Boost metaparse how to return custom types

Question

I'm trying to create a simple parser with metaparse to demonstrate how I can return my own custom types.

If I had a custom type ValNum, how can I integrate this into a basic parser such that it is returned by the parser after matching a number? Apologies if this is trivial but I've been struggling to achieve this.

template<int Value>
struct ValNum {
    constexpr static int value = Value;
};

Answer 1

You can return custom types by using the transform function along with a metafunction.

Here's a really quick example of returning a custom type (butchering the identity metafunction class they gave in the link and a couple over examples I found in their offical GitHub)

// custom type here
template<int Value>
struct ValNum {
    constexpr static int value = Value;
};

struct identity
{
  template <class T>
  struct apply
  {
    // ValNum is my custom type
    using type = ValNum<T::type::value>; 
  };
  using type = identity;
};

typedef
  grammar<_STR("int_token")>
    ::import<_STR("int_token"), token<transform<int_, identity>>>::type
  expression;

typedef build_parser<entire_input<expression>> calculator_parser;

int main()
{

  // hello is now our custom ValNum type
  using hello = apply_wrap1<calculator_parser, _STR("13")>::type;

  using std::cout;
  using std::endl;

  cout << hello::value << endl;
}

Hope this helps someone.