for_each()
#include <vector>
#include <iostream>
using namespace std;
int main() {
int ia[] = {1, 2, 3};
vector<int> ivec(ia, ia + sizeof(ia) / sizeof(int));
for(vector<int>::const_iterator iter = ivec.begin(); iter != ivec.end(); ++iter)
cout << *iter;
}//输出123
使用for_each语句,简化语句;for_each其实是个函数模板
template<typename InputIterator, typename Function> Function for_each(InputIterator beg, InputIterator end, Function f) { while(beg != end) f(*beg++); }
for_each()只能配合global function和function object。
以下我們將對procedure based、object oriented、generics三種paradigm與for_each()搭配做探討。
Procedure Based與for_each()搭配
1.不傳入參數
1 /* 2 (C) OOMusou 2007 http://oomusou.cnblogs.com 3 Filename : GenericAlgo_for_each_GlobalFunction.cpp 4 Compiler : Visual C++ 8.0 / BCB 6.0 / gcc 3.4.2 / ISO C++ 5 Description : Demo how to use for_each with global function 6 Release : 05/11/2007 1.0 7 */ 8 #include <iostream> 9 #include <vector> 10 #include <iostream> 11 #include <algorithm> 12 13 using namespace std; 14 15 void printElem(int& elem) { 16 cout << elem << endl; 17 } 18 19 int main() { 20 int ia[] = {1, 2, 3}; 21 vector<int> ivec(ia, ia + sizeof(ia) / sizeof(int)); 22 23 for_each(ivec.begin(), ivec.end(), printElem); 24 }
執行結果
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23行 for_each(ivec.begin(), ivec.end(), printElem);
只需將vector::begin(),vector::end()和 global function name 傳給for_each()即可,再也不用for迴圈那種複雜的語法了。
2. 传入参数
若要傳參數給global function,就不能再只傳global function name而已,必須通過ptr_fun()這個function adapter將global function轉成function object,然後再用bind2nd()將參數bind成一個function object。
/*
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(C) OOMusou 2007 http://oomusou.cnblogs.com
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Filename : GenericAlgo_for_each_GlobalFunctionWithParameter.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 with global function with Parameter
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Release : 05/11/2007 1.0
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*/
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#include <iostream>
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#include <vector>
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#include <iostream>
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#include <algorithm>
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#include <functional>
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using namespace std;
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void printElem(int elem, const char* prefix) {
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cout << prefix << elem << endl;
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}
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int main() {
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int ia[] = {1, 2, 3};
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vector<int> ivec(ia, ia + sizeof(ia) / sizeof(int));
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for_each(ivec.begin(), ivec.end(), bind2nd(ptr_fun(printElem), "Element:"));
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}
執行結果
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Object Oriented與for_each()搭配
1.不傳入參數
使用function object
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執行結果
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2.傳入參數
若使用function object,也可以將參數傳給printElem(),透過constructor的技巧接收參數。
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執行結果
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function object有很多種寫法,但只要是function object都可以跟for_each()合作。
3.member_function與for_each()搭配
3.1 不傳入參數
本文的重點來了,在物件導向世界裡,最常用的就是for_each()配合member function,這該怎麼寫呢?直覺會這樣子寫
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由於global function name本身就是一個pointer,所以想藉由&Door::open傳進一個address,但這樣compile並不會過,正確解法是
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透過mem_fun_ref()這個function adapter將member function轉成function object。
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執行結果
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值得注意的是,mem_fun_ref()用在object的member function。若要搭配多型,vector必須放pointer,也就是得使用object pointer的member function,此時得使用mem_fun()將member function轉成function object。
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執行結果
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使用了mem_fun()。
3.2傳入參數
問題又來了,若要使member function也傳入參數呢?這時得使用bind2nd將function object和參數bind在一起,變成另外一個新的function object。
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透過了bind2nd將參數結合後,成為一個新的function object。
Generics與for_each()搭配
1.Function Template
1.1不傳入參數
在泛型世界裡,那for_each()該怎麼配合function template呢?
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若使用function template,有兩種寫法
一種是
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由於template function需要在compile時確定型別,所以要加上<int>確定為int型別。
另外一種寫法
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template function並沒有確定型別,但轉成function pointer時,並須明確轉成int型別的function pointer。
1.2 傳入參數
若要如function object那樣能傳參數呢?funtion template是可以,不過有些限制,若使用nontype parameter,只能使用以下三種型別
1.int或enum
2.pointer:pointer to object,pointer to function,pointer to member。
3.reference:reference to object,reference to function。
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所以無法如function object那樣可以傳入字串或任意型別,最少在目前ISO C++標準是做不到的。
既然討論了function template,那最具威力的class template是否也能搭配for_each()?
2.Class Template
2.1 不傳入參數
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因為printElem只接受for_each()所傳的參數,算是單參數而已,所以public繼承了unary_function<T,void>,因為for_each的定義
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傳進去的是UnaryFunction型別,第一個type parameter T表示傳入的型別,第二個type parameter void,表示回傳的型別,最後重新定義operator()。
2.2 傳入參數
若要使class template也能傳入參數,一樣利用function object的技巧,借用constructor。
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Conclusion
STL的for_each()事實上很好用,不過由於限制很多,所以常令很多新手卻步,本文試著將所有會遇到問題的地方都提出來討論,包括procedure based、object oriented、generics三種paradigm與for_each()的搭配都涵蓋了,希望對各位有幫助。