之前在(原創) 如何使用for_each() algorithm? (初级) (C++) 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。
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>
![](https://www.cnblogs.com/Images/OutliningIndicators/None.gif)
using namespace std;
![](https://www.cnblogs.com/Images/OutliningIndicators/None.gif)
![](https://www.cnblogs.com/Images/OutliningIndicators/ExpandedBlockStart.gif)
int main()
{
![](https://www.cnblogs.com/Images/OutliningIndicators/ExpandedSubBlockStart.gif)
int ia[] =
{1, 2, 3};
vector<int> ivec(ia, ia + sizeof(ia) / sizeof(int));
![](https://www.cnblogs.com/Images/OutliningIndicators/ExpandedSubBlockStart.gif)
for(vector<int>::const_iterator iter = ivec.begin(); iter != ivec.end(); ++iter)
{
cout << *iter << endl;
}
}
執行結果
當時我覺得STL什麼都好,就是以下這一串又臭又長
![](https://www.cnblogs.com/Images/OutliningIndicators/ExpandedBlockStart.gif)
for(vector<int>::const_iterator iter = ivec.begin(); iter != ivec.end(); ++iter)
{
![](https://www.cnblogs.com/Images/OutliningIndicators/InBlock.gif)
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>
![](https://www.cnblogs.com/Images/OutliningIndicators/ExpandedBlockStart.gif)
Function for_each(InputIterator beg, InputIterator end, Function f)
{
while(beg != end)
f(*beg++);
}
![](https://www.cnblogs.com/Images/OutliningIndicators/None.gif)
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![](https://www.cnblogs.com/Images/OutliningIndicators/ExpandedBlockStart.gif)
/**//*
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![](https://www.cnblogs.com/Images/OutliningIndicators/None.gif)
13
using namespace std;
14![](https://www.cnblogs.com/Images/OutliningIndicators/None.gif)
15![](https://www.cnblogs.com/Images/OutliningIndicators/ExpandedBlockStart.gif)
void printElem(int& elem)
{
16
cout << elem << endl;
17
}
18![](https://www.cnblogs.com/Images/OutliningIndicators/None.gif)
19![](https://www.cnblogs.com/Images/OutliningIndicators/ExpandedBlockStart.gif)
int main()
{
20![](https://www.cnblogs.com/Images/OutliningIndicators/ExpandedSubBlockStart.gif)
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
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![](https://www.cnblogs.com/Images/OutliningIndicators/ExpandedBlockStart.gif)
/**//*
2
(C) OOMusou 2007 http://oomusou.cnblogs.com
3
Filename : GenericAlgo_for_each_GlobalFunctionWithParameter.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 with Parameter
6
Release : 05/11/2007 1.0
7
*/
8
#include <iostream>
9
#include <vector>
10
#include <iostream>
11
#include <algorithm>
12
#include <functional>
13![](https://www.cnblogs.com/Images/OutliningIndicators/None.gif)
14
using namespace std;
15![](https://www.cnblogs.com/Images/OutliningIndicators/None.gif)
16![](https://www.cnblogs.com/Images/OutliningIndicators/ExpandedBlockStart.gif)
void printElem(int elem, const char* prefix)
{
17
cout << prefix << elem << endl;
18
}
19![](https://www.cnblogs.com/Images/OutliningIndicators/None.gif)
20![](https://www.cnblogs.com/Images/OutliningIndicators/ExpandedBlockStart.gif)
int main()
{
21![](https://www.cnblogs.com/Images/OutliningIndicators/ExpandedSubBlockStart.gif)
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![](https://www.cnblogs.com/Images/OutliningIndicators/ExpandedBlockStart.gif)
