Follow Techotopia on Twitter

On-line Guides
All Guides
eBook Store
iOS / Android
Linux for Beginners
Office Productivity
Linux Installation
Linux Security
Linux Utilities
Linux Virtualization
Linux Kernel
System/Network Admin
Scripting Languages
Development Tools
Web Development
GUI Toolkits/Desktop
Mail Systems
Eclipse Documentation

How To Guides
General System Admin
Linux Security
Linux Filesystems
Web Servers
Graphics & Desktop
PC Hardware
Problem Solutions
Privacy Policy




Thinking in C++ Vol 2 - Practical Programming
Prev Home Next

Function objects

As you study some of the examples earlier in this chapter, you will probably notice the limited utility of the function gt15( ). What if you want to use a number other than 15 as a comparison threshold? You may need a gt20( ) or gt25( ) or others as well. Having to write a separate function is time consuming, but also unreasonable because you must know all required values when you write your application code.

The latter limitation means that you can t use runtime values[88] to govern your searches, which is unacceptable. Overcoming this difficulty requires a way to pass information to predicates at runtime. For example, you would need a greater-than function that you can initialize with an arbitrary comparison value. Unfortunately, you can t pass that value as a function parameter because unary predicates, such as our gt15( ), are applied to each value in a sequence individually and must therefore take only one parameter.

The way out of this dilemma is, as always, to create an abstraction. Here, we need an abstraction that can act like a function as well as store state, without disturbing the number of function parameters it accepts when used. This abstraction is called a function object.[89]

A function object is an instance of a class that overloads operator( ), the function call operator. This operator allows an object to be used with function call syntax. As with any other object, you can initialize it via its constructors. Here is a function object that can be used in place of gt15( ):

//: C06:GreaterThanN.cpp
#include <iostream>
using namespace std;
class gt_n {
int value;
gt_n(int val) : value(val) {}
bool operator()(int n) { return n > value; }
int main() {
gt_n f(4);
cout << f(3) << endl; // Prints 0 (for false)
cout << f(5) << endl; // Prints 1 (for true)
} ///:~

The fixed value to compare against (4) is passed when the function object f is created. The expression f(3) is then evaluated by the compiler as the following function call:


which returns the value of the expression 3 > value, which is false when value is 4, as it is in this example.

Since such comparisons apply to types other than int, it would make sense to define gt_n( ) as a class template. It turns out you don t need to do it yourself, though the standard library has already done it for you. The following descriptions of function objects should not only make that topic clear, but also give you a better understanding of how the generic algorithms work.

Thinking in C++ Vol 2 - Practical Programming
Prev Home Next

   Reproduced courtesy of Bruce Eckel, MindView, Inc. Design by Interspire