Advanced C++ - Function object

Function object is object which has function-call operator (operator()) defined or overloaded. Thanks to that objects have behaviour of function which means that you can call such object by using paranthenses and passing arguments. Let's see and example:

The output of this example is:
In point (II) we are defining function object according to class defined in point (I).
In point (III) we are using our function object like a normal function.
You may be considering what is advantage of usage function object instead of normal function. Here are few advantages of function object:
1. Function object can be used as normal function but can has also additional abilities related to being object ex. the same function, represente by different function object, can has different states at the same time.
2. Function object has own types. What is more different function objects can has different states even if they signature is the same. This is very useful in STL library. Thanks to that, for containers of different types we can use the same kind of function object like a sorting function for example.
3. Function objects are faster than normal function in STL library. It is because thanks to template programming (will be described in one of next articles) more detailes can be defined in compiled time instead of run-time.
Above example you can find in git repository related to this blog here:
https://github.com/xmementoit/CppAdventureExamples/tree/master/advancedCpp/functionObjects

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