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21

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 2 days ago

r3mus n0xr3mus n0x

4,25011 gold badge44 silver badges2727 bronze badges

add a comment | 

21

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 2 days ago

r3mus n0xr3mus n0x

4,25011 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 2 days ago

r3mus n0xr3mus n0x

4,25011 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 2 days ago

r3mus n0xr3mus n0x

4,25011 gold badge44 silver badges2727 bronze badges

share|improve this question

asked 2 days ago

r3mus n0xr3mus n0x

4,25011 gold badge44 silver badges2727 bronze badges

asked 2 days ago

r3mus n0xr3mus n0x

4,25011 gold badge44 silver badges2727 bronze badges

asked 2 days ago

r3mus n0xr3mus n0x

4,25011 gold badge44 silver badges2727 bronze badges

1 Answer
1

active

oldest

votes

23

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) : u{o} {}

    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 2 days ago

answered 2 days ago

Guillaume RacicotGuillaume Racicot

19.8k55 gold badges4141 silver badges7979 bronze badges

• 
  
  
  

  
  7
  

  
  
  
  
  
  

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

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

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

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

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

  
  
  
  

add a comment | 

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23

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) : u{o} {}

    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 2 days ago

answered 2 days ago

Guillaume RacicotGuillaume Racicot

19.8k55 gold badges4141 silver badges7979 bronze badges

• 
  
  
  

  
  7
  

  
  
  
  
  
  

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

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

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

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

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

  
  
  
  

add a comment | 

23

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) : u{o} {}

    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 2 days ago

answered 2 days ago

Guillaume RacicotGuillaume Racicot

19.8k55 gold badges4141 silver badges7979 bronze badges

• 
  
  
  

  
  7
  

  
  
  
  
  
  

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

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

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

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  @r3musn0x 9.1.1. I don't think 7.4.0 implements LWG 2900.
  
  – Ted Lyngmo
  2 days 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
  2 days 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
  2 days 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) : u{o} {}

    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 2 days ago

answered 2 days ago

Guillaume RacicotGuillaume Racicot

19.8k55 gold badges4141 silver badges7979 bronze badges

struct nontrivial {

    constexpr nontrivial(int o) : u{o} {}

    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 2 days ago

answered 2 days ago

Guillaume RacicotGuillaume Racicot

19.8k55 gold badges4141 silver badges7979 bronze badges

answered 2 days ago

Guillaume RacicotGuillaume Racicot

19.8k55 gold badges4141 silver badges7979 bronze badges

answered 2 days ago

Guillaume RacicotGuillaume Racicot

19.8k55 gold badges4141 silver badges7979 bronze badges