"functors" in c++, ML, haskell, prolog

catonmat.net

53 points·gtani··4 comments

This is an old article, but I love this blog. Peteris is so prolific you can't help but get inspired.

> Take C++ for example. Everyone who has mastered C++ knows that you call a class that implements operator() a functor.

Nope. This is what dilettantes call them. Capable C++ programmers (I don't claim to be a master, that would be foolish) know how to grep the standard. The term "functor" is not found in it, at least not that draft that comes up first on Google. The plain and transparent term "function object" has its own section, 20.3.

The term "functor" is widely used -- including such places as SGI's STL documentation (http://www.sgi.com/tech/stl/functors.html) and numerous other reference sites.

According to Bruce Eckel's Thinking in C++ Vol 2 (in the section describing the Command Pattern), the term dates to 1992:

> In Advanced C++: Programming Styles And Idioms (Addison Wesley, 1992), Jim Coplien coins the term functor which is an object whose sole purpose is to encapsulate a function (since “functor” has a meaning in mathematics, we shall use the more explicit term function object). The point is to decouple the choice of function to be called from the site where that function is called.

Unfortunately, I don't have a copy of Advanced C++: Programming Styles and Idioms, so I can't verify this myself. But, if it's accurate, that means the term predates the standard by at least six years since ISO/IEC 14882 was released in 1998... :)

Personally, I do prefer "function object" though, since it seems like a clearer term.

Bjarne Stroustrup's own "The C++ Programming Language" introduces them that way on page 515 as "a function-like object, a functor, or simply a function object". I don't like the term either, but that seems like a pretty authoritative source giving its blessing.

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