Structured binding on const












7















Is the following code supposed to compile?



void foo() {
const std::pair<int, int> x = {1, 2};

auto [a, b] = x;

static_assert(std::is_const_v<decltype(a)>);
static_assert(std::is_const_v<decltype(b)>);
}




  • MSVC says "yes!".


  • GCC says "oh no, man!".


  • Clang says "no way!".




So, is this an MSVC bug?



The standard is not straightforward here (I had a quick look), but considering the rules for auto, I suppose, a and b should be copied discarding cv-qualifier.










share|improve this question



























    7















    Is the following code supposed to compile?



    void foo() {
    const std::pair<int, int> x = {1, 2};

    auto [a, b] = x;

    static_assert(std::is_const_v<decltype(a)>);
    static_assert(std::is_const_v<decltype(b)>);
    }




    • MSVC says "yes!".


    • GCC says "oh no, man!".


    • Clang says "no way!".




    So, is this an MSVC bug?



    The standard is not straightforward here (I had a quick look), but considering the rules for auto, I suppose, a and b should be copied discarding cv-qualifier.










    share|improve this question

























      7












      7








      7








      Is the following code supposed to compile?



      void foo() {
      const std::pair<int, int> x = {1, 2};

      auto [a, b] = x;

      static_assert(std::is_const_v<decltype(a)>);
      static_assert(std::is_const_v<decltype(b)>);
      }




      • MSVC says "yes!".


      • GCC says "oh no, man!".


      • Clang says "no way!".




      So, is this an MSVC bug?



      The standard is not straightforward here (I had a quick look), but considering the rules for auto, I suppose, a and b should be copied discarding cv-qualifier.










      share|improve this question














      Is the following code supposed to compile?



      void foo() {
      const std::pair<int, int> x = {1, 2};

      auto [a, b] = x;

      static_assert(std::is_const_v<decltype(a)>);
      static_assert(std::is_const_v<decltype(b)>);
      }




      • MSVC says "yes!".


      • GCC says "oh no, man!".


      • Clang says "no way!".




      So, is this an MSVC bug?



      The standard is not straightforward here (I had a quick look), but considering the rules for auto, I suppose, a and b should be copied discarding cv-qualifier.







      c++ c++17 structured-bindings






      share|improve this question













      share|improve this question











      share|improve this question




      share|improve this question










      asked 6 hours ago









      Biagio FestaBiagio Festa

      5,19321239




      5,19321239
























          2 Answers
          2






          active

          oldest

          votes


















          8















          Is the following code supposed to compile?




          It is not. This is an MSVC bug.



          A structured binding declaration introduces a new name (for specification only), e, that is declared like:



          auto e = x;


          The type of e is called E, and since the initializer is tuple-like, the types of the bindings are given by tuple_element_t<i, E>. In this case E is pair<int, int>, so the two types are just int. The rule for decltype of a structured binding is to give the referenced type, so decltype(a) and decltype(b) are both int.



          The important part here is that a and b (the structured bindings) come from the invented variable (e), and not its initializer (x). e is not const because you just declared it auto. What we're doing is copying x, and then taking bindings into this (non-const) copy.






          share|improve this answer































            3














            The static assertions should fail (so this would be an MSVC bug I guess). Why? Because it's basically the same as the case of:



            void foo() {
            const int x_1 = 1;
            const int x_2 = 2;

            auto a = x_1;
            auto b = x_2;

            static_assert(std::is_const_v<decltype(a)>);
            static_assert(std::is_const_v<decltype(b)>);
            }


            which does indeed fail on MSVC as well.



            In C++ expression types decay on assignment: the auto sees an int, not a const int. Structured binding simply lets you do more than a single auto binding at a time.






            share|improve this answer

























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              2 Answers
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              active

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              2 Answers
              2






              active

              oldest

              votes









              active

              oldest

              votes






              active

              oldest

              votes









              8















              Is the following code supposed to compile?




              It is not. This is an MSVC bug.



              A structured binding declaration introduces a new name (for specification only), e, that is declared like:



              auto e = x;


              The type of e is called E, and since the initializer is tuple-like, the types of the bindings are given by tuple_element_t<i, E>. In this case E is pair<int, int>, so the two types are just int. The rule for decltype of a structured binding is to give the referenced type, so decltype(a) and decltype(b) are both int.



              The important part here is that a and b (the structured bindings) come from the invented variable (e), and not its initializer (x). e is not const because you just declared it auto. What we're doing is copying x, and then taking bindings into this (non-const) copy.






              share|improve this answer




























                8















                Is the following code supposed to compile?




                It is not. This is an MSVC bug.



