Re: [boost] Functions as Filters (was Program Reuse ...)

From: Matt Austern (austern_at_[hidden])
Date: 2005-01-08 17:33:39

On Sat, 08 Jan 2005 14:24:51 -0500, christopher diggins
<cdiggins_at_[hidden]> wrote:
> Thank you very much to everyone who has been helping me out, and have made
> some suggestions. It took me a bit to realize the implications of what some
> people were suggesting, so I apologize if I am a bit slow to catch up.
> 
> I now have code which allows two arbitrary functions ( which have no
> parameters and return void ) to be chained together from cout of one to the
> cin of the other:
> 
> void HelloWorld() {
>   cout << "hello world" << endl;
> }
> 
> void CountChars() {
>   string s;
>   s = cin.getline();
>   cout << static_cast<int>(s.size()) << endl;
> }
> 
> int main() {
>   filter(HelloWorld) | filter(CountChars);
>   return 0;
> }
> 
> This is done by redirecting cout for the first function to a temporary
> stringstream buffer, and then redirecting the output of the second stream.
> The code to accomplish this is trivial, but potentially interesting for the
> community:
> 
> #include <iostream>
> #include <sstream>
> 
> using namespace std;
> 
> typedef void(*procedure)();
> 
> class filter {
>   public:
>     filter(procedure x) : proc(x) { }
>     void operator()(istream& in, ostream& out) {
>       streambuf* inbuf = cin.rdbuf();
>       streambuf* outbuf = cout.rdbuf();
>       cin.rdbuf(in.rdbuf());
>       cout.rdbuf(out.rdbuf());
>       proc();
>       cin.rdbuf(inbuf);
>       cout.rdbuf(outbuf);
>     }
>   private:
>     procedure proc;
> };
> 
> void operator|(filter f1, filter f2) {
>   stringstream s;
>   f1(cin, s);
>   s.seekg(0);
>   f2(s, cout);
> }
> 
> Many good suggestions have been made so far. Some possibilities for more
> features are:
> 
> 1) create wrapper objects around the filters, this would allow the passing
> of data to functions, like a command line string
> 2) chain arbitrarily long sequences by creating a pipeline object, and doing
> the piping in its destructor
> 3) allow the chaining of streams
> 4) allow the chaining of the various iostreams concepts
> 5) allow the chaining of FILE* (i.e. popen, etc.)
> 6) allow the chaining of processes
> 7) allow threading of functions
> 
> Any comments or suggestions? Is this a direction people would like to see
> continued? Also is anyone interested in collaborating?

Allow the two functions to be run as coroutines, and you'll have
something really interesting.

        --Matt