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std::is_constructible on incomplete types

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22

2

I have the following code:

#include <iostream>

class A;

int main()

 std::cout << std::is_constructible<A>::value << std::endl;

When I use GCC 8.3, this code compiles. However, when I use Clang 8.0, I get a compilation error that incomplete types cannot be used in type traits.

Which one is correct? Am I allowed to use is_constructible on an incomplete type (with an expected value of false), or am I not allowed to?

c++ c++11 language-lawyer

share|improve this question

edited Apr 24 at 14:02

StoryTeller

107k15224288

asked Apr 24 at 13:42

R_KappR_Kapp

1,94711124

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Related question SFINAE with std::enable_if and std::is_default_constructible for incomplete type in libc++
  
  – t.niese
  Apr 24 at 13:47
  
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  How would is_constructible determine if A is constructible if A is not a complete type?
  
  – François Andrieux
  Apr 24 at 13:56
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  @FrançoisAndrieux: If A isn't complete, we can't construct it (A a; on an incomplete class A causes a compiler error) - that's the way it was working in our code base in the past, but it appears that was just luck of the draw due to undefined behavior.
  
  – R_Kapp
  Apr 24 at 13:59
  

  
  
  
  


• 
  
  
  

  
  3
  

  
  
  
  
  
  

  
  
  Re: "we can't construct it" -- type traits are about the properties of a type. It would be really disturbing if std::is_constuctible<A> changed its result depending on what headers you included. In one place it's constructible and in another it isn't?
  
  – Pete Becker
  Apr 24 at 14:33
  
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  @PeteBecker: That's a fair point that I hadn't considered - the original use case was a templated class A that only had specific parameters instantiated. We then used SFINAE with std::is_constructible so that only certain types of types were allowed through. This is easily done correctly by explicitly instantiating a default class A where constructors are explicitly deleted (which is what I've done), but the thought process was from that angle. Hadn't considered the (what should have been obvious) "forward-declared-class-that-actually-exists" angle...
  
  – R_Kapp
  Apr 24 at 19:10
  

  
  
  
  

add a comment | 

22

2

I have the following code:

#include <iostream>

class A;

int main()

 std::cout << std::is_constructible<A>::value << std::endl;

When I use GCC 8.3, this code compiles. However, when I use Clang 8.0, I get a compilation error that incomplete types cannot be used in type traits.

Which one is correct? Am I allowed to use is_constructible on an incomplete type (with an expected value of false), or am I not allowed to?

c++ c++11 language-lawyer

share|improve this question

edited Apr 24 at 14:02

StoryTeller

107k15224288

asked Apr 24 at 13:42

R_KappR_Kapp

1,94711124

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Related question SFINAE with std::enable_if and std::is_default_constructible for incomplete type in libc++
  
  – t.niese
  Apr 24 at 13:47
  
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  How would is_constructible determine if A is constructible if A is not a complete type?
  
  – François Andrieux
  Apr 24 at 13:56
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  @FrançoisAndrieux: If A isn't complete, we can't construct it (A a; on an incomplete class A causes a compiler error) - that's the way it was working in our code base in the past, but it appears that was just luck of the draw due to undefined behavior.
  
  – R_Kapp
  Apr 24 at 13:59
  

  
  
  
  


• 
  
  
  

  
  3
  

  
  
  
  
  
  

  
  
  Re: "we can't construct it" -- type traits are about the properties of a type. It would be really disturbing if std::is_constuctible<A> changed its result depending on what headers you included. In one place it's constructible and in another it isn't?
  
  – Pete Becker
  Apr 24 at 14:33
  
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  @PeteBecker: That's a fair point that I hadn't considered - the original use case was a templated class A that only had specific parameters instantiated. We then used SFINAE with std::is_constructible so that only certain types of types were allowed through. This is easily done correctly by explicitly instantiating a default class A where constructors are explicitly deleted (which is what I've done), but the thought process was from that angle. Hadn't considered the (what should have been obvious) "forward-declared-class-that-actually-exists" angle...
  
