Abstract
for_each()是STL中少數可以回傳值的algorithm,此範例在展示for_each()此特殊功能與function object可以留住state的特性。
Introduction
function object與global function的差別除了function object可以傳入參數外,還可以不使用static就可以留住state。
一個簡單的需求,想要每印n個數字就換行,並且統計出所有iterator的和,所以function object必須能留住state才知道目前印了幾個數字,且統計sum為多少。
Sample Code
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/*
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(C) OOMusou 2007 http://oomusou.cnblogs.com
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Filename : GenericAlgo_for_each_state_returnValue.cpp
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Compiler : Visual C++ 8.0 / BCB 6.0 / gcc 3.4.2 / ISO C++
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Description : Demo how to use for_each to return value and remain state.
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Release : 05/13/2007 1.0
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05/15/2007 2.0
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*/
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#include <iostream>
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#include <algorithm>
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#include <vector>
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using namespace std;
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class printElem {
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private:
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int _n;
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int _cnt;
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int _sum;
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public:
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printElem(int n = 5) : _n(n), _cnt(0), _sum(0) {}
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void operator() (int elem) {
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++_cnt;
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_sum += elem;
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cout << elem;
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(_cnt % _n) ? cout << " " : cout << endl;
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}
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operator int() {
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return _sum;
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}
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};
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int main() {
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vector<int> ivec;
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for(int i = 0; i != 20; ++i)
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ivec.push_back(i);
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int sum = for_each(ivec.begin(), ivec.end(), printElem(5));
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cout << "sum is " << sum << endl;
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}
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執行結果
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_n為設定幾個字跳行
_cnt統計目前已經印了幾個字
_sum統計目前加總結果
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若每印n個字,就加印換行
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為了讓for_each()能傳回值,特別改寫operator int(),讓function object能夠傳回值。
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這樣for_each()就能風風光光的每n個字就換行,還可以順便加總結果。
Conclusion
STL真的很神奇,以上的程式想一行一行翻成C#還真的做不到呢!!