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C++ copy constructor called at return

When to use virtual destructors?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?

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;

edited 47 mins ago

asked 1 hour ago

Sobuch

385

New contributor

see: In which situations is the C++ copy constructor called?

– kmdreko
1 hour ago

3

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
1 hour 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;

edited 47 mins ago

asked 1 hour ago

Sobuch

385

New contributor

see: In which situations is the C++ copy constructor called?

– kmdreko
1 hour ago

3

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
1 hour 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;

edited 47 mins ago

asked 1 hour ago

Sobuch

385

New contributor

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++

edited 47 mins ago

asked 1 hour ago

Sobuch

385

New contributor

edited 47 mins ago

asked 1 hour ago

Sobuch

385

New contributor

edited 47 mins ago

asked 1 hour ago

Sobuch

385

New contributor

asked 1 hour ago

Sobuch

385

asked 1 hour ago

Sobuch

385

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.

see: In which situations is the C++ copy constructor called?

– kmdreko
1 hour ago

3

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
1 hour ago

add a comment |

see: In which situations is the C++ copy constructor called?

– kmdreko
1 hour ago

3

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
1 hour ago

see: In which situations is the C++ copy constructor called?

– kmdreko
1 hour ago

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
1 hour ago

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;
;

edited 1 hour ago

answered 1 hour ago

NathanOliver

96k16135209

5

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
1 hour 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
1 hour 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
1 hour 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
1 hour 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
1 hour ago

|
show 23 more comments

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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;
;

edited 1 hour ago

answered 1 hour ago

NathanOliver

96k16135209

5

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
1 hour 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
1 hour 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
1 hour 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
1 hour 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
1 hour ago

|
show 23 more comments

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;
;

edited 1 hour ago

answered 1 hour ago

NathanOliver

96k16135209

5

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
1 hour 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
1 hour 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
1 hour 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
1 hour 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
1 hour ago

|
show 23 more comments

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;
;

edited 1 hour ago

answered 1 hour ago

NathanOliver

96k16135209

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;
;

edited 1 hour ago

answered 1 hour ago

NathanOliver

96k16135209

edited 1 hour ago

answered 1 hour ago

NathanOliver

96k16135209

answered 1 hour ago

NathanOliver

96k16135209

answered 1 hour ago

NathanOliver

96k16135209

5

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
1 hour 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
1 hour 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
1 hour 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
1 hour 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
1 hour ago

|
show 23 more comments

5

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
1 hour 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
1 hour 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
1 hour 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
1 hour 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
1 hour ago

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
1 hour ago

@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
1 hour ago

@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
1 hour ago

@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
1 hour ago

@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
1 hour ago

|
show 23 more comments

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