Friday, March 31, 2017

Structured Binding (C++17 inside)

Let's see how structured binding introduced in C++17 will change the way to interact with std::pair, std::tuple, std::array and such:
     std::pair<int,float> foo();  
     auto [a,b] = foo();  
it will replaces:
     std::pair<int,float> foo();  
     int a;  
     float b;  
     std::tie(a,b) = foo();  
in case you are running an obfuscated code contest this new swap can scrub up well (please don't try this at home):
     std::tie(a,b) = std::make_pair(b,a);  
The decomposition works with c array and std::array as well:
    int a[4] = { 1, 2, 3, 4};  
    auto [b,c,d,e] = a;  
    std::array<int, 4> a;  
    auto [b,c,d,e] = a;  
and this is what you can do using a ranged for loop:
    std::map myMap;  
    ...  
    for (const auto & [k,v] : myMap) {  
    }  
I bet someone in c++ committee has become recently a python enthusiast. Now if you wonder what your structures like this:
   stuct X {  
    int theInt = 3;  
    float thePi = 3.14;  
   };  
   auto [a,b] = x;  
shall provide to make the decomposition working the response is: a plain nothing. That will work indeed off the shelf.

Unfortunately if you need to do something more fancy with your class it has to support the get<>() functions, and you need to reopen the std namespace to specialize std::tuple_size and std::tuple_element.

Given the following user defined type (note a and b here are private members):
   class Y {  
   public:  
    int foo() const {  
     return a;  
    }  
    float bar() const {  
     return b;  
    }  
   private:  
    int a = 3;  
    float b = 3.14;  
   };  
you need to provide the gets<>() functions:
   template <int N> auto get(Y const &);  
   template <> auto get<0>(Y const & aY) {  
    return aY.foo();  
   }  
   template <> auto get<1>(Y const & aY) {  
    return aY.bar();  
   }  
and then you need to reopen the std namespace (one of those few allowed cases):
   namespace std {  
    template<>  
    struct std::tuple_size<Y> {  
      static const size_t value = 2;  
    };  
    template<size_t I>  
    struct std::tuple_element<I, Y> {  
     using type = decltype(get<I>(declval<Y>()));  
    };  
   }  
Note the partial specialization for std::tuple_element, you don't need to hard code the type of each index, it's enough to "deduce" it using the get function. You did a lot of work in order to have your class supporting the decomposition, in this case c++17 can save you some work taking advantage of a new c++17 feature, the "constexpr if", just writing a single version of get<>():

   template<int N>   
   auto get(Y const & aY) {  
     static_assert(N==0 || N==1);  
     if constexpr (N == 0) {  
       return aY.foo();  
     } else if constexpr (N == 1) {  
       return aY.bar();  
     }  
   }  
If you want use/experiment with those new language features go for clang++ (I tried only version 5.0 but it should work with the 4.0 as well) and you need to specify -std=gnu++1z

2 comments:

Hariom Singh said...

Not able to use your class example .Has problem in calling get

Hariom Singh said...

int main()
{
Y aY;
auto [n,s] =get(aY);
}

No matching function for call to 'get'