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C++ copy constructor called at return

In which situations is the C++ copy constructor called?What are the differences between a pointer variable and a reference variable in C++?Why don't C++ compilers define operator== and operator!=?What is the lifetime of a static variable in a C++ function?Can I call a constructor from another constructor (do constructor chaining) in C++?Inheriting constructorsHow can I profile C++ code running on Linux?The Definitive C++ Book Guide and ListWhen to use virtual destructors?What is the “-->” operator in C++?What is the copy-and-swap idiom?

6

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++

share|improve this question

edited 6 mins ago

Sobuch

asked 1 hour ago

SobuchSobuch

335

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• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  At the moment, I can't verify that snippet because there's no B given.
  
  – Tzalumen
  57 mins ago
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  see: In which situations is the C++ copy constructor called?
  
  – kmdreko
  55 mins ago
  

  
  
  
  


• 
  
  
  

  
  2
  

  
  
  
  
  
  

  
  
  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
  55 mins ago
  

  
  
  
  

add a comment | 

6

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++

share|improve this question

edited 6 mins ago

Sobuch

asked 1 hour ago

SobuchSobuch

335

New contributor

Sobuch is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  At the moment, I can't verify that snippet because there's no B given.
  
  – Tzalumen
  57 mins ago
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  see: In which situations is the C++ copy constructor called?
  
  – kmdreko
  55 mins ago
  

  
  
  
  


• 
  
  
  

  
  2
  

  
  
  
  
  
  

  
  
  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
  55 mins ago
  

  
  
  
  

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++

share|improve this question

edited 6 mins ago

Sobuch

asked 1 hour ago

SobuchSobuch

335

New contributor

Sobuch is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.

error: use of deleted function 'A::A(const A&)'
 return tmp;
 ^~~

struct B ;

struct A
 std::unique_ptr<B> x;
 virtual ~A() = default;
;

A f() 
 A tmp;
 return tmp;

share|improve this question

edited 6 mins ago

Sobuch

asked 1 hour ago

SobuchSobuch

335

New contributor

Sobuch is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.

share|improve this question

edited 6 mins ago

Sobuch

asked 1 hour ago

SobuchSobuch

335

New contributor

Sobuch is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.

asked 1 hour ago

SobuchSobuch

335

New contributor

Sobuch is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.

asked 1 hour ago

SobuchSobuch

335

1 Answer
1

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15

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;
;

share|improve this answer

edited 35 mins ago

answered 52 mins ago

NathanOliverNathanOliver

96k16135209

• 
  
  
  

  
  4
  

  
  
  
  
  
  

  
  
  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
  50 mins ago
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  @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
  49 mins ago
  

  
  
  
  


• 
  
  
  

  
  2
  

  
  
  
  
  
  

  
  
  @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
  44 mins ago
  
  

  
  
  
  


• 
  
  
  

  
  2
  

  
  
  
  
  
  

  
  
  @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
  43 mins ago
  

  
  
  
  


• 
  
  
  

  
  2
  

  
  
  
  
  
  

  
  
  @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
  42 mins ago
  
  

  
  
  
  

 | 
show 15 more comments

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15

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;
;

share|improve this answer

edited 35 mins ago

answered 52 mins ago

NathanOliverNathanOliver

96k16135209

• 
  
  
  

  
  4
  

  
  
  
  
  
  

  
  
  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
  50 mins ago
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  @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
  49 mins ago
  

  
  
  
  


• 
  
  
  

  
  2
  

  
  
  
  
  
  

  
  
  @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
  44 mins ago
  
  

  
  
  
  


• 
  
  
  

  
  2
  

  
  
  
  
  
  

  
  
  @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
  43 mins ago
  

  
  
  
  


• 
  
  
  

  
  2
  

  
  
  
  
  
  

  
  
  @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
  42 mins ago
  
  

  
  
  
  

 | 
show 15 more comments

15

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;
;

share|improve this answer

edited 35 mins ago

answered 52 mins ago

NathanOliverNathanOliver

96k16135209

• 
  
  
  

  
  4
  

  
  
  
  
  
  

  
  
  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
  50 mins ago
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  @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
  49 mins ago
  

  
  
  
  


• 
  
  
  

  
  2
  

  
  
  
  
  
  

  
  
  @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
  44 mins ago
  
  

  
  
  
  


• 
  
  
  

  
  2
  

  
  
  
  
  
  

  
  
  @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
  43 mins ago
  

  
  
  
  


• 
  
  
  

  
  2
  

  
  
  
  
  
  

  
  
  @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
  42 mins ago
  
  

  
  
  
  

 | 
show 15 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;
;

share|improve this answer

edited 35 mins ago

answered 52 mins ago

NathanOliverNathanOliver

96k16135209

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;
;

struct C 
 virtual ~C() = default;
;

struct A : C 
 std::unique_ptr<B> x;
;

share|improve this answer

edited 35 mins ago

answered 52 mins ago

NathanOliverNathanOliver

96k16135209

answered 52 mins ago

NathanOliverNathanOliver

96k16135209

answered 52 mins ago

NathanOliverNathanOliver

96k16135209