/**//*
2
(C) OOMusou 2007 http://oomusou.cnblogs.com
3
Filename : GenericAlgo_for_each_FunctionObject.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 function object
6
Release : 05/11/2007 1.0
7
*/
8
#include <iostream>
9
#include <vector>
10
#include <iostream>
11
#include <algorithm>
12![](https://www.cnblogs.com/Images/OutliningIndicators/None.gif)
13
using namespace std;
14![](https://www.cnblogs.com/Images/OutliningIndicators/None.gif)
15![](https://www.cnblogs.com/Images/OutliningIndicators/ExpandedBlockStart.gif)
struct printElem
{
16![](https://www.cnblogs.com/Images/OutliningIndicators/ExpandedSubBlockStart.gif)
void operator() (int elem)
{
17
cout << elem << endl;
18
}
19
};
20![](https://www.cnblogs.com/Images/OutliningIndicators/None.gif)
21![](https://www.cnblogs.com/Images/OutliningIndicators/ExpandedBlockStart.gif)
int main()
{
22![](https://www.cnblogs.com/Images/OutliningIndicators/ExpandedSubBlockStart.gif)
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
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![](https://www.cnblogs.com/Images/OutliningIndicators/ExpandedBlockStart.gif)
/**//*
2
(C) OOMusou 2007 http://oomusou.cnblogs.com
3
Filename : GenericAlgo_for_each_FunctionObjectWithParameter.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 function object with parameter
6
Release : 05/11/2007 1.0
7
*/
8
#include <iostream>
9
#include <vector>
10
#include <iostream>
11
#include <algorithm>
12![](https://www.cnblogs.com/Images/OutliningIndicators/None.gif)
13
using namespace std;
14![](https://www.cnblogs.com/Images/OutliningIndicators/None.gif)
15![](https://www.cnblogs.com/Images/OutliningIndicators/ExpandedBlockStart.gif)
struct printElem
{
16
const char* _prefix;
17![](https://www.cnblogs.com/Images/OutliningIndicators/InBlock.gif)
18![](https://www.cnblogs.com/Images/OutliningIndicators/ExpandedSubBlockStart.gif)
printElem(const char* prefix) : _prefix(prefix)
{}
19
20![](https://www.cnblogs.com/Images/OutliningIndicators/ExpandedSubBlockStart.gif)
void operator() (int elem)
{
21
cout << _prefix << elem << endl;
22
}
23
};
24![](https://www.cnblogs.com/Images/OutliningIndicators/None.gif)
25![](https://www.cnblogs.com/Images/OutliningIndicators/ExpandedBlockStart.gif)
int main()
{
26![](https://www.cnblogs.com/Images/OutliningIndicators/ExpandedSubBlockStart.gif)
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。
![](https://www.cnblogs.com/images/cnblogs_com/oomusou/for_each_MemberFunctionObject2.jpg)
1![](https://www.cnblogs.com/Images/OutliningIndicators/ExpandedBlockStart.gif)
/**//*
2
(C) OOMusou 2007 http://oomusou.cnblogs.com
3![](https://www.cnblogs.com/Images/OutliningIndicators/InBlock.gif)
4
Filename : GenericAlgo_for_each_MemberFunctionObject.cpp
5
Compiler : Visual C++ 8.0 / BCB 6.0 / gcc 3.4.2 / ISO C++
6
Description : Demo how to use for_each with member function with object
7
Release : 05/11/2007 1.0
8
*/
9
#include <vector>
10
#include <iostream>
11
#include <algorithm>
12
#include <functional>
13![](https://www.cnblogs.com/Images/OutliningIndicators/None.gif)
14
using namespace std;
15![](https://www.cnblogs.com/Images/OutliningIndicators/None.gif)
16![](https://www.cnblogs.com/Images/OutliningIndicators/ExpandedBlockStart.gif)
