STL Algorithm design Concepts-function objects and function objects when parameters and return values

Function object:
The class that overloads the function call operator. Their objects are often called function objects, meaning they are objects that behave like functions.

A class object that shows a feature of a function that uses a class object in the form of an object name + (List of tables), assuming that there is no context, it can be treated as a function.
This is done by overloading the operator () of the class.

"In the standard library. Function objects are widely used to obtain elasticity. " Very many algorithms in the standard library can use function objects or functions as custom callback behaviors.
Demo

#include <iostream> #include <cstdio>using namespace std;//function object. Class overloads () template <typename t>  class showelement{public://overload () void operator () (t &t) {cout << T < < Endl;} Protected:private:int n;};/ /function template Template <typename t>void funcshowelement (t &t) {cout << t << Endl;} normal function void FuncShowElement2 (int &t) {cout << t << Endl;} void Play01 () {int a = 10; Showelement<int> Showelement;showelement (a); The invocation of a function object, much like a function, is called a functor//10funcshowelement<int> (a); FuncShowElement2 (a);} int main () {play01 (); return 0;}

The above simply demonstrates the use of function objects and similarities and differences with the definition of common functions, see the following demo.

#include <iostream> #include <cstdio> #include <vector> #include <algorithm>using namespace std ;//Function object, class overloaded () template <typename T>class showelement{public:showelement () {n = 0;} overloaded () void operator () (t &t) {++n;cout << T << ';} void Printn () {cout << "N:" << n << Endl;} Protected:private:int n;};/ /function template Template <typename t>void funcshowelement (t &t) {cout << T << ';} normal function void FuncShowElement2 (int &t) {cout << T << ';} void Play01 () {int a = 10; Showelement<int> Showelement;showelement (a); The invocation of a function object, much like a function, is called a functor//10funcshowelement<int> (a); FuncShowElement2 (a);} The advantage of Function object//function object belongs to class object, can break the concept of function. Can keep the call state information void play02 () {vector<int> v;v.push_back (1); V.push_back (3); V.push_back (5); For_each (V.begin (), V.end (), showelement<int> ()); Anonymous function object, anonymous functor cout << endl;//1 3 5for_each (V.begin (), V.end (), FuncShowElement2); Rewrite class Showelem with callback function cout << endl;//1 3 5//Entshowelement<int> showelement;/* for_each function prototype Template<class _init,class _Fn1> inline_Fn1 For_each (_ InIt _first, _init _last, _fn1 _func) {//Perform function for each element_debug_range (_first, _last); _debug_pointer (_fun c); _for_each (_unchecked (_first), _unchecked (_last), _func); return (_std Move (_func));} *///can see that the function object passing of the for_each algorithm is the element value passing. Not quoted pass For_each (V.begin (), V.end (), showelement); cout << endl;showelement.printn (); So the print results here are not the 3//n:0//solution we expected. For_each finally the function object passed in the value returned showelement = For_each (V.begin (), V.end (), showelement); cout << Endl; SHOWELEMENT.PRINTN (); bingo//N:3}int Main () {//play01 ();p lay02 (); return 0;}

Conclusion: It is very important to know whether the value returned by the STL algorithm is an iterator or a predicate (function object).

STL Algorithm design Concepts-function objects and function objects when parameters and return values