janvy

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STL for_each用法(转载)

Posted on 2010-03-25 11:34 janvy 阅读(4707) 评论(0)  编辑 收藏 引用 所属分类: STL

Abstract
之前在(原創) 如何使用for_each() algorithm? (C/C++) (STL) 曾經討論過for_each(),不過當時功力尚淺,只談到了皮毛而已,這次看了effective STL的item 41、43後,對for_each()又有了更深入的了解,因此做了本篇心得報告。

Motivation
看到了eXile的C++中实现 foreach使用了巨集對foreach做改善,也看到了很多人對STL style的for_each()做討論,使我想對STL的for_each()再做了一次研究。

Introduction
學習過STL的container後,想要存取每一個iterator,你一定寫過以下的程式


#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 << endl;
  }
}

執行結果


1
2
3

當時我覺得STL什麼都好,就是以下這一串又臭又長


for(vector<int>::const_iterator iter = ivec.begin(); iter != ivec.end(); ++iter) {


若不常寫,一時還會寫不出來,其實若配合container,C++其實不應該這樣寫迴圈,正確的方式該使用for_each(),語法會變的相當簡單。

for_each()事實上是個function template,其實做如下[effective STL item 41]


template<typename InputIterator, typename Function>
Function for_each(InputIterator beg, InputIterator end, Function f) {
  while(beg != end)
    f(*beg++);
}


由以上source可知,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
 3Filename    : GenericAlgo_for_each_GlobalFunction.cpp
 4Compiler    : Visual C++ 8.0 / BCB 6.0 / gcc 3.4.2 / ISO C++
 5Description : Demo how to use for_each with global function
 6Release     : 05/11/2007 1.0
 7*/
 8#include <iostream>
 9#include <vector>
10#include <iostream>
11#include <algorithm>
12
13using namespace std;
14
15void printElem(int& elem) {
16  cout << elem << endl;
17}
18
19int 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


1
2
3

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.傳入參數
若要傳參數給global function,就不能再只傳global function name而已,必須透過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。


 1/**//*
 2(C) OOMusou 2007 http://oomusou.cnblogs.com
 3Filename    : GenericAlgo_for_each_GlobalFunctionWithParameter.cpp
 4Compiler    : Visual C++ 8.0 / BCB 6.0 / gcc 3.4.2 / ISO C++
 5Description : Demo how to use for_each with global function with Parameter
 6Release     : 05/11/2007 1.0
 7*/
 8#include <iostream>
 9#include <vector>
10#include <iostream>
11#include <algorithm>
12#include <functional>
13
14using namespace std;
15
16void printElem(int elem, const char* prefix) {
17  cout << prefix << elem << endl;
18}
19
20int main() {
21  int ia[] = {1, 2, 3};
22  vector<int> ivec(ia, ia + sizeof(ia) / sizeof(int));
23 
24  for_each(ivec.begin(), ivec.end(), bind2nd(ptr_fun(printElem), "Element:"));
25}

執行結果


Element:1
Element:2
Element:3

Object Oriented與for_each()搭配
1.不傳入參數
使用function object


 1/**//*
 2(C) OOMusou 2007 http://oomusou.cnblogs.com
 3Filename    : GenericAlgo_for_each_FunctionObject.cpp
 4Compiler    : Visual C++ 8.0 / BCB 6.0 / gcc 3.4.2 / ISO C++
 5Description : Demo how to use for_each with function object
 6Release     : 05/11/2007 1.0
 7*/
 8#include <iostream>
 9#include <vector>
10#include <iostream>
11#include <algorithm>
12
13using namespace std;
14
15struct printElem {
16  void operator() (int elem) {
17    cout << elem << endl;
18  }
19};
20
21int main() {
22  int ia[] = {1, 2, 3};
23  vector<int> ivec(ia, ia + sizeof(ia) / sizeof(int));
24 
25  for_each(ivec.begin(), ivec.end(), printElem());
26}

執行結果


1
2
3

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。


 1/**//*
 2(C) OOMusou 2007 http://oomusou.cnblogs.com
 3Filename    : GenericAlgo_for_each_FunctionObjectWithParameter.cpp
 4Compiler    : Visual C++ 8.0 / BCB 6.0 / gcc 3.4.2 / ISO C++
 5Description : Demo how to use for_each with function object with parameter
 6Release     : 05/11/2007 1.0
 7*/
 8#include <iostream>
 9#include <vector>
10#include <iostream>
11#include <algorithm>
12
13using namespace std;
14
15struct printElem {
16  const char* _prefix;
17
18  printElem(const char* prefix) : _prefix(prefix) {}
19 
20  void operator() (int elem) {
21    cout << _prefix << elem << endl;
22  }
23};
24
25int main() {
26  int ia[] = {1, 2, 3};
27  vector<int> ivec(ia, ia + sizeof(ia) / sizeof(int));
28 
29  for_each(ivec.begin(), ivec.end(), printElem("Element:"));
30}
執行結果

