C++ Multiple Dispatch?
Thu, 2008-04-10 07:54 — jason
I clearly must be missing something. I just noticed this article on the O'Reilly ONLamp blog, discussing multiple dispatch in Perl.
The example code given:
class Thing {}
class Rock is Thing {}
class Paper is Thing {}
class Scissors is Thing {}
multi sub defeats(Thing $t1, Thing $t2) { 0 };
multi sub defeats(Paper $t1, Rock $t2) { 1 };
multi sub defeats(Rock $t1, Scissors $t2) { 1 };
multi sub defeats(Scissors $t1, Paper $t2) { 1 };
my $paper = Paper.new;
my $rock = Rock.new;
say defeats($paper, $rock);
say defeats($rock, $paper);Absolutely, positively translates directly into C++:
#include <iostream>
class Thing{};
class Rock : public Thing {};
class Paper : public Thing {};
class Scissors : public Thing {};
bool defeats(const Thing &, const Thing &) { return false; }
bool defeats(const Paper &, const Rock &) { return true; }
bool defeats(const Rock &, const Scissors &) { return true; }
bool defeats(const Scissors &, const Paper &) { return true; }
Paper paper;
Rock rock;
int main(int, char **)
{
std::cout << std::boolalpha;
std::cout << defeats(paper, rock) << std::endl;
std::cout << defeats(rock, paper) << std::endl;
}Is multiple dispatch nothing more than just function overloading?
Comments
Use the base class
paper and rock have to be declared as type Thing* in order for the problem/difference to show up in C++ (or Java). Overloading is resolved at compile time, so if you switch the compile-time types, you switch which function gets called.
I see that now, after looking
I see that now, after looking at the linked documents. But the specific example in the ONLamp article does not clearly show that, nor does it even give a compelling case for why you would want to use runtime dispatch instead of compile time, except, I guess "compile time" doesn't really exist in Perl?
C++ Multiple Dispatch Update
I threw together a quick C++ example of how one would do runtime function dispatch in C++.
C++ Multiple Dispatch!