Generalized chaining of non-member functions in C++

Question

I don't know if this can even be achivieable, but given these set of functions\class:

float plus1(float x) { return x+1; }
float div2(float x) { return x/2.0f; }
template <typename T>
class chain {
public:
    chain(const T& val = T()) : val_(val) {}
    chain& operator<<( std::function<float (float)> func ) {
    val_ = func(val_);
    return *this;
  }
  operator T() const {
    return val_;
  }
  T val_;
};

I can chain functions operating on floats like this:

float x = chain<float>(3.0f) << div2 << plus1 << div2 << plus1;

However, I'd like to generalize\extend this to being able to convert between types and have functions with arguments. Unfortunately I'm not smart enough to figure out how, or if, this can be done. Too be more specific I'd like to be able to do something like this (Where operator<< is just an arbitary choice, and preferably I dont even have to write "chain" at the beginning); Also, these are just dummy examples, I do not intend to use it for arithmetics.

std::string str = chain<float>(3.0) << mul(2.0f) << sqrt << to_string << to_upper;

or

vec3d v = chain<vec3i>(vec3i(1,1,1)) << normalize << to_vec3<double>;

Any ideas?

Answer 1

A general and extendable solution using boost::proto :

#include <iostream>
#include <boost/proto/proto.hpp>

namespace bp = boost::proto;

// -----------------------------------------------------------------------------
// perform is a callable transform that take a function_ terminal and execute it
// -----------------------------------------------------------------------------
struct perform : bp::callable
{
  template<class Sig> struct result;
  template<class This, class Func, class In>
  struct result<This(Func,In)> 
       : boost::result_of<typename boost::remove_reference<Func>::type(In)> {};

  template<class Func, class In>
  typename result<perform(Func &,In)>::type
  operator()( Func& f, In& in ) const
  {
    return f(in);
  }
};

// -----------------------------------------------------------------------------
// Grammar for chaining pipe of functions
// -----------------------------------------------------------------------------
struct pipeline_grammar
: bp::or_<
    bp::when<
        bp::bitwise_or<pipeline_grammar,pipeline_grammar>
          , pipeline_grammar(
                bp::_right
              , pipeline_grammar(bp::_left,bp::_state)
                )
        >
      , bp::when<
            bp::terminal<bp::_>
          , perform(bp::_value, bp::_state) 
    >
> {};

// -----------------------------------------------------------------------------
// Forward declaration of the pipeline domain
// -----------------------------------------------------------------------------
struct pipeline_domain;

// -----------------------------------------------------------------------------
// A pipeline is the top level DS entity
// -----------------------------------------------------------------------------
template<class Expr>
struct  pipeline : bp::extends<Expr,pipeline<Expr>, pipeline_domain>
{
  typedef bp::extends<Expr, pipeline<Expr>, pipeline_domain> base_type;
  pipeline(Expr const &expr = Expr()) : base_type(expr) {}

  // ---------------------------------------------------------------------------
  // A pipeline is an unary callable object
  // ---------------------------------------------------------------------------
  template<class Input>
  typename boost::result_of<pipeline_grammar(pipeline,Input)>::type
  operator()(Input const& in) const
  {
    pipeline_grammar evaluator;
    return evaluator(*this,in);
  }
};

// -----------------------------------------------------------------------------
// the pipeline_domain make pipeline expression macthes pipeline_grammar
// -----------------------------------------------------------------------------
struct pipeline_domain 
     : bp::domain<bp::generator<pipeline>,pipeline_grammar>
{};

// -----------------------------------------------------------------------------
// Takes a PFO instance and make it a pipeline terminal
// -----------------------------------------------------------------------------
template<class Func>
typename bp::result_of::
make_expr<bp::tag::terminal, pipeline_domain,Func>::type
task( Func const& f )
{
  return bp::make_expr<bp::tag::terminal,pipeline_domain>( f );
}

//--------------------------- Examples --------------------

struct return_value
{  
  template<class Sig> struct result;
  template<class This, class T>
  struct result<This(T)> : bp::detail::uncvref<T>
  {};

  return_value(int i = 1) : factor(i) {}

  template<class T> 
  T operator()(T const& in) const
  {
    return in*factor;
  }

  int factor;
};

struct say_hi
{
  typedef void result_type;

  template<class T> 
  void operator()(T const& in) const
  {
    std::cout << "Hi from value = " << in << "\n";
  }
};

int main()
{
  return_value r1,r2(5);
  (task(r1) | task(r2) | task(say_hi())) (7); // SHould print 35

  float k = 10,r;
  r = (task(r2) | task(r2) | task(r2) | task(r2))(k);
  std::cout << r << "\n"; // Should print 6250
}

The basic idea is to wrap function objects as proto terminals, build a small | based grammar and let the proto system deals with the composition.