[STL]for_each详细用法[转]

转自： http://www.cppblog.com/janvy/archive/2010/03/25/110499.htmlAbstract
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。