Chapter 22. Function Objects and Callbacks

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A function object (also called a functor ) is any object that can be called using the function call syntax . In the C programming language, three kinds of entities can lead to syntax that looks like a function call: functions, function-like macros, and pointers to functions. Because functions and macros are not objects, this implies that only pointers to functions are available as functors in C. In C++, additional possibilities are added: The function call operator can be overloaded for class types, a concept of references to functions exists, and member functions and pointer-to-member functions have a call syntax of their own. Not all of these concepts are equally useful, but the combination of the concept of a functor with the compile-time parameterization offered by templates leads to powerful programming techniques.

Besides developing functor types, this chapter also delves into some usage idioms for functors. Nearly all uses end up being a form of callback : The client of a library wants that library to call back some function of the client code. The classic example is a sorting routine that needs a function to compare two elements in the set being sorted. The comparison routine is passed as a functor in this case. Traditionally, the term callback has been reserved for functors that are passed as function call arguments (as opposed to, for example, template arguments), and we maintain this tradition.

The terms function object and functor are unfortunately a little fuzzy in the sense that different members of the C++ programming community may give slightly different meanings to these terms. A common variation of the definition we have given is to include only objects of class types in the functor or function object concept; function pointers are then excluded. In addition, it is not uncommon to read or hear discussions referring to the class type of a function object as a "function object." In other words, the phrase "class of function objects so and so " is shortened to "function objects so and so ." Although we sometimes handle this terminology somewhat sloppily in our own daily work, we have made it a point to stick to our initial definitions in this chapter.

Before digging into the use of templates to implement useful functors, we discuss some properties of function calls that motivate some of the advantages of template-based functors.

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