On Thu, Feb 26, 2004 at 12:04:19PM -0600, Larry Evans wrote:
<boost_files>/regexp_first.zip , but I was hoping FC++ could maybe do better. One problem (maybe not too big of a problem) with the regexp_first code is that there's a pointer from subtype to supertype and back (more memory) and the virtual functions in supertype must be delegated to in the subtype (more programmer time).
I don't know if FC++ can improve on that, but I'd like to see if it can. Brian, do you have any ideas on how to do it better in FC++?
Brian wrote: Sorry, no. Indeed, I cannot imagine _any_ solution in C++ which doesn't involve "manually" writing the delegation methods as well as keeping pointers in both directions.
I changed my mind, after talking to Yannis about it in front of a white board. :) Ok, so you can do something clever with "delegation" or "dynamic inheritance" or the "state pattern" or whatever you want to call it if we change the constraints of the problem some. I am still fooling with the solution, so I'm not sure of all the design constraints yet. I also want help smoothing it out. If class "C" wants to delegate method "f" to class B, then below is code to do it. I used boost::{lambda,function} rather than FC++ in the example. The basic idea is to get rid of member functions (a recurring motif) in favor of function objects. Annoyingly, you have to pass a pointer-to-the-current-object as an extra argument, which I can't seem to get rid of right now. (Ideas?) #include <iostream> using std::cout; using std::endl; #include "boost/lambda/lambda.hpp" #include "boost/lambda/bind.hpp" #include "boost/function.hpp" using boost::function; using boost::lambda::_1; using boost::lambda::_2; using boost::lambda::constant; struct B { int x; function<void(B*,int)> f; // instead of void f(int); B( int x_, function<void(B*,int)> f_ ) : x(x_), f(f_) {} }; struct C : B { int y; function<void(C*,int)> g; // instead of void g(int); C( const B& b, int y_, function<void(C*,int)> g_ ) : B(b), y(y_), g(g_) {} }; int main() { // make two different Bs with different behavior B b1( 1, cout << constant("x is ") << bind(&B::x,_1) << " and arg is " << _2 << "\n" ); B b2( 2, cout << constant("my x is ") << bind(&B::x,_1) << " and my arg is " << _2 << "\n" ); // make a C that delegates f() to b1 C c( b1, 3, cout << constant("y is ") << bind(&C::y,_1) << " and arg is " << _2 << "\n" ); // the &c extra argument is annoying, grr c.f(&c,5); c.g(&c,5); // now delegate to b2, instead dynamic_cast<B&>(c) = b2; c.f(&c,5); c.g(&c,5); } -- -Brian McNamara (lorgon@cc.gatech.edu)