函數(shù)指針的一種替代策略是Function object（函數(shù)對象）。
函數(shù)對象與函數(shù)指針相比較有兩個方面的優(yōu)點：首先如果被重載的調(diào)用操作符是inline函數(shù)則編譯器能夠執(zhí)行內(nèi)聯(lián)編譯，提供可能的性能好處；其次函數(shù)對象可以擁有任意數(shù)目的額外數(shù)據(jù)，用這些數(shù)據(jù)可以緩沖結(jié)果，也可以緩沖有助于當(dāng)前操作的數(shù)據(jù)。
函數(shù)對象是一個類，它重載了函數(shù)調(diào)用操作符operator() ,該操作符封裝了一個函數(shù)的功能。典型情況下函數(shù)對象被作為實參傳遞給泛型算法，當(dāng)然我們也可以定義獨立的函數(shù)對象實例。
來看下面的二個例子： 比較理解會更好些：
#include<vector>
#include<string>
#include<iostream>
#include<algorithm>
using namespace std;
class Sum {
int val;
public:
Sum(int i) :val(i) { }
//當(dāng)在需要int的地方，Sum將自動轉(zhuǎn)換為int類型
//這里是為了方便cout<<Sum的實例；
operator int() const { return val; }
//寫在類中的函數(shù)代碼一般默認(rèn)為內(nèi)聯(lián)代碼
int operator()(int i) { return val+=i; }
};
void f(vector<int> v)
{
Sum s = 0; //Sum s = 0等價于Sum s(0),不等價于Sum s;s = 0;
//對vector<int>中的元素求和
//函數(shù)對象被作為實參傳遞給泛型算法
s = for_each(v.begin(), v.end(), s);
cout << "the sum is " << s << "\n";
//更簡單的寫法，定義獨立的函數(shù)對象實例
cout << "the sum is " << for_each(v.begin(), v.end(), Sum(0)) << "\n";
}
int main()
{
vector<int> v;
v.push_back(3); v.push_back(2); v.push_back(1);
f(v);
system("pause");
return 0;
}
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#include <iostream>
#include <list>
#include <algorithm>
#include "print.hpp"
using namespace std;
// function object that adds the value with which it is initialized
class AddValue {
private:
int theValue; // the value to add
public:
// constructor initializes the value to add
AddValue(int v) : theValue(v) {
}
// the ``function call'' for the element adds the value
void operator() (int& elem) const {
elem += theValue;
}
};
int main()
{
list<int> coll;
// insert elements from 1 to 9
for (int i=1; i<=9; ++i) {
coll.push_back(i);
}
PRINT_ELEMENTS(coll,"initialized: ");
// add value 10 to each element
for_each (coll.begin(), coll.end(), // range
AddValue(10)); // operation
PRINT_ELEMENTS(coll,"after adding 10: ");
// add value of first element to each element
for_each (coll.begin(), coll.end(), // range
AddValue(*coll.begin())); // operation
PRINT_ELEMENTS(coll,"after adding first element: ");
}
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operator()中的參數(shù)為container中的元素
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另外的實例：
Function Objects as Sorting Criteria
Programmers often need a sorted collection of elements that have a special class (for example, a collection of persons). However, you either don't want to use or you can't use the usual operator < to sort the objects. Instead, you sort the objects according to a special sorting criterion based on some member function. In this regard, a function object can help. Consider the following example:
// fo/sortl.cpp
#include <iostream>
#include <string>
#include <set>
#include <algorithm>
using namespace std;
class Person {
public:
string firstname() const;
string lastname() const;
...
};
/* class for function predicate
* - operator() returns whether a person is less than another person
*/
class PersonSortCriterion {
public:
bool operator() (const Person& p1, const Person& p2) const {
/* a person is less than another person
* - if the last name is less
* - if the last name is equal and the first name is less
*/
return p1.lastname()<p2.1astname() ||
(! (p2.1astname()<p1.lastname()) &&
p1.firstname()<p2.firstname());
}
};
int main()
{
//declare set type with special sorting criterion
typedef set<Person,PersonSortCriterion> PersonSet;
//create such a collection
PersonSet coll;
...
//do something with the elements
PersonSet::iterator pos;
for (pos = coll.begin(); pos != coll.end();++pos) {
...
}
...
}
//fo/foreach3.cpp
#include <iostream>
#include <vector>
#include <algorithm>
using namespace std;
//function object to process the mean value
class MeanValue {
private:
long num; //number of elements
long sum; //sum of all element values
public:
//constructor
MeanValue() : num(0), sum(0) {
}
//"function call"
//-process one more element of the sequence
void operator() (int elem) {
num++; //increment count
sum += elem; //add value
}
//return mean value
double value() {
return static_cast<double>(sum) / static_cast<double>(num);
}
};
int main()
{
vector<int> coll;
//insert elments from 1 to 8
for (int i=1; i<=8; ++i) {
coll.push_back(i);
}
//process and print mean value
MeanValue mv = for_each (coll.begin(), coll.end(), //range
MeanValue()); //operation
cout << "mean value: " << mv.value() << endl;
}