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c++boostc++14fusion

Initialise const fusion boost list from derived class

Is it possible to initialise a member fusion vector to a value specified in a derived class without making the base class a template class?

like this:

class container
{
const auto children;
container (auto children):children (children){}
}

class derived : public container
{
derived():container(make_vector(string("test1"),string("test"))){} // http://www.boost.org/doc/libs/1_57_0/libs/fusion/doc/html/fusion/container/generation/functions/make_vector.html
}

I know that will not work- but I hope it will make it easier to understand my goal.

  1. delaying the specification of the types the vector will contain until a class derives from it.
  2. Without specifying the types the vector should contain by making the base class a template class

If not- what is closest thing to it?

Solution

  • The closest thing that doesn't require the base class to be a template is to use type erasure. You can roll your own ¹ or use Boost Type Erasure etc. Pick what suits you best.

    The simplest way to achieve it would be boost::any:

    Sample

    Live On Coliru

    #include <boost/any.hpp>
#include <boost/fusion/include/io.hpp>
#include <boost/fusion/include/vector.hpp>
#include <boost/fusion/include/make_vector.hpp>
#include <string>

namespace fus = boost::fusion;

class container
{
  protected:
    boost::any children;

    template <typename T>
    container (T const& children) : children(children) {}
};

class derived : public container
{
    using V = boost::fusion::vector2<std::string, std::string>;
  public:
    derived() : 
        container(fus::make_vector(std::string("test1"),std::string("test"))){} 

    friend std::ostream& operator<<(std::ostream& os, derived const& d) {
        return os << boost::any_cast<V const&>(d.children);
    }
};

#include <iostream>

int main() {
    derived d;
    std::cout << d;
}

    Prints

    (test1 test)

    ¹ e.g.