Why is boost::optional not convertable to bool for the purposes of std::is_convertible












4















I have



auto result = std::is_convertible
< boost::optional<int>
, bool
>::value;

static_assert( result , "task should return bool" );


and it fails to compile. The definition of std::is_convertible is



template< class From, class To > struct is_convertible;


and optional is clearly convertible to boolean because we always use it like



void(boost::optional<int> const & value){
if(value){
std::cerr << *value << endl;
}
}


what am I missing here?










share|improve this question



























    4















    I have



    auto result = std::is_convertible
    < boost::optional<int>
    , bool
    >::value;

    static_assert( result , "task should return bool" );


    and it fails to compile. The definition of std::is_convertible is



    template< class From, class To > struct is_convertible;


    and optional is clearly convertible to boolean because we always use it like



    void(boost::optional<int> const & value){
    if(value){
    std::cerr << *value << endl;
    }
    }


    what am I missing here?










    share|improve this question

























      4












      4








      4








      I have



      auto result = std::is_convertible
      < boost::optional<int>
      , bool
      >::value;

      static_assert( result , "task should return bool" );


      and it fails to compile. The definition of std::is_convertible is



      template< class From, class To > struct is_convertible;


      and optional is clearly convertible to boolean because we always use it like



      void(boost::optional<int> const & value){
      if(value){
      std::cerr << *value << endl;
      }
      }


      what am I missing here?










      share|improve this question














      I have



      auto result = std::is_convertible
      < boost::optional<int>
      , bool
      >::value;

      static_assert( result , "task should return bool" );


      and it fails to compile. The definition of std::is_convertible is



      template< class From, class To > struct is_convertible;


      and optional is clearly convertible to boolean because we always use it like



      void(boost::optional<int> const & value){
      if(value){
      std::cerr << *value << endl;
      }
      }


      what am I missing here?







      c++ boost-optional






      share|improve this question













      share|improve this question











      share|improve this question




      share|improve this question










      asked Nov 15 '18 at 12:11









      bradgonesurfingbradgonesurfing

      16.4k1084151




      16.4k1084151
























          1 Answer
          1






          active

          oldest

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          10














          boost::optional's operator bool is explicit. It works inside an if's condition, because it is a contextual conversion.



          You need std::is_constructible, which tries to perform an explicit conversion.



          The following compiles



          static_assert
          ( std::is_constructible<bool, boost::optional<int>>::value
          , "msg" );


          and the following fails to compile because optional is not convertible to int



          static_assert
          ( std::is_constructible<int, boost::optional<int>>::value
          , "msg" );





          share|improve this answer


























          • Could you please elaborate on how to use std::is_constructible to detect if I can convert the type to bool? I'm not sure how to go about it.

            – bradgonesurfing
            Nov 15 '18 at 12:25











          • @bradgonesurfing There's a gotcha in that the arguments are reversed compared to std::is_convertible, but it should be as simple as std::is_constructible<bool, boost::optional<int>>.

            – Quentin
            Nov 15 '18 at 12:26






          • 1





            Thanks @quentin I added the examples to the answer so it's clear to the next person coming this way :)

            – bradgonesurfing
            Nov 15 '18 at 12:32











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          1 Answer
          1






          active

          oldest

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          1 Answer
          1






          active

          oldest

          votes









          active

          oldest

          votes






          active

          oldest

          votes









          10














          boost::optional's operator bool is explicit. It works inside an if's condition, because it is a contextual conversion.



          You need std::is_constructible, which tries to perform an explicit conversion.



          The following compiles



          static_assert
          ( std::is_constructible<bool, boost::optional<int>>::value
          , "msg" );


          and the following fails to compile because optional is not convertible to int



          static_assert
          ( std::is_constructible<int, boost::optional<int>>::value
          , "msg" );





          share|improve this answer


























          • Could you please elaborate on how to use std::is_constructible to detect if I can convert the type to bool? I'm not sure how to go about it.

            – bradgonesurfing
            Nov 15 '18 at 12:25











          • @bradgonesurfing There's a gotcha in that the arguments are reversed compared to std::is_convertible, but it should be as simple as std::is_constructible<bool, boost::optional<int>>.

            – Quentin
            Nov 15 '18 at 12:26






          • 1





            Thanks @quentin I added the examples to the answer so it's clear to the next person coming this way :)

            – bradgonesurfing
            Nov 15 '18 at 12:32
















          10














          boost::optional's operator bool is explicit. It works inside an if's condition, because it is a contextual conversion.