  – R_Kapp
  Apr 24 at 19:10
  

  
  
  
  

add a comment | 

I have the following code:

#include <iostream>

class A;

int main()

 std::cout << std::is_constructible<A>::value << std::endl;

When I use GCC 8.3, this code compiles. However, when I use Clang 8.0, I get a compilation error that incomplete types cannot be used in type traits.

Which one is correct? Am I allowed to use is_constructible on an incomplete type (with an expected value of false), or am I not allowed to?

c++ c++11 language-lawyer

share|improve this question

edited Apr 24 at 14:02

StoryTeller

107k15224288

asked Apr 24 at 13:42

R_KappR_Kapp

1,94711124

#include <iostream>

class A;

int main()

 std::cout << std::is_constructible<A>::value << std::endl;

share|improve this question

edited Apr 24 at 14:02

StoryTeller

107k15224288

asked Apr 24 at 13:42

R_KappR_Kapp

1,94711124

share|improve this question

edited Apr 24 at 14:02

StoryTeller

107k15224288

asked Apr 24 at 13:42

R_KappR_Kapp

1,94711124

edited Apr 24 at 14:02

StoryTeller

107k15224288

edited Apr 24 at 14:02

StoryTeller

107k15224288

asked Apr 24 at 13:42

R_KappR_Kapp

1,94711124

asked Apr 24 at 13:42

R_KappR_Kapp

1,94711124

3 Answers
3

active

oldest

votes

22

The behavior is undefined.

[meta.unary.prop]

template <class T, class... Args> struct is_constructible;

T and all types in the parameter pack Args shall be complete types,
(possibly cv-qualified) void, or arrays of unknown bound.

That's a precondition of the meta-function. A contract that your code violates. libc++ is being generous by notifying you.

---

Mind you, that putting that precondition there and leaving it undefined otherwise is for a reason. A program where two points of instantiation of a template have different meanings is ill-formed NDR. The only sane course of action is demand complete types. And after all, that's when the trait is most useful anyway.

share|improve this answer

edited Apr 24 at 18:23

answered Apr 24 at 13:47

StoryTellerStoryTeller

107k15224288

add a comment | 

16

Your code causes undefined behavior.

Cppreference states:

template< class T, class... Args >
struct is_constructible;

T and all types in the parameter pack Args shall each be a complete type, (possibly cv-qualified) void, or an array of unknown bound. Otherwise, the behavior is undefined.

share|improve this answer

answered Apr 24 at 13:47

Nellie DanielyanNellie Danielyan

807517

add a comment | 

7

Your code has undefined behavior. Per [meta.unary.prop] table 47 std::is_constructible requires

T and all types in the template parameter pack Args shall be complete types, cv void, or arrays of unknown bound.

^{emphasis mine}

share|improve this answer

answered Apr 24 at 13:48

NathanOliverNathanOliver

100k16139221

add a comment | 

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22

The behavior is undefined.

[meta.unary.prop]

template <class T, class... Args> struct is_constructible;

T and all types in the parameter pack Args shall be complete types,
(possibly cv-qualified) void, or arrays of unknown bound.

That's a precondition of the meta-function. A contract that your code violates. libc++ is being generous by notifying you.

---

Mind you, that putting that precondition there and leaving it undefined otherwise is for a reason. A program where two points of instantiation of a template have different meanings is ill-formed NDR. The only sane course of action is demand complete types. And after all, that's when the trait is most useful anyway.

share|improve this answer

edited Apr 24 at 18:23

answered Apr 24 at 13:47

StoryTellerStoryTeller

107k15224288

add a comment | 

22

The behavior is undefined.

[meta.unary.prop]

template <class T, class... Args> struct is_constructible;

T and all types in the parameter pack Args shall be complete types,
(possibly cv-qualified) void, or arrays of unknown bound.

That's a precondition of the meta-function. A contract that your code violates. libc++ is being generous by notifying you.

