A Framework for Higher-Order Functions in C++

C and C++ allow passing functions as arguments to other functions in the form of function pointers. However, since function pointers can refer only to existing functions declared at global or file scope, these function arguments cannot capture local environments. This leads to the common misconception that C and C++ do not support function closures. In fact, function closures can be modeled directly in C++ by enclosing a function inside an object such that the local environment is captured by data members of the object. This idiom is described in advanced C++ texts and is used, for example, to implement callbacks. The purpose of this paper is twofold: First, we demonstrate how this idiom can be generalized to a type-safe framework of C++ class templates for higher-order functions that support composition and partial application. Second, we explore the expressiveness of the framework and compare it with that of existing functional programming languages. We illustrate by means of various examples that object-oriented and functional idioms can coexist productively and can be used to enhance the functionality of common classes, for example, of nonlinear collections such as trees. A C++ implementation of the framework is available on request.