Element:1
Element:2
Element:3

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


for_each(_doorVec.begin(), _doorVec.end(),&Door::open);

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


for_each(_doorVec.begin(), _doorVec.end(), mem_fun_ref(&Door::open));

透過mem_fun_ref()這個function adapter將member function轉成function object。

 

 1/**//*
 2(C) OOMusou 2007 http://oomusou.cnblogs.com
 3
 4Filename    : GenericAlgo_for_each_MemberFunctionObject.cpp
 5Compiler    : Visual C++ 8.0 / BCB 6.0 / gcc 3.4.2 / ISO C++
 6Description : Demo how to use for_each with member function with object
 7Release     : 05/11/2007 1.0
 8*/
 9#include <vector>
10#include <iostream>
11#include <algorithm>
12#include <functional>
13
14using namespace std;
15
16class Door {
17public:
18  void open() const {
19    cout << "open door horizontally" << endl;
20  }
21 
22  void close() const {
23    cout << "close door horizontally" << endl;
24  }
25};
26
27class DoorController {
28protected:
29  vector<Door> _doorVec;
30 
31public:
32  void addDoor(Door aDoor) {
33    _doorVec.push_back(aDoor);
34  }
35 
36  void openDoor() const {
37    for_each(_doorVec.begin(), _doorVec.end(), mem_fun_ref(&Door::open));
38  }
39};
40
41int main() {
42  DoorController dc;
43  dc.addDoor(Door());
44  dc.addDoor(Door());
45  dc.openDoor();
46}

執行結果


open door horizontally
open door horizontally

37行


for_each(_doorVec.begin(), _doorVec.end(), mem_fun_ref(&Door::open));

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。

 

 1/**//*
 2(C) OOMusou 2007 http://oomusou.cnblogs.com
 3
 4Filename    : GenericAlgo_for_each_MemberFunctionObjectPointer.cpp
 5Compiler    : Visual C++ 8.0 / BCB 6.0 / gcc 3.4.2 / ISO C++
 6Description : Demo how to use for_each with member function with object pointer
 7Release     : 05/11/2007 1.0
 8*/
 9#include <vector>
10#include <iostream>
11#include <algorithm>
12#include <functional>
13
14using namespace std;
15
16class AbstractDoor {
17public:
18  virtual void open() const {
19    cout << "open door horizontally" << endl;
20  }
21 
22  virtual void close() const {
23    cout << "close door horizontally" << endl;
24  }
25};
26
27class HorizontalDoor : public AbstractDoor {
28};
29
30class VerticalDoor : public AbstractDoor {
31public:
32  void open() const {
33    cout << "open door vertically" << endl;
34  }
35 
36  void close() const {
37    cout << "close door vertically" << endl;
38  }
39};
40
41class DoorController {
42protected:
43  vector<AbstractDoor*> _doorVec;
44 
45public:
46  void addDoor(AbstractDoor& aDoor) {
47    _doorVec.push_back(&aDoor);
48  }
49 
50  void openDoor() const {
51    for_each(_doorVec.begin(), _doorVec.end(), mem_fun(&AbstractDoor::open));
52  }
53};
54
55int main() {
56  DoorController dc;
57  dc.addDoor(HorizontalDoor());
58  dc.addDoor(VerticalDoor());
59  dc.openDoor();
60}

執行結果


open door horizontally
open door vertically

51行


for_each(_doorVec.begin(), _doorVec.end(), mem_fun(&AbstractDoor::open));

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。


 1/**//*
 2(C) OOMusou 2007 http://oomusou.cnblogs.com
 3
 4Filename    : GenericAlgo_for_each_MemberFunctionObjectPointerWithParameter.cpp
 5Compiler    : Visual C++ 8.0 / BCB 6.0 / gcc 3.4.2 / ISO C++
 6Description : Demo how to use for_each with member function with object pointer
 7Release     : 05/11/2007 1.0
 8*/
 9#include <iostream>
10#include <vector>
11#include <algorithm>
12#include <functional>
13
14using namespace std;
15
16class AbstractDoor {
17public:
18  virtual void open() const {
19    cout << "open door horizontally" << endl;
20  }
21 
22  virtual void close() const {
23    cout << "close door horizontally" << endl;
24  }
25 
26  virtual void openDoorBy(const char* name) const {
27    cout << name << " ";
28    open();
29  }
30};
31
32class HorizontalDoor : public AbstractDoor {
33};
34
35class VerticalDoor : public AbstractDoor {
36public:
37  void open() const {
38    cout << "open door vertically" << endl;
39  }
40 
41  void close() const {
42    cout << "close door vertically" << endl;
43  }
44};
45
46class DoorController {
47protected:
48  vector<AbstractDoor*> _doorVec;
49 
50public:
51  void addDoor(AbstractDoor& aDoor) {
52    _doorVec.push_back(&aDoor);
53  }
54 
55  void openDoor() const {
56    for_each(_doorVec.begin(), _doorVec.end(), bind2nd(mem_fun(&AbstractDoor::openDoorBy), "John"));
57  }
58};
59
60int main() {
61  DoorController dc;
62  dc.addDoor(HorizontalDoor());
63  dc.addDoor(VerticalDoor());
64  dc.openDoor();
65}

執行結果


1John open door horizontally
2John open door vertically

56行


for_each(_doorVec.begin(), _doorVec.end(), bind2nd(mem_fun(&AbstractDoor::openDoorBy), "John"));

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?


