CODEWITHSUNDEEP
c++c++17perfect-forwardingstdtuple
Abruzzo Forte e Gentile
Abruzzo Forte e Gentile

Reputation: 14869

I cannot access element by type in tuple created with forward_as_tuple

I am using C++17 with Visual C++ 2017 and I am creating a std::tuple of references using std::forward_as_tuple.

Since C++14 it is possible to access an element of a tuple by using a class type instead of an index.

When I try the code below I have compilation error

 error C2338: duplicate type T in get<T>(tuple)

Do you know how to access an element in a tuple created in such way?

Below the sample code

struct CA { 
    int data_ = 0; 
};

struct CB { 
    int data_ = 0; 
};

int main()
{
    CA a;
    CA b;

    auto joined_objects = std::forward_as_tuple(a, b);  

    std::cout << std::get<0>(joined_objects).data_ << std::endl; // works
    std::cout << std::get<CA &>(joined_objects).data_ << std::endl; // fails
 }

Upvotes: 1

Views: 153

Answers (1)

luk32
luk32

Reputation: 16070

It's because the compiler doesn't know which element you really want.

There is ambiguity because a and b have same type. get helper for types cannot work if tuple contains same type twice. It's pretty understandable.

Did you perhaps mean to write CB b? This works.

Upvotes: 5

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