
StackExchange.helpers.onClickDraftSave('#login-link');
});
Sign up using Google
Sign up using Facebook
Sign up using Email and Password
Sign up using Google
Sign up using Facebook
Sign up using Email and Password
Post as a guest
Required, but never shown
Required, but never shown
Required, but never shown
Required, but never shown
Required, but never shown
Required, but never shown
Required, but never shown
Required, but never shown
Required, but never shown
1
see: In which situations is the C++ copy constructor called?
– kmdreko
Mar 21 at 20:29
5
C++ handles objects different than C#/Java. When an instance goes out of scope (
tmphere) its destructor must be called. Therefore, when youreturn tmpthen you're asking it to make a copy oftmpto be return to whomever calls the function. Once copied,tmpwill be destroyed and its copy will be available for use.– Everyone
Mar 21 at 20:29
3
@Everyone except that it is usually a move rather than a copy, which is what the question is about.
– Quentin
Mar 22 at 9:24