 1/**//*
 2(C) OOMusou 2007 http://oomusou.cnblogs.com
 3Filename    : GenericAlgo_for_each_FunctionTemplate.cpp
 4Compiler    : Visual C++ 8.0 / BCB 6.0 / gcc 3.4.2 / ISO C++
 5Description : Demo how to use for_each with function template
 6Release     : 05/11/2007 1.0
 7*/
 8#include <iostream>
 9#include <vector>
10#include <iostream>
11#include <algorithm>
12
13using namespace std;
14
15template<typename T>
16void printElem(T elem) {
17  cout << elem << endl;
18}
19
20int main() {
21  int ia[] = {1, 2, 3};
22  vector<int> ivec(ia, ia + sizeof(ia) / sizeof(int));
23 
24  for_each(ivec.begin(), ivec.end(), printElem<int>);
25  //for_each(ivec.begin(), ivec.end(), (void(*)(int))printElem);
26}

執行結果


1
2
3

若使用function template,有兩種寫法
一種是


for_each(ivec.begin(), ivec.end(), printElem<int>);

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


for_each(ivec.begin(), ivec.end(), (void(*)(int))printElem);

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。


 1/**//*
 2(C) OOMusou 2007 http://oomusou.cnblogs.com
 3Filename    : GenericAlgo_for_each_FunctionTemplateWithNontypeParameter.cpp
 4Compiler    : Visual C++ 8.0 / BCB 6.0 / gcc 3.4.2 / ISO C++
 5Description : Demo how to use for_each with function template with nontype parameter
 6Release     : 05/11/2007 1.0
 7*/
 8#include <iostream>
 9#include <vector>
10#include <iostream>
11#include <algorithm>
12
13using namespace std;
14
15template<typename T, int i>
16void printElem(T elem) {
17  cout << i << ":"  << elem << endl;
18}
19
20int main() {
21  int ia[] = {1, 2, 3};
22  vector<int> ivec(ia, ia + sizeof(ia) / sizeof(int));
23 
24  for_each(ivec.begin(), ivec.end(), printElem<int, 5>);
25}

執行結果


5:1
5:2
5:3

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


 1/**//*
 2(C) OOMusou 2007 http://oomusou.cnblogs.com
 3Filename    : GenericAlgo_for_each_ClassTemplate.cpp
 4Compiler    : Visual C++ 8.0 / BCB 6.0 / gcc 3.4.2 / ISO C++
 5Description : Demo how to use for_each with class template
 6Release     : 05/11/2007 1.0
 7*/
 8#include <iostream>
 9#include <vector>
10#include <iostream>
11#include <algorithm>
12#include <functional>
13
14using namespace std;
15
16template<typename T>
17class printElem : public unary_function<T, void> {
18public:
19  void operator() (T elem) {
20    cout << elem << endl;
21  }
22};
23
24int main() {
25  int ia[] = {1, 2, 3};
26  vector<int> ivec(ia, ia + sizeof(ia) / sizeof(int));
27 
28  for_each(ivec.begin(), ivec.end(), printElem<int>());
29}

執行結果


1
2
3

17行


class printElem : public unary_function<T, void> {

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


template <class InputIterator, class UnaryFunction>
UnaryFunction for_each(InputIterator first, InputIterator last, UnaryFunction f);

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。


 1/**//*
 2(C) OOMusou 2007 http://oomusou.cnblogs.com
 3Filename    : GenericAlgo_for_each_ClassTemplateWithParameter.cpp
 4Compiler    : Visual C++ 8.0 / BCB 6.0 / gcc 3.4.2 / ISO C++
 5Description : Demo how to use for_each with class template & parameter
 6Release     : 05/11/2007 1.0
 7*/
 8#include <iostream>
 9#include <vector>
10#include <iostream>
11#include <algorithm>
12#include <functional>
13
14using namespace std;
15
16template<typename T, typename U>
17class printElem : public unary_function<T, void> {
18private:
19  U _prefix;
20 
21public:
22  printElem(U prefix) : _prefix(prefix) {}
23 
24  void operator() (T elem) {
25    cout << _prefix << elem << endl;
26  }
27};
28
29int main() {
30  int ia[] = {1, 2, 3};
31  vector<int> ivec(ia, ia + sizeof(ia) / sizeof(int));
32 
33  for_each(ivec.begin(), ivec.end(), printElem<int, const char*>("Element:"));
34}

執行結果

 

Element:1
Element:2
Element:3

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。


本文来自CSDN博客,转载请标明出处:http://blog.csdn.net/hionceshine/archive/2008/11/19/3336318.aspx


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