class Door
{
17
public:
18![](https://www.cnblogs.com/Images/OutliningIndicators/ExpandedSubBlockStart.gif)
void open() const
{
19
cout << "open door horizontally" << endl;
20
}
21
22![](https://www.cnblogs.com/Images/OutliningIndicators/ExpandedSubBlockStart.gif)
void close() const
{
23
cout << "close door horizontally" << endl;
24
}
25
};
26![](https://www.cnblogs.com/Images/OutliningIndicators/None.gif)
27![](https://www.cnblogs.com/Images/OutliningIndicators/ExpandedBlockStart.gif)
class DoorController
{
28
protected:
29
vector<Door> _doorVec;
30
31
public:
32![](https://www.cnblogs.com/Images/OutliningIndicators/ExpandedSubBlockStart.gif)
void addDoor(Door aDoor)
{
33
_doorVec.push_back(aDoor);
34
}
35
36![](https://www.cnblogs.com/Images/OutliningIndicators/ExpandedSubBlockStart.gif)
void openDoor() const
{
37
for_each(_doorVec.begin(), _doorVec.end(), mem_fun_ref(&Door::open));
38
}
39
};
40![](https://www.cnblogs.com/Images/OutliningIndicators/None.gif)
41![](https://www.cnblogs.com/Images/OutliningIndicators/ExpandedBlockStart.gif)
int 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。
![](https://www.cnblogs.com/images/cnblogs_com/oomusou/for_each_MemberFunctionObjectPointer.jpg)
1![](https://www.cnblogs.com/Images/OutliningIndicators/ExpandedBlockStart.gif)
/**//*
2
(C) OOMusou 2007 http://oomusou.cnblogs.com
3![](https://www.cnblogs.com/Images/OutliningIndicators/InBlock.gif)
4
Filename : GenericAlgo_for_each_MemberFunctionObjectPointer.cpp
5
Compiler : Visual C++ 8.0 / BCB 6.0 / gcc 3.4.2 / ISO C++
6
Description : Demo how to use for_each with member function with object pointer
7
Release : 05/11/2007 1.0
8
*/
9
#include <vector>
10
#include <iostream>
11
#include <algorithm>
12
#include <functional>
13![](https://www.cnblogs.com/Images/OutliningIndicators/None.gif)
14
using namespace std;
15![](https://www.cnblogs.com/Images/OutliningIndicators/None.gif)
16![](https://www.cnblogs.com/Images/OutliningIndicators/ExpandedBlockStart.gif)
class AbstractDoor
{
17
public:
18![](https://www.cnblogs.com/Images/OutliningIndicators/ExpandedSubBlockStart.gif)
virtual void open() const
{
19
cout << "open door horizontally" << endl;
20
}
21
22![](https://www.cnblogs.com/Images/OutliningIndicators/ExpandedSubBlockStart.gif)
virtual void close() const
{
23
cout << "close door horizontally" << endl;
24
}
25
};
26![](https://www.cnblogs.com/Images/OutliningIndicators/None.gif)
27![](https://www.cnblogs.com/Images/OutliningIndicators/ExpandedBlockStart.gif)
class HorizontalDoor : public AbstractDoor
{
28
};
29![](https://www.cnblogs.com/Images/OutliningIndicators/None.gif)
30![](https://www.cnblogs.com/Images/OutliningIndicators/ExpandedBlockStart.gif)
class VerticalDoor : public AbstractDoor
{
31
public:
32![](https://www.cnblogs.com/Images/OutliningIndicators/ExpandedSubBlockStart.gif)
void open() const
{
33
cout << "open door vertically" << endl;
34
}
35
36![](https://www.cnblogs.com/Images/OutliningIndicators/ExpandedSubBlockStart.gif)
void close() const
{
37
cout << "close door vertically" << endl;
38
}
39
};
40![](https://www.cnblogs.com/Images/OutliningIndicators/None.gif)
41![](https://www.cnblogs.com/Images/OutliningIndicators/ExpandedBlockStart.gif)
class DoorController
{
42
protected:
43