---

Mind you, that putting that precondition there and leaving it undefined otherwise is for a reason. A program where two points of instantiation of a template have different meanings is ill-formed NDR. The only sane course of action is demand complete types. And after all, that's when the trait is most useful anyway.

share|improve this answer

edited Apr 24 at 18:23

answered Apr 24 at 13:47

StoryTellerStoryTeller

107k15224288

add a comment | 

The behavior is undefined.

[meta.unary.prop]

template <class T, class... Args> struct is_constructible;

T and all types in the parameter pack Args shall be complete types,
(possibly cv-qualified) void, or arrays of unknown bound.

That's a precondition of the meta-function. A contract that your code violates. libc++ is being generous by notifying you.

---

Mind you, that putting that precondition there and leaving it undefined otherwise is for a reason. A program where two points of instantiation of a template have different meanings is ill-formed NDR. The only sane course of action is demand complete types. And after all, that's when the trait is most useful anyway.

share|improve this answer

edited Apr 24 at 18:23

answered Apr 24 at 13:47

StoryTellerStoryTeller

107k15224288

share|improve this answer

edited Apr 24 at 18:23

answered Apr 24 at 13:47

StoryTellerStoryTeller

107k15224288

answered Apr 24 at 13:47

StoryTellerStoryTeller

107k15224288

answered Apr 24 at 13:47

StoryTellerStoryTeller

107k15224288

16

Your code causes undefined behavior.

Cppreference states:

template< class T, class... Args >
struct is_constructible;

T and all types in the parameter pack Args shall each be a complete type, (possibly cv-qualified) void, or an array of unknown bound. Otherwise, the behavior is undefined.

share|improve this answer

answered Apr 24 at 13:47

Nellie DanielyanNellie Danielyan

807517

add a comment | 

16

Your code causes undefined behavior.

Cppreference states:

template< class T, class... Args >
struct is_constructible;

T and all types in the parameter pack Args shall each be a complete type, (possibly cv-qualified) void, or an array of unknown bound. Otherwise, the behavior is undefined.

share|improve this answer

answered Apr 24 at 13:47

Nellie DanielyanNellie Danielyan

807517

add a comment | 

Your code causes undefined behavior.

Cppreference states:

template< class T, class... Args >
struct is_constructible;

T and all types in the parameter pack Args shall each be a complete type, (possibly cv-qualified) void, or an array of unknown bound. Otherwise, the behavior is undefined.

share|improve this answer

answered Apr 24 at 13:47

Nellie DanielyanNellie Danielyan

807517

share|improve this answer

answered Apr 24 at 13:47

Nellie DanielyanNellie Danielyan

807517

answered Apr 24 at 13:47

Nellie DanielyanNellie Danielyan

807517

answered Apr 24 at 13:47

Nellie DanielyanNellie Danielyan

807517

7

Your code has undefined behavior. Per [meta.unary.prop] table 47 std::is_constructible requires

T and all types in the template parameter pack Args shall be complete types, cv void, or arrays of unknown bound.

^{emphasis mine}

share|improve this answer

answered Apr 24 at 13:48

NathanOliverNathanOliver

100k16139221

add a comment | 

7

Your code has undefined behavior. Per [meta.unary.prop] table 47 std::is_constructible requires

T and all types in the template parameter pack Args shall be complete types, cv void, or arrays of unknown bound.

^{emphasis mine}

share|improve this answer

answered Apr 24 at 13:48

NathanOliverNathanOliver

100k16139221

add a comment | 

Your code has undefined behavior. Per [meta.unary.prop] table 47 std::is_constructible requires

T and all types in the template parameter pack Args shall be complete types, cv void, or arrays of unknown bound.

^{emphasis mine}

share|improve this answer

answered Apr 24 at 13:48

NathanOliverNathanOliver

100k16139221

share|improve this answer

answered Apr 24 at 13:48

NathanOliverNathanOliver

100k16139221

answered Apr 24 at 13:48

NathanOliverNathanOliver

100k16139221

answered Apr 24 at 13:48

NathanOliverNathanOliver

100k16139221