Consider a class hierarchy that represents and evaluates arithmetic expressions:
#include <iostream>
struct Node {
virtual ~Node() {}
virtual int value() const = 0;
};
struct Variable : Node {
Variable(int value) : v(value) {}
int value() const override { return v; }
int v;
};
struct Plus : Node {
Plus(const Node& left, const Node& right) : left(left), right(right) {}
int value() const override { return left.value() + right.value(); }
const Node& left; const Node& right;
};
struct Times : Node {
Times(const Node& left, const Node& right) : left(left), right(right) {}
int value() const override { return left.value() * right.value(); }
const Node& left; const Node& right;
};
int main() {
Variable a{2}, b{3}, c{4};
Plus d{a, b}; Times e{d, c};
std::cout << e.value() << "\n"; // 20
}We want to add a new operation that prints the expression in postfix notation. How do we do it?
The simple way is: add a new virtual function. Like so:
#include <iostream>
struct Node {
// as before
virtual void postfix(std::ostream& os) const = 0;
};
struct Variable : Node {
// as before
virtual void postfix(std::ostream& os) const override { os << v; }
};
struct Plus : Node {
// as before
void postfix(std::ostream& os) const override {
left.postfix(os); os << ' '; right.postfix(os); os << " +";
}
};
struct Times : Node {
// as before
void postfix(std::ostream& os) const override {
left.postfix(os); os << ' '; right.postfix(os); os << " *";
}
};
int main() {
Variable a{2}, b{3}, c{4};
Plus d{a, b}; Times e{d, c};
e.postfix(std::cout); // 2 3 + 4 *
std::cout << " = " << e.value() << "\n"; // = 20
}If the classes are part of a small, self-contained program, this may be a reasonable solution. Otherwise, there are serious problems with this approach.
-
We need to modify all the classes in the hierarchy, which, in a real program, may be numerous and spread across multiple files. This assumes that we have access to their source code.
-
We need to recompile all the files that use the Node classes, because the layout of their virtual function tables (v-tables) has changed.
-
Programs that use the Node classes get the overriders of
postfix, even those that don’t callpostfixanywhere. -
They also pull in
postfix's transitive dependencies, in this casestd::ostream, locales, facets, exceptions, etc. Note that now we need to include<iostream>before defining the classes.
In C++, this problem is often "solved" with the Visitor design pattern. We may still need to modify Node to equip it with Visitor support, if it is not there already. But once that is done, we can add all the operations we want, albeit in a clumsy fashion.
Sadly, it is now difficult to add new Node classes, because we have to modify the Visitor’s interface, and all its implementations, and recompile all the files that use the Visitor. We traded one problem for another.
Paul Graham said that design patterns are essentially workarounds for features that are missing in less expressive languages. Indeed, in some languages, our problem does not even exist. C# has extension classes. Lisp, Clojure, Dylan, Julia, Cecil, TADS, and others, have multi-methods.
But wait! What do multi-methods have to do with our problem? There is no multiple dispatch going on here! The thing is, "multi-methods" is not a very good name. Even in languages that support them, many "multi-methods" have a single virtual parameter - they are uni-methods [1]! For that reason, we prefer the term "open-methods", to emphasize the important feature: openness. Multiple dispatch is just a bonus.
An open-method is like a virtual function, but it exists outside of a class, as a free-standing function. We can create all the open-methods we need, without ever needing to modify existing classes.
This library implements open-methods for C++17 and above. Next we will see how to use them in the Node example.