std::move doesn't work when the derived class' destructor is specified












7














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 the 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++







share|improve this question
























  • You wrote a move constructor but not a matching move assignment operator. Why not?
    – Jesper Juhl
    1 hour ago








  • 2




    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
    1 hour ago










  • @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
    55 mins ago
















7














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 the 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++







share|improve this question
























  • You wrote a move constructor but not a matching move assignment operator. Why not?
    – Jesper Juhl
    1 hour ago








  • 2




    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
    1 hour ago










  • @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
    55 mins ago














7












7








7


5





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 the 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++







share|improve this question















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 the 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++






share|improve this question















share|improve this question













share|improve this question




share|improve this question








edited 44 mins ago

























asked 1 hour ago









Searene

6,5502380111




6,5502380111












  • You wrote a move constructor but not a matching move assignment operator. Why not?
    – Jesper Juhl
    1 hour ago








  • 2




    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
    1 hour ago










  • @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
    55 mins ago


















  • You wrote a move constructor but not a matching move assignment operator. Why not?
    – Jesper Juhl
    1 hour ago








  • 2




    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
    1 hour ago










  • @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
    55 mins ago
















You wrote a move constructor but not a matching move assignment operator. Why not?
– Jesper Juhl
1 hour ago






You wrote a move constructor but not a matching move assignment operator. Why not?
– Jesper Juhl
1 hour ago






2




2




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
1 hour ago




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
1 hour ago












@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
55 mins ago




@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
55 mins ago












1 Answer
1






active

oldest

votes


















14














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:



enter image description here






share|improve this answer

















  • 1




    Thank you for that table! Mind if I save and use intensively?
    – YSC
    1 hour ago










  • @YSC: I take no credit for the table, it was made by Howard Hinnant.
    – Vittorio Romeo
    1 hour ago






  • 3




    The slides from the host: accu.org/content/conf2014/Howard_Hinnant_Accu_2014.pdf
    – YSC
    1 hour ago










  • The easier to remember rule: =default or =delete or implement the 5 special member functions whenever you touch any of them.
    – Yakk - Adam Nevraumont
    21 mins ago











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14














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:



enter image description here






share|improve this answer

















  • 1




    Thank you for that table! Mind if I save and use intensively?
    – YSC
    1 hour ago










  • @YSC: I take no credit for the table, it was made by Howard Hinnant.
    – Vittorio Romeo
    1 hour ago






  • 3




    The slides from the host: accu.org/content/conf2014/Howard_Hinnant_Accu_2014.pdf
    – YSC
    1 hour ago










  • The easier to remember rule: =default or =delete or implement the 5 special member functions whenever you touch any of them.
    – Yakk - Adam Nevraumont
    21 mins ago
















14














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:



enter image description here






share|improve this answer

















  • 1




    Thank you for that table! Mind if I save and use intensively?
    – YSC
    1 hour ago










  • @YSC: I take no credit for the table, it was made by Howard Hinnant.
    – Vittorio Romeo
    1 hour ago






  • 3




    The slides from the host: accu.org/content/conf2014/Howard_Hinnant_Accu_2014.pdf
    – YSC
    1 hour ago










  • The easier to remember rule: =default or =delete or implement the 5 special member functions whenever you touch any of them.
    – Yakk - Adam Nevraumont
    21 mins ago














14












14








14






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:



enter image description here






share|improve this answer












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:



enter image description here







share|improve this answer












share|improve this answer



share|improve this answer










answered 1 hour ago









Vittorio Romeo

57.1k17153293




57.1k17153293








  • 1




    Thank you for that table! Mind if I save and use intensively?
    – YSC
    1 hour ago










  • @YSC: I take no credit for the table, it was made by Howard Hinnant.
    – Vittorio Romeo
    1 hour ago






  • 3




    The slides from the host: accu.org/content/conf2014/Howard_Hinnant_Accu_2014.pdf
    – YSC
    1 hour ago










  • The easier to remember rule: =default or =delete or implement the 5 special member functions whenever you touch any of them.
    – Yakk - Adam Nevraumont
    21 mins ago














  • 1




    Thank you for that table! Mind if I save and use intensively?
    – YSC
    1 hour ago










  • @YSC: I take no credit for the table, it was made by Howard Hinnant.
    – Vittorio Romeo
    1 hour ago






  • 3




    The slides from the host: accu.org/content/conf2014/Howard_Hinnant_Accu_2014.pdf
    – YSC
    1 hour ago










  • The easier to remember rule: =default or =delete or implement the 5 special member functions whenever you touch any of them.
    – Yakk - Adam Nevraumont
    21 mins ago








1




1




Thank you for that table! Mind if I save and use intensively?
– YSC
1 hour ago




Thank you for that table! Mind if I save and use intensively?
– YSC
1 hour ago












@YSC: I take no credit for the table, it was made by Howard Hinnant.
– Vittorio Romeo
1 hour ago




@YSC: I take no credit for the table, it was made by Howard Hinnant.
– Vittorio Romeo
1 hour ago




3




3




The slides from the host: accu.org/content/conf2014/Howard_Hinnant_Accu_2014.pdf
– YSC
1 hour ago




The slides from the host: accu.org/content/conf2014/Howard_Hinnant_Accu_2014.pdf
– YSC
1 hour ago












The easier to remember rule: =default or =delete or implement the 5 special member functions whenever you touch any of them.
– Yakk - Adam Nevraumont
21 mins ago




The easier to remember rule: =default or =delete or implement the 5 special member functions whenever you touch any of them.
– Yakk - Adam Nevraumont
21 mins ago


















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