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Why is there a dummy union member in some implemetations of std::optional?

Why is there a dummy union member in some implemetations of std::optional?

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18

1

Both libstdc++ (GNU) and libc++ (LLVM) implement std::optional value storage using a union and both of them include a dummy member.

GNU implementation:

using _Stored_type = remove_const_t<_Tp>;
struct _Empty_byte ;
union 
 _Empty_byte _M_empty;
 _Stored_type _M_payload;
;

LLVM implementation:

union

 char __null_state_;
 value_type __val_;
;

My question is: Why do we need these _M_empty/__null_state_ members? Is there something wrong with a single-member union?

c++ c++17 optional unions

share|improve this question

asked 11 hours ago

r3mus n0xr3mus n0x

4,21511 gold badge44 silver badges2727 bronze badges

add a comment | 

18

1

Both libstdc++ (GNU) and libc++ (LLVM) implement std::optional value storage using a union and both of them include a dummy member.

GNU implementation:

using _Stored_type = remove_const_t<_Tp>;
struct _Empty_byte ;
union 
 _Empty_byte _M_empty;
 _Stored_type _M_payload;
;

LLVM implementation:

union

 char __null_state_;
 value_type __val_;
;

My question is: Why do we need these _M_empty/__null_state_ members? Is there something wrong with a single-member union?

c++ c++17 optional unions

share|improve this question

asked 11 hours ago

r3mus n0xr3mus n0x

4,21511 gold badge44 silver badges2727 bronze badges

add a comment | 

Both libstdc++ (GNU) and libc++ (LLVM) implement std::optional value storage using a union and both of them include a dummy member.

GNU implementation:

using _Stored_type = remove_const_t<_Tp>;
struct _Empty_byte ;
union 
 _Empty_byte _M_empty;
 _Stored_type _M_payload;
;

LLVM implementation:

union

 char __null_state_;
 value_type __val_;
;

My question is: Why do we need these _M_empty/__null_state_ members? Is there something wrong with a single-member union?

c++ c++17 optional unions

share|improve this question

asked 11 hours ago

r3mus n0xr3mus n0x

4,21511 gold badge44 silver badges2727 bronze badges

using _Stored_type = remove_const_t<_Tp>;
struct _Empty_byte ;
union 
 _Empty_byte _M_empty;
 _Stored_type _M_payload;
;

union

 char __null_state_;
 value_type __val_;
;

share|improve this question

asked 11 hours ago

r3mus n0xr3mus n0x

4,21511 gold badge44 silver badges2727 bronze badges

share|improve this question

asked 11 hours ago

r3mus n0xr3mus n0x

4,21511 gold badge44 silver badges2727 bronze badges

asked 11 hours ago

r3mus n0xr3mus n0x

4,21511 gold badge44 silver badges2727 bronze badges

asked 11 hours ago

r3mus n0xr3mus n0x

4,21511 gold badge44 silver badges2727 bronze badges

1 Answer
1

active

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20

This is because when using a non-trivial type in a union, suddently the default constructor is deleted.

This creates all sorts of problem when trying to be constexpr compatible.

Consider this code:

struct nontrivial 
 constexpr nontrivial(int o) : uo 
 int u;
;

union storage 
 nontrivial nt;
;

struct optional 
 storage s;
;

constexpr auto run() -> int 
 optional o;
 return o.s.nt.u;


int main() 
 constexpr int t = run();

This is ill formed because optional has a deleted constructor.

Then a simple fix would be to add a constructor that initialize no union member:

union storage 
 constexpr storage() // standard says no
 nontrivial nt;
;

But it won't work. Constexpr unions must have at least one active member. It cannot be an empty union. To workaround this limitation, a dummy member is added. This makes std::optional useable in constexpr context.

share|improve this answer

edited 10 hours ago

answered 10 hours ago

Guillaume RacicotGuillaume Racicot

19.7k55 gold badges4141 silver badges7979 bronze badges

• 
  
  
  

  
  6
  

  
  
  
  
  
  

  
  
  For those hunting for the standard quote: eel.is/c++draft/dcl.dcl#dcl.constexpr-4.1
  
  – Barry
  10 hours ago
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  @TedLyngmo, which version? Tried on 7.4.0 - got an error.
  
  – r3mus n0x
  9 hours ago
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  @r3musn0x 9.1.1. I don't think 7.4.0 implements LWG 2900.
  
  – Ted Lyngmo
  9 hours ago
  
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  @TedLyngmo, it looks to me that LWG 2900 is only related to the library implementation of optional, so I don't really see the connection... Anyway look here - an error! :)
  
  – r3mus n0x
  9 hours ago
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  @r3musn0x You are probably correct. Here's a comparison between g++ and clang++ and also a bugreport that seems to be connected to this: bugzilla #886581 It seems to hit my example when templates are involved, but in the bugreport they've managed to trigger the bug without templates.
  
  – Ted Lyngmo
  9 hours ago
  
  

  
  
  
  

add a comment | 

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20

This is because when using a non-trivial type in a union, suddently the default constructor is deleted.

This creates all sorts of problem when trying to be constexpr compatible.

Consider this code:

struct nontrivial 
 constexpr nontrivial(int o) : uo 
 int u;
;

union storage 
 nontrivial nt;
;

struct optional 
 storage s;
;

constexpr auto run() -> int 
 optional o;
 return o.s.nt.u;


int main() 
 constexpr int t = run();

This is ill formed because optional has a deleted constructor.

