std::move doesn't work when the derived class' destructor is specified
19
I was trying to move an instance to another one, as follows:
#include <iostream>
class student {
public:
student() = default;
student(const student& student) {
std::cout << "copy student" << std::endl;
}
student(student&& student) {
std::cout << "move student" << std::endl;
}
virtual ~student() = default;
};
class undergraduate: public student {
public:
undergraduate(): student() {}
~undergraduate() override = default; // removing this line will make std::move work
};
int main() {
undergraduate student;
undergraduate student1 = std::move(student);
}
Here is the output:
copy student
As you can see, std::move didn't work, student was copied instead of moved, however, if I remove undergraduate's destructor, i.e. the following line:
~undergraduate() override = default; // removing this line will make std::move work
The output will become move student, which means std::move works. Why? Why didn't std::move work when the derived class' destructor is specified?
c++
asked Dec 27 '18 at 14:52
SeareneSearene
6,9722383119
add a comment |
19
I was trying to move an instance to another one, as follows:
#include <iostream>
class student {
public:
student() = default;
student(const student& student) {
std::cout << "copy student" << std::endl;
}
student(student&& student) {
std::cout << "move student" << std::endl;
}
virtual ~student() = default;
};
class undergraduate: public student {
public:
undergraduate(): student() {}
~undergraduate() override = default; // removing this line will make std::move work
};
int main() {
undergraduate student;
undergraduate student1 = std::move(student);
}
Here is the output:
copy student
As you can see, std::move didn't work, student was copied instead of moved, however, if I remove undergraduate's destructor, i.e. the following line:
~undergraduate() override = default; // removing this line will make std::move work
The output will become move student, which means std::move works. Why? Why didn't std::move work when the derived class' destructor is specified?
c++
asked Dec 27 '18 at 14:52
SeareneSearene
6,9722383119
You wrote a move constructor but not a matching move assignment operator. Why not?
– Jesper Juhl
Dec 27 '18 at 15:06
4
std::movealways work. In can't not work, it is a cast in disguise. The question is, why move constructor is not called - which is answered below.
– SergeyA
Dec 27 '18 at 15:11
@JesperJuhl Because move constructor is used here instead of move assignment. Move constructor is called when an object is initialized, likeT a = std::move(b);orT a(std::move(b));, check en.cppreference.com/w/cpp/language/move_constructor for more details.
– Searene
Dec 27 '18 at 15:30
add a comment |
19
19
19
12
I was trying to move an instance to another one, as follows:
#include <iostream>
class student {
public:
student() = default;
student(const student& student) {
std::cout << "copy student" << std::endl;
}
student(student&& student) {
std::cout << "move student" << std::endl;
}
virtual ~student() = default;
};
class undergraduate: public student {
public:
undergraduate(): student() {}
~undergraduate() override = default; // removing this line will make std::move work
};
int main() {
undergraduate student;
undergraduate student1 = std::move(student);
}
Here is the output:
copy student
As you can see, std::move didn't work, student was copied instead of moved, however, if I remove undergraduate's destructor, i.e. the following line:
~undergraduate() override = default; // removing this line will make std::move work
The output will become move student, which means std::move works. Why? Why didn't std::move work when the derived class' destructor is specified?
c++
asked Dec 27 '18 at 14:52
SeareneSearene
6,9722383119
I was trying to move an instance to another one, as follows:
#include <iostream>
class student {
public:
student() = default;
student(const student& student) {
std::cout << "copy student" << std::endl;
}
student(student&& student) {
std::cout << "move student" << std::endl;
}
virtual ~student() = default;
};
class undergraduate: public student {
public:
undergraduate(): student() {}
~undergraduate() override = default; // removing this line will make std::move work
};
int main() {
undergraduate student;
undergraduate student1 = std::move(student);
}
Here is the output:
copy student
As you can see, std::move didn't work, student was copied instead of moved, however, if I remove undergraduate's destructor, i.e. the following line:
~undergraduate() override = default; // removing this line will make std::move work
The output will become move student, which means std::move works. Why? Why didn't std::move work when the derived class' destructor is specified?
c++
c++
asked Dec 27 '18 at 14:52
SeareneSearene
6,9722383119
asked Dec 27 '18 at 14:52
SeareneSearene
6,9722383119
edited Dec 28 '18 at 0:43
Searene
asked Dec 27 '18 at 14:52
SeareneSearene
6,9722383119
asked Dec 27 '18 at 14:52
SeareneSearene
6,9722383119
asked Dec 27 '18 at 14:52
SeareneSearene
6,9722383119
6,9722383119
You wrote a move constructor but not a matching move assignment operator. Why not?
– Jesper Juhl
Dec 27 '18 at 15:06
4
std::movealways work. In can't not work, it is a cast in disguise. The question is, why move constructor is not called - which is answered below.
– SergeyA
Dec 27 '18 at 15:11
@JesperJuhl Because move constructor is used here instead of move assignment. Move constructor is called when an object is initialized, likeT a = std::move(b);orT a(std::move(b));, check en.cppreference.com/w/cpp/language/move_constructor for more details.
