Functional (C++)

In the context of the programming language C++, functional refers to a header file that is part of the C++ Standard Library and provides a set of predefined class templates for function objects, including operations for arithmetic, comparisons, and logic. Instances of these class templates are C++ classes that define a function call operator, and the instances of these classes can be called as if they were functions.^[1] It is possible to perform very sophisticated operations without writing a new function object, simply by combining predefined function objects and function object adaptors.

The class template std::function provided by C++11 is a general-purpose polymorphic function wrapper. Instances of std::function can store, copy, and invoke any callable target—functions, lambda expressions (expressions defining anonymous functions), bind expressions (instances of function adapters that transform functions to other functions of smaller arity by providing values for some of the arguments), or other function objects.

The algorithms provided by the C++ Standard Library do not require function objects of more than two arguments. Function objects that return Boolean values are an important special case. A unary function whose return type is bool is called a predicate, and a binary function whose return type is bool is called a binary predicate.

Adaptable function objects

In general, a function object has restrictions on the type of its argument. The type restrictions need not be simple, though: operator() may be overloaded or may be a member template. Similarly, there need be no way for a program to determine what those restrictions are. An adaptable function object, however, does specify what the argument and return types are, and provides nested typedefs so that those types can be named and used in programs. If a type F0 is a model of an adaptable generator, then it must define F0::result_type. Similarly, if F1 is a model of the adaptable unary function, it must define F1::argument_type and F1::result_type, and if F2 is a model of the adaptable binary function, it must define F2::first_argument_type, F2::second_argument_type, and F2::result_type. The C++ Standard Library provides base classes unary_function and binary_function to simplify the definition of adaptable unary functions and adaptable binary functions.

Adaptable function objects are important, because they can be used by function object adaptors: function objects that transform or manipulate other function objects. The C++ Standard Library provides many different function object adaptors, including unary_negate (that returns the logical complement of the value returned by a particular adaptable predicate), and unary_compose and binary_compose, which perform composition of function object.

Predefined function objects

The C++ Standard Library includes in the header file functional many different predefined function objects, including arithmetic operations (plus, minus, multiplies, divides, modulus, and negate), comparisons (equal_to, not_equal_to, greater, less, greater_equal, and less_equal), and logical operations (logical_and, logical_or, and logical_not).^[1]

Examples

Function wrappers can be used to make calls to ordinary functions or to functions objects created by lambda expressions.

import std;

// Define a template function 
template <typename T>
void printValue(T value) {
    std::println("{}", value);
}

int main(int argc, char* argv[]) {
    // A function wrapper to a function 
    std::function<void(int)> func_a = printValue<int>;
    func_a(2015);

    // A function wrapper to a function pointer
    std::function<void(int)> func_b = &printValue<int>;
    func_b(2016);

    // A function wrapper to a lambda function.
    std::function<void(int)> func_c = [](int value) -> void {
        std::println("{}", value);
    };
    func_c(2017);

    /* A function wrapper generated by std::bind().
     * Pass a pre-defined parameter when binding.
     */
    std::function<void(void)> func_d = std::bind(printValue<std::string>, "PI is");
    func_d();

    /* A function wrapper generated by std::bind().
     * Pass a parameter when calling the function.
     */
    std::function<void(float)> func_e = std::bind(printValue<float>, std::placeholders::_1);
    func_e(3.14159);
}

Function wrappers also can be used to access member variables and member functions of classes.

import std;

template <typename T>
class AnyData {
public:
    T value;

    AnyData(T value): 
        value{value} {}

    void print() { 
        std::println("{}", value); 
    }
    void printAfterAdd(T value) { 
        std::println("{}", this->value + value); 
    }
};

int main(int argc, char* argv[]) {
    // A function wrapper to a member variable of a class
    AnyData<int> data_a{2016};
    std::function<int(AnyData<int> &)> func_a = &AnyData<int>::value;
    std::println("{}", func_a(data_a));

    // A function wrapper to member function without parameter passing 
    AnyData<float> data_b{2016.1};
    std::function<void(AnyData<float>&)> func_b = &AnyData<float>::print;
    func_b(data_b);

    // A function wrapper to member function with passing a parameter
    std::function<void(AnyData<float>&, float)> func_c = &AnyData<float>::printAfterAdd;
    func_c(data_b, 0.1);

    // A function wrapper to member function generated by std::bind
    std::function<void(float)> func_d = std::bind(
        &CAnyData<float>::printAfterAdd,
        &data_b, 
        std::placeholders::_1
    );
    func_d(0.2);
}