vector<AbstractDoor*> _doorVec;
44
45
public:
46![](https://www.cnblogs.com/Images/OutliningIndicators/ExpandedSubBlockStart.gif)
void addDoor(AbstractDoor& aDoor)
{
47
_doorVec.push_back(&aDoor);
48
}
49
50![](https://www.cnblogs.com/Images/OutliningIndicators/ExpandedSubBlockStart.gif)
void openDoor() const
{
51
for_each(_doorVec.begin(), _doorVec.end(), mem_fun(&AbstractDoor::open));
52
}
53
};
54![](https://www.cnblogs.com/Images/OutliningIndicators/None.gif)
55![](https://www.cnblogs.com/Images/OutliningIndicators/ExpandedBlockStart.gif)
int 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![](https://www.cnblogs.com/Images/OutliningIndicators/ExpandedBlockStart.gif)
/**//*
2
(C) OOMusou 2007 http://oomusou.cnblogs.com
3![](https://www.cnblogs.com/Images/OutliningIndicators/InBlock.gif)
4
Filename : GenericAlgo_for_each_MemberFunctionObjectPointerWithParameter.cpp
5
Compiler : Visual C++ 8.0 / BCB 6.0 / gcc 3.4.2 / ISO C++
6
Description : Demo how to use for_each with member function with object pointer
7
Release : 05/11/2007 1.0
8
*/
9
#include <iostream>
10
#include <vector>
11
#include <algorithm>
12
#include <functional>
13![](https://www.cnblogs.com/Images/OutliningIndicators/None.gif)
14
using namespace std;
15![](https://www.cnblogs.com/Images/OutliningIndicators/None.gif)
16![](https://www.cnblogs.com/Images/OutliningIndicators/ExpandedBlockStart.gif)
class AbstractDoor
{
17
public:
18![](https://www.cnblogs.com/Images/OutliningIndicators/ExpandedSubBlockStart.gif)
virtual void open() const
{
19
cout << "open door horizontally" << endl;
20
}
21
22![](https://www.cnblogs.com/Images/OutliningIndicators/ExpandedSubBlockStart.gif)
virtual void close() const
{
23
cout << "close door horizontally" << endl;
24
}
25
26![](https://www.cnblogs.com/Images/OutliningIndicators/ExpandedSubBlockStart.gif)
virtual void openDoorBy(const char* name) const
{
27
cout << name << " ";
28
open();
29
}
30
};
31![](https://www.cnblogs.com/Images/OutliningIndicators/None.gif)
32![](https://www.cnblogs.com/Images/OutliningIndicators/ExpandedBlockStart.gif)
class HorizontalDoor : public AbstractDoor
{
33
};
34![](https://www.cnblogs.com/Images/OutliningIndicators/None.gif)
35![](https://www.cnblogs.com/Images/OutliningIndicators/ExpandedBlockStart.gif)
class VerticalDoor : public AbstractDoor
{
36
public:
37![](https://www.cnblogs.com/Images/OutliningIndicators/ExpandedSubBlockStart.gif)
void open() const
{
38
cout << "open door vertically" << endl;
39
}
40
41![](https://www.cnblogs.com/Images/OutliningIndicators/ExpandedSubBlockStart.gif)
void close() const
{
42
cout << "close door vertically" << endl;
43
}
44
};
45![](https://www.cnblogs.com/Images/OutliningIndicators/None.gif)
46![](https://www.cnblogs.com/Images/OutliningIndicators/ExpandedBlockStart.gif)
class DoorController
{
47
protected:
48
vector<AbstractDoor*> _doorVec;
49
50
public:
51![](https://www.cnblogs.com/Images/OutliningIndicators/ExpandedSubBlockStart.gif)
void addDoor(AbstractDoor& aDoor)
{
52
_doorVec.push_back(&aDoor);
53
}
54
55![](https://www.cnblogs.com/Images/OutliningIndicators/ExpandedSubBlockStart.gif)
void openDoor() const
{
56
for_each(_doorVec.begin(), _doorVec.end(), bind2nd(mem_fun(&AbstractDoor::openDoorBy), "John"));
57
