c++ - Translating a std::tuple into a template parameter pack

Question

I have a

typedef std::tuple<A, B> TupleType;

and would like to use the list of classes for a "template".

Suppose I have:

template<typename... args>
std::tuple<args...> parse(std::istream &stream) {
  return std::make_tuple(args(stream)...);
}

and that I can successfully use it with:

auto my_tuple = parse<A, B>(ifs);

is it possible to avoid having to specify the class list A,B if I already have a

typedef std::tuple<A,B> TupleType;

where the list A,B is already present?

an example:

#include <cstdlib>  // EXIT_SUCCESS, EXIT_FAILURE
#include <iostream> // std::cerr
#include <fstream>  // std::ifstream
#include <tuple>    // std::tuple

class A {
public:
  A(std::istream &);  // May throw FooBaarException 
};

class B {
public:
  B(std::istream &); // May throw FooBaarException 
};

template<typename... args>
std::tuple<args...> parse(std::istream &stream) {
  return std::make_tuple(args(stream)...);
}

int main() {
  std::ifstream ifs;
  ifs.exceptions(ifstream::eofbit | ifstream::failbit | ifstream::badbit);
  int res = EXIT_FAILURE;
  try {
    ifs.open("/some/file/path", std::ios::in | std::ios::binary);
    auto my_tuple = parse<A, B>(ifs); // my_tuple is of the type std::tuple<A,B>
    /* Here do something interesting with my_tuple */ 
    res = EXIT_SUCCESS;
  } catch (ifstream::failure e) {
    std::cerr << "error: opening or reading file failed
";
  } catch (FooBaarException e) {
    std::cerr << "error: parsing in a constructor failed
";
  }
  return res;
}

Answer 1

The underlying problem in your situation seems to be that you'd like to specialize the function template parse for the special case when the template argument is a std::tuple. Unfortunately, this kind of specialization isn't possible with function templates.

However, it is possible with class templates.

So, as a first step, you could define parse as a static function of a struct, like this:

using std::istream;
using std::tuple;
using std::make_tuple;

struct A { A(const istream &) {} };
struct B { B(const istream &) {} };

template <typename... Args>
struct parser
{
  /* Your original function, now inside a struct.
     I'm using direct tuple construction and an
     initializer list to circumvent the order-of-
     construction problem mentioned in the comment
     to your question. */
  static tuple<Args...> parse(const istream &strm)
  { return tuple<Args...> {Args(strm)...}; }
};

template <typename... Args>
struct parser<tuple<Args...>>
{
  /* Specialized for tuple. */
  static tuple<Args...> parse(const istream &strm)
  { return parser<Args...>::parse(strm); }
};

You can then call it in the desired way:

int main()
{
  typedef tuple<A,B> tuple_type;
  auto tup = parser<tuple_type>::parse(std::cin);
  return 0;
}

As a second step, you can define a function template (again) which passes the arguments on to the right specialization of the struct:

template <typename... Args>
auto parse(const istream &strm) -> decltype(parser<Args...>::parse(strm))
{ return parser<Args...>::parse(strm); }

And now you can use it in exactly the way you wanted:

int main()
{
  typedef tuple<A,B> tuple_type;
  auto tup = parse<tuple_type>(std::cin);
  return 0;
}

(And you can still use it in the old way, too: auto tup = parse<A,B>(std::cin).)

---

Remark. As mentioned in the comment to parser::parse(), I used direct tuple construction instead of make_tuple to avoid problems with the order of construction of the tuple elements. This is not directly related to your question, but a good thing to do. See how to avoid undefined execution order for the constructors when using std::make_tuple.