for

it2022-05-05 111

http://www.cnblogs.com/Purple_Xiapei/archive/2012/05/27/2520477.html

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>1213using namespace std;1415void printElem(int& elem) {16 cout << elem << endl;17}1819int 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>1314using namespace std;1516void printElem(int elem, const char* prefix) {17 cout << prefix << elem << endl;18}1920int 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>1213using namespace std;1415struct printElem {16 void operator() (int elem) {17 cout << elem << endl;18 } 19};2021int 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>1213using namespace std;1415struct printElem {16 const char* _prefix;1718 printElem(const char* prefix) : _prefix(prefix) {}19 20 void operator() (int elem) {21 cout << _prefix << elem << endl;22 } 23};2425int 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>1314using namespace std;1516class 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};2627class 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};4041int 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>1314using namespace std;1516class 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};2627class HorizontalDoor : public AbstractDoor {28};2930class 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};4041class 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};5455int 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>1314using namespace std;1516class 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};3132class HorizontalDoor : public AbstractDoor {33};3435class 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};4546class 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};5960int main() {61 DoorController dc;62 dc.addDoor(HorizontalDoor());63 dc.addDoor(VerticalDoor());64 dc.openDoor();65}

執行結果

1 John open door horizontally 2 John 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 Template1.1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>1213using namespace std;1415template<typename T>16void printElem(T elem) {17 cout << elem << endl;18}1920int 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enum2.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>1213using namespace std;1415template<typename T, int i>16void printElem(T elem) {17 cout << i << ":" << elem << endl;18}1920int 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 Template2.1 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>1314using namespace std;1516template<typename T>17class printElem : public unary_function<T, void> {18public:19 void operator() (T elem) {20 cout << elem << endl;21 }22};2324int 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>1314using namespace std;1516template<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};2829int 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

ConclusionSTL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。

转载于:https://www.cnblogs.com/virusolf/p/4906654.html

专利

最新回复(0)