Question

I have a set of classes that have the following structure:

class U
{
public:
    explicit U(int) { ... }
    U() {...}
    Init(int) {...}
};

I need to be able to compose 1 or more of these classes into a class X. Pseudocode:

template<class TypeSequence>
class X that derives publicly from all the classes in TypeSequence
{
    X(int): all bases are initialized with the integer passed 
    {}
    //if the above constructor is impossible, then the following will do as well:
    X(int)
    {
        Call Init on all bases and pass the given int to them.
    }
};

I think I need a lot of mpl, but I’m not really good at it. Is what am I trying to do doable? A code sample would be great.

MY FAULT: Forgot to mention I can’t use C++11 features. I am looking for an MPL solution.

Answer 1

Well, Boost.MPL contains metafunctions inherit and inherit_linearly you can combine them with for_each to get the second variant (with init functions). Or using just boost::mpl::fold and custom metafunctions:

struct Null_IntConstructor
{
  Null_IntConstructor(int) { }
};

struct InheritFrom_IntConstructor_Folder
{
  template<typename T1, typename T2>
  struct apply
  {
    struct type : T1, T2
    {
      type(int x) : T1(x), T2(x) { }
    };
  };
};

template<typename Bases>
struct InheritFrom_IntConstructor 
  : boost::mpl::fold<Bases, 
                     Null_IntConstructor,
                     InheritFrom_IntConstructor_Folder>::type
{
  InheritFrom_IntConstructor(int x) 
    : boost::mpl::fold<Bases, 
                       Null_IntConstructor,
                       InheritFrom_IntConstructor_Folder>::type(x) 
  { }
};

Usage example:

#include <iostream>
#include <boost/mpl/fold.hpp>
#include <boost/mpl/vector.hpp>

struct A
{
  A(int x) { std::cout << "A::A " << x << std::endl; }
};

struct B
{
  B(int x) { std::cout << "B::B " << x << std::endl; }
};

struct C
{
  C(int x) { std::cout << "C::C " << x << std::endl; }
};

int main()
{
  InheritFrom_IntConstructor< boost::mpl::vector<A, B, C> >(1);
}

Metafunction InheritFrom_IntConstructor can generalized to accept arbitrary type as constructor parameter and I’m not sure if can generalized to accept arbitrary number of arguments.