– Searene
Dec 27 '18 at 15:30
add a comment |
You wrote a move constructor but not a matching move assignment operator. Why not?
– Jesper Juhl
Dec 27 '18 at 15:06
4
std::movealways work. In can't not work, it is a cast in disguise. The question is, why move constructor is not called - which is answered below.
– SergeyA
Dec 27 '18 at 15:11
@JesperJuhl Because move constructor is used here instead of move assignment. Move constructor is called when an object is initialized, likeT a = std::move(b);orT a(std::move(b));, check en.cppreference.com/w/cpp/language/move_constructor for more details.
– Searene
Dec 27 '18 at 15:30
You wrote a move constructor but not a matching move assignment operator. Why not?
– Jesper Juhl
Dec 27 '18 at 15:06
You wrote a move constructor but not a matching move assignment operator. Why not?
– Jesper Juhl
Dec 27 '18 at 15:06
4
4
std::move always work. In can't not work, it is a cast in disguise. The question is, why move constructor is not called - which is answered below.– SergeyA
Dec 27 '18 at 15:11
std::move always work. In can't not work, it is a cast in disguise. The question is, why move constructor is not called - which is answered below.– SergeyA
Dec 27 '18 at 15:11
@JesperJuhl Because move constructor is used here instead of move assignment. Move constructor is called when an object is initialized, like
T a = std::move(b); or T a(std::move(b));, check en.cppreference.com/w/cpp/language/move_constructor for more details.– Searene
Dec 27 '18 at 15:30
@JesperJuhl Because move constructor is used here instead of move assignment. Move constructor is called when an object is initialized, like
T a = std::move(b); or T a(std::move(b));, check en.cppreference.com/w/cpp/language/move_constructor for more details.– Searene
Dec 27 '18 at 15:30
add a comment |
1 Answer
1
active
oldest
votes
39
Specifying a class's destructor inhibits automatic generation of the move constructor and move assignment. You can restore them by using = default:
class undergraduate: public student {
public:
undergraduate(): student() {}
~undergraduate() override = default;
undergraduate(const undergraduate&) = default;
undergraduate& operator=(const undergraduate&) = default;
undergraduate(undergraduate&&) = default;
undergraduate& operator=(undergraduate&&) = default;
};
Howard Hinnant created an excellent table for his Everything You Ever Wanted To Know About Move Semantics (and then some)" presentation:
answered Dec 27 '18 at 14:53
Vittorio RomeoVittorio Romeo
58.1k17158300
3
Thank you for that table! Mind if I save and use intensively?
– YSC
Dec 27 '18 at 15:06
@YSC: I take no credit for the table, it was made by Howard Hinnant.
– Vittorio Romeo
Dec 27 '18 at 15:16
7
The slides from the host: accu.org/content/conf2014/Howard_Hinnant_Accu_2014.pdf
– YSC
Dec 27 '18 at 15:18
8
The easier to remember rule:=defaultor=deleteor implement the 5 special member functions whenever you touch any of them.
– Yakk - Adam Nevraumont
Dec 27 '18 at 16:03
add a comment |
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39
Specifying a class's destructor inhibits automatic generation of the move constructor and move assignment. You can restore them by using = default:
class undergraduate: public student {
public:
undergraduate(): student() {}
~undergraduate() override = default;
undergraduate(const undergraduate&) = default;
undergraduate& operator=(const undergraduate&) = default;
undergraduate(undergraduate&&) = default;
undergraduate& operator=(undergraduate&&) = default;
};
Howard Hinnant created an excellent table for his Everything You Ever Wanted To Know About Move Semantics (and then some)" presentation:
answered Dec 27 '18 at 14:53
Vittorio RomeoVittorio Romeo
58.1k17158300
3
Thank you for that table! Mind if I save and use intensively?
– YSC
Dec 27 '18 at 15:06
@YSC: I take no credit for the table, it was made by Howard Hinnant.
– Vittorio Romeo
Dec 27 '18 at 15:16
7
The slides from the host: accu.org/content/conf2014/Howard_Hinnant_Accu_2014.pdf
– YSC
Dec 27 '18 at 15:18
8
The easier to remember rule:=defaultor=deleteor implement the 5 special member functions whenever you touch any of them.
– Yakk - Adam Nevraumont
Dec 27 '18 at 16:03
add a comment |
39
Specifying a class's destructor inhibits automatic generation of the move constructor and move assignment. You can restore them by using = default:
class undergraduate: public student {
public:
undergraduate(): student() {}
~undergraduate() override = default;
undergraduate(const undergraduate&) = default;
undergraduate& operator=(const undergraduate&) = default;
undergraduate(undergraduate&&) = default;
undergraduate& operator=(undergraduate&&) = default;
};
Howard Hinnant created an excellent table for his Everything You Ever Wanted To Know About Move Semantics (and then some)" presentation:
answered Dec 27 '18 at 14:53
Vittorio RomeoVittorio Romeo
58.1k17158300
3
Thank you for that table! Mind if I save and use intensively?