                A structured binding declaration introduces a new name (for specification only), e, that is declared like:



                auto e = x;


                The type of e is called E, and since the initializer is tuple-like, the types of the bindings are given by tuple_element_t<i, E>. In this case E is pair<int, int>, so the two types are just int. The rule for decltype of a structured binding is to give the referenced type, so decltype(a) and decltype(b) are both int.



                The important part here is that a and b (the structured bindings) come from the invented variable (e), and not its initializer (x). e is not const because you just declared it auto. What we're doing is copying x, and then taking bindings into this (non-const) copy.






                share|improve this answer


























                  8












                  8








                  8








                  Is the following code supposed to compile?




                  It is not. This is an MSVC bug.



                  A structured binding declaration introduces a new name (for specification only), e, that is declared like:



                  auto e = x;


                  The type of e is called E, and since the initializer is tuple-like, the types of the bindings are given by tuple_element_t<i, E>. In this case E is pair<int, int>, so the two types are just int. The rule for decltype of a structured binding is to give the referenced type, so decltype(a) and decltype(b) are both int.



                  The important part here is that a and b (the structured bindings) come from the invented variable (e), and not its initializer (x). e is not const because you just declared it auto. What we're doing is copying x, and then taking bindings into this (non-const) copy.






                  share|improve this answer














                  Is the following code supposed to compile?




                  It is not. This is an MSVC bug.



                  A structured binding declaration introduces a new name (for specification only), e, that is declared like:



                  auto e = x;


                  The type of e is called E, and since the initializer is tuple-like, the types of the bindings are given by tuple_element_t<i, E>. In this case E is pair<int, int>, so the two types are just int. The rule for decltype of a structured binding is to give the referenced type, so decltype(a) and decltype(b) are both int.



                  The important part here is that a and b (the structured bindings) come from the invented variable (e), and not its initializer (x). e is not const because you just declared it auto. What we're doing is copying x, and then taking bindings into this (non-const) copy.







                  share|improve this answer












                  share|improve this answer



                  share|improve this answer










                  answered 6 hours ago









                  BarryBarry

                  185k21325600




                  185k21325600

























                      3














                      The static assertions should fail (so this would be an MSVC bug I guess). Why? Because it's basically the same as the case of:



                      void foo() {
                      const int x_1 = 1;
                      const int x_2 = 2;

                      auto a = x_1;
                      auto b = x_2;

                      static_assert(std::is_const_v<decltype(a)>);
                      static_assert(std::is_const_v<decltype(b)>);
                      }


                      which does indeed fail on MSVC as well.



                      In C++ expression types decay on assignment: the auto sees an int, not a const int. Structured binding simply lets you do more than a single auto binding at a time.






                      share|improve this answer






























                        3














                        The static assertions should fail (so this would be an MSVC bug I guess). Why? Because it's basically the same as the case of:



                        void foo() {
                        const int x_1 = 1;
                        const int x_2 = 2;

                        auto a = x_1;
                        auto b = x_2;

                        static_assert(std::is_const_v<decltype(a)>);
                        static_assert(std::is_const_v<decltype(b)>);
                        }


                        which does indeed fail on MSVC as well.



                        In C++ expression types decay on assignment: the auto sees an int, not a const int. Structured binding simply lets you do more than a single auto binding at a time.






                        share|improve this answer




























                          3












                          3








                          3







                          The static assertions should fail (so this would be an MSVC bug I guess). Why? Because it's basically the same as the case of:



                          void foo() {
                          const int x_1 = 1;
                          const int x_2 = 2;

                          auto a = x_1;
                          auto b = x_2;

                          static_assert(std::is_const_v<decltype(a)>);
                          static_assert(std::is_const_v<decltype(b)>);
                          }


                          which does indeed fail on MSVC as well.



                          In C++ expression types decay on assignment: the auto sees an int, not a const int. Structured binding simply lets you do more than a single auto binding at a time.






                          share|improve this answer















                          The static assertions should fail (so this would be an MSVC bug I guess). Why? Because it's basically the same as the case of:



                          void foo() {
                          const int x_1 = 1;
                          const int x_2 = 2;

                          auto a = x_1;
                          auto b = x_2;

                          static_assert(std::is_const_v<decltype(a)>);
                          static_assert(std::is_const_v<decltype(b)>);
                          }


                          which does indeed fail on MSVC as well.



                          In C++ expression types decay on assignment: the auto sees an int, not a const int. Structured binding simply lets you do more than a single auto binding at a time.







                          share|improve this answer














                          share|improve this answer



                          share|improve this answer








                          edited 6 hours ago

























                          answered 6 hours ago









                          einpoklumeinpoklum

                          36.1k28132260




                          36.1k28132260






























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