CODEWITHSUNDEEP
c++templates
Adam Ryczkowski
Adam Ryczkowski

Reputation: 8064

How to transform boost::mpl::vector into another boost::mpl::vector?

Suppose I have an boost::mpl::vector "myvec", defined for example like this:

using myvec = boost::mpl::vector<int, double, double>;

Now I want to define another type, myvecex, that transforms each myvec member into std::tuple with added string. I want to get a type defined like this:

using myvecex = boost::mpl::vector<std::tuple<int, std::string>, 
                                   std::tuple<double, std::string>,
                                   std::tuple<double, std::string> >;

But I don't want to repeat myself and name all the vector members. Instead I want to define some_smart_template template type where I will somehow put the logic of converting each member type into a tuple.

using myvecex2 = some_smart_template<myvec>; 
static_assert(std::is_same<myvecex, myvecex2>::value);

Is it doable in C++ at all?

Upvotes: 1

Views: 513

Answers (1)

Barry
Barry

Reputation: 302718

Boost.MPL doesn't just give you containers, it gives you algorithms over those containers too. In this case, what you want is transform:

template<
      typename Sequence
    , typename Op
    , typename In = unspecified
    >
struct transform
{
    typedef unspecified type;
};

The semantics are you give it a sequence and what MPL refers to as a Lambda Expression and you get out another sequence. Specifically:

using B = mpl::transform<A,
    std::tuple<mpl::_1, std::string>
    >::type;

Or at least that would work if apply supported variadic class templates like std::tuple. So you'll need to just write an operation that's either a metafunction class:

struct tuple_of_strings {
    template <class T>
    struct apply {
        using type = std::tuple<T, std::string>;
    };
};

using B = mpl::transform<A, tuple_of_strings>::type;

or a metafunction:

template <class T>
struct tuple_of_strings {
    using type = std::tuple<T, std::string>;
};

using B = mpl::transform<A, tuple_of_strings<_1>>::type;

Upvotes: 1

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