– YSC
Dec 27 '18 at 15:06
@YSC: I take no credit for the table, it was made by Howard Hinnant.
– Vittorio Romeo
Dec 27 '18 at 15:16
7
The slides from the host: accu.org/content/conf2014/Howard_Hinnant_Accu_2014.pdf
– YSC
Dec 27 '18 at 15:18
8
The easier to remember rule:=defaultor=deleteor implement the 5 special member functions whenever you touch any of them.
– Yakk - Adam Nevraumont
Dec 27 '18 at 16:03
add a comment |
39
39
39
Specifying a class's destructor inhibits automatic generation of the move constructor and move assignment. You can restore them by using = default:
class undergraduate: public student {
public:
undergraduate(): student() {}
~undergraduate() override = default;
undergraduate(const undergraduate&) = default;
undergraduate& operator=(const undergraduate&) = default;
undergraduate(undergraduate&&) = default;
undergraduate& operator=(undergraduate&&) = default;
};
Howard Hinnant created an excellent table for his Everything You Ever Wanted To Know About Move Semantics (and then some)" presentation:
answered Dec 27 '18 at 14:53
Vittorio RomeoVittorio Romeo
58.1k17158300
Specifying a class's destructor inhibits automatic generation of the move constructor and move assignment. You can restore them by using = default:
class undergraduate: public student {
public:
undergraduate(): student() {}
~undergraduate() override = default;
undergraduate(const undergraduate&) = default;
undergraduate& operator=(const undergraduate&) = default;
undergraduate(undergraduate&&) = default;
undergraduate& operator=(undergraduate&&) = default;
};
Howard Hinnant created an excellent table for his Everything You Ever Wanted To Know About Move Semantics (and then some)" presentation:
answered Dec 27 '18 at 14:53
Vittorio RomeoVittorio Romeo
58.1k17158300
answered Dec 27 '18 at 14:53
Vittorio RomeoVittorio Romeo
58.1k17158300
answered Dec 27 '18 at 14:53
Vittorio RomeoVittorio Romeo
58.1k17158300
answered Dec 27 '18 at 14:53
Vittorio RomeoVittorio Romeo
58.1k17158300
58.1k17158300
3
Thank you for that table! Mind if I save and use intensively?
– YSC
Dec 27 '18 at 15:06
@YSC: I take no credit for the table, it was made by Howard Hinnant.
– Vittorio Romeo
Dec 27 '18 at 15:16
7
The slides from the host: accu.org/content/conf2014/Howard_Hinnant_Accu_2014.pdf
– YSC
Dec 27 '18 at 15:18
8
The easier to remember rule:=defaultor=deleteor implement the 5 special member functions whenever you touch any of them.
– Yakk - Adam Nevraumont
Dec 27 '18 at 16:03
add a comment |
3
Thank you for that table! Mind if I save and use intensively?
– YSC
Dec 27 '18 at 15:06
@YSC: I take no credit for the table, it was made by Howard Hinnant.
– Vittorio Romeo
Dec 27 '18 at 15:16
7
The slides from the host: accu.org/content/conf2014/Howard_Hinnant_Accu_2014.pdf
– YSC
Dec 27 '18 at 15:18
8
The easier to remember rule:=defaultor=deleteor implement the 5 special member functions whenever you touch any of them.
– Yakk - Adam Nevraumont
Dec 27 '18 at 16:03
3
3
Thank you for that table! Mind if I save and use intensively?
– YSC
Dec 27 '18 at 15:06
Thank you for that table! Mind if I save and use intensively?
– YSC
Dec 27 '18 at 15:06
@YSC: I take no credit for the table, it was made by Howard Hinnant.
– Vittorio Romeo
Dec 27 '18 at 15:16
@YSC: I take no credit for the table, it was made by Howard Hinnant.
– Vittorio Romeo
Dec 27 '18 at 15:16
7
7
The slides from the host: accu.org/content/conf2014/Howard_Hinnant_Accu_2014.pdf
– YSC
Dec 27 '18 at 15:18
The slides from the host: accu.org/content/conf2014/Howard_Hinnant_Accu_2014.pdf
– YSC
Dec 27 '18 at 15:18
8
8
The easier to remember rule:
=default or =delete or implement the 5 special member functions whenever you touch any of them.– Yakk - Adam Nevraumont
Dec 27 '18 at 16:03
The easier to remember rule:
=default or =delete or implement the 5 special member functions whenever you touch any of them.– Yakk - Adam Nevraumont
Dec 27 '18 at 16:03
add a comment |
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You wrote a move constructor but not a matching move assignment operator. Why not?
– Jesper Juhl
Dec 27 '18 at 15:06
4
std::movealways work. In can't not work, it is a cast in disguise. The question is, why move constructor is not called - which is answered below.– SergeyA
Dec 27 '18 at 15:11
@JesperJuhl Because move constructor is used here instead of move assignment. Move constructor is called when an object is initialized, like
T a = std::move(b);orT a(std::move(b));, check en.cppreference.com/w/cpp/language/move_constructor for more details.– Searene
Dec 27 '18 at 15:30