Then a simple fix would be to add a constructor that initialize no union member:

union storage 
 constexpr storage() // standard says no
 nontrivial nt;
;

But it won't work. Constexpr unions must have at least one active member. It cannot be an empty union. To workaround this limitation, a dummy member is added. This makes std::optional useable in constexpr context.

share|improve this answer

edited 10 hours ago

answered 10 hours ago

Guillaume RacicotGuillaume Racicot

19.7k55 gold badges4141 silver badges7979 bronze badges

• 
  
  
  

  
  6
  

  
  
  
  
  
  

  
  
  For those hunting for the standard quote: eel.is/c++draft/dcl.dcl#dcl.constexpr-4.1
  
  – Barry
  10 hours ago
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  @TedLyngmo, which version? Tried on 7.4.0 - got an error.
  
  – r3mus n0x
  9 hours ago
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  @r3musn0x 9.1.1. I don't think 7.4.0 implements LWG 2900.
  
  – Ted Lyngmo
  9 hours ago
  
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  @TedLyngmo, it looks to me that LWG 2900 is only related to the library implementation of optional, so I don't really see the connection... Anyway look here - an error! :)
  
  – r3mus n0x
  9 hours ago
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  @r3musn0x You are probably correct. Here's a comparison between g++ and clang++ and also a bugreport that seems to be connected to this: bugzilla #886581 It seems to hit my example when templates are involved, but in the bugreport they've managed to trigger the bug without templates.
  
  – Ted Lyngmo
  9 hours ago
  
  

  
  
  
  

add a comment | 

20

This is because when using a non-trivial type in a union, suddently the default constructor is deleted.

This creates all sorts of problem when trying to be constexpr compatible.

Consider this code:

struct nontrivial 
 constexpr nontrivial(int o) : uo 
 int u;
;

union storage 
 nontrivial nt;
;

struct optional 
 storage s;
;

constexpr auto run() -> int 
 optional o;
 return o.s.nt.u;


int main() 
 constexpr int t = run();

This is ill formed because optional has a deleted constructor.

Then a simple fix would be to add a constructor that initialize no union member:

union storage 
 constexpr storage() // standard says no
 nontrivial nt;
;

But it won't work. Constexpr unions must have at least one active member. It cannot be an empty union. To workaround this limitation, a dummy member is added. This makes std::optional useable in constexpr context.

share|improve this answer

edited 10 hours ago

answered 10 hours ago

Guillaume RacicotGuillaume Racicot

19.7k55 gold badges4141 silver badges7979 bronze badges

• 
  
  
  

  
  6
  

  
  
  
  
  
  

  
  
  For those hunting for the standard quote: eel.is/c++draft/dcl.dcl#dcl.constexpr-4.1
  
  – Barry
  10 hours ago
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  @TedLyngmo, which version? Tried on 7.4.0 - got an error.
  
  – r3mus n0x
  9 hours ago
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  @r3musn0x 9.1.1. I don't think 7.4.0 implements LWG 2900.
  
  – Ted Lyngmo
  9 hours ago
  
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  @TedLyngmo, it looks to me that LWG 2900 is only related to the library implementation of optional, so I don't really see the connection... Anyway look here - an error! :)
  
  – r3mus n0x
  9 hours ago
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  @r3musn0x You are probably correct. Here's a comparison between g++ and clang++ and also a bugreport that seems to be connected to this: bugzilla #886581 It seems to hit my example when templates are involved, but in the bugreport they've managed to trigger the bug without templates.
  
  – Ted Lyngmo
  9 hours ago
  
  

  
  
  
  

add a comment | 

This is because when using a non-trivial type in a union, suddently the default constructor is deleted.

This creates all sorts of problem when trying to be constexpr compatible.

Consider this code:

struct nontrivial 
 constexpr nontrivial(int o) : uo 
 int u;
;

union storage 
 nontrivial nt;
;

struct optional 
 storage s;
;

constexpr auto run() -> int 
 optional o;
 return o.s.nt.u;


int main() 
 constexpr int t = run();

This is ill formed because optional has a deleted constructor.

Then a simple fix would be to add a constructor that initialize no union member:

union storage 
 constexpr storage() // standard says no
 nontrivial nt;
;

But it won't work. Constexpr unions must have at least one active member. It cannot be an empty union. To workaround this limitation, a dummy member is added. This makes std::optional useable in constexpr context.

share|improve this answer

edited 10 hours ago

answered 10 hours ago

Guillaume RacicotGuillaume Racicot

19.7k55 gold badges4141 silver badges7979 bronze badges

struct nontrivial 
 constexpr nontrivial(int o) : uo 
 int u;
;

union storage 
 nontrivial nt;
;

struct optional 
 storage s;
;

constexpr auto run() -> int 
 optional o;
 return o.s.nt.u;


int main() 
 constexpr int t = run();

union storage 
 constexpr storage() // standard says no
 nontrivial nt;
;

share|improve this answer

edited 10 hours ago

answered 10 hours ago

Guillaume RacicotGuillaume Racicot

19.7k55 gold badges4141 silver badges7979 bronze badges

answered 10 hours ago

Guillaume RacicotGuillaume Racicot

19.7k55 gold badges4141 silver badges7979 bronze badges

answered 10 hours ago

Guillaume RacicotGuillaume Racicot

19.7k55 gold badges4141 silver badges7979 bronze badges