          You need std::is_constructible, which tries to perform an explicit conversion.



          The following compiles



          static_assert
          ( std::is_constructible<bool, boost::optional<int>>::value
          , "msg" );


          and the following fails to compile because optional is not convertible to int



          static_assert
          ( std::is_constructible<int, boost::optional<int>>::value
          , "msg" );





          share|improve this answer


























          • Could you please elaborate on how to use std::is_constructible to detect if I can convert the type to bool? I'm not sure how to go about it.

            – bradgonesurfing
            Nov 15 '18 at 12:25











          • @bradgonesurfing There's a gotcha in that the arguments are reversed compared to std::is_convertible, but it should be as simple as std::is_constructible<bool, boost::optional<int>>.

            – Quentin
            Nov 15 '18 at 12:26






          • 1





            Thanks @quentin I added the examples to the answer so it's clear to the next person coming this way :)

            – bradgonesurfing
            Nov 15 '18 at 12:32














          10












          10








          10







          boost::optional's operator bool is explicit. It works inside an if's condition, because it is a contextual conversion.



          You need std::is_constructible, which tries to perform an explicit conversion.



          The following compiles



          static_assert
          ( std::is_constructible<bool, boost::optional<int>>::value
          , "msg" );


          and the following fails to compile because optional is not convertible to int



          static_assert
          ( std::is_constructible<int, boost::optional<int>>::value
          , "msg" );





          share|improve this answer















          boost::optional's operator bool is explicit. It works inside an if's condition, because it is a contextual conversion.



          You need std::is_constructible, which tries to perform an explicit conversion.



          The following compiles



          static_assert
          ( std::is_constructible<bool, boost::optional<int>>::value
          , "msg" );


          and the following fails to compile because optional is not convertible to int



          static_assert
          ( std::is_constructible<int, boost::optional<int>>::value
          , "msg" );






          share|improve this answer














          share|improve this answer



          share|improve this answer








          edited Nov 15 '18 at 12:31









          bradgonesurfing

          16.4k1084151




          16.4k1084151










          answered Nov 15 '18 at 12:19









          QuentinQuentin

          46.5k589146




          46.5k589146













          • Could you please elaborate on how to use std::is_constructible to detect if I can convert the type to bool? I'm not sure how to go about it.

            – bradgonesurfing
            Nov 15 '18 at 12:25











          • @bradgonesurfing There's a gotcha in that the arguments are reversed compared to std::is_convertible, but it should be as simple as std::is_constructible<bool, boost::optional<int>>.

            – Quentin
            Nov 15 '18 at 12:26






          • 1





            Thanks @quentin I added the examples to the answer so it's clear to the next person coming this way :)

            – bradgonesurfing
            Nov 15 '18 at 12:32



















          • Could you please elaborate on how to use std::is_constructible to detect if I can convert the type to bool? I'm not sure how to go about it.

            – bradgonesurfing
            Nov 15 '18 at 12:25











          • @bradgonesurfing There's a gotcha in that the arguments are reversed compared to std::is_convertible, but it should be as simple as std::is_constructible<bool, boost::optional<int>>.

            – Quentin
            Nov 15 '18 at 12:26






          • 1





            Thanks @quentin I added the examples to the answer so it's clear to the next person coming this way :)

            – bradgonesurfing
            Nov 15 '18 at 12:32

















          Could you please elaborate on how to use std::is_constructible to detect if I can convert the type to bool? I'm not sure how to go about it.

          – bradgonesurfing
          Nov 15 '18 at 12:25





          Could you please elaborate on how to use std::is_constructible to detect if I can convert the type to bool? I'm not sure how to go about it.

          – bradgonesurfing
          Nov 15 '18 at 12:25













          @bradgonesurfing There's a gotcha in that the arguments are reversed compared to std::is_convertible, but it should be as simple as std::is_constructible<bool, boost::optional<int>>.

          – Quentin
          Nov 15 '18 at 12:26





          @bradgonesurfing There's a gotcha in that the arguments are reversed compared to std::is_convertible, but it should be as simple as std::is_constructible<bool, boost::optional<int>>.

          – Quentin
          Nov 15 '18 at 12:26




          1




          1





          Thanks @quentin I added the examples to the answer so it's clear to the next person coming this way :)

          – bradgonesurfing
          Nov 15 '18 at 12:32





          Thanks @quentin I added the examples to the answer so it's clear to the next person coming this way :)

          – bradgonesurfing
          Nov 15 '18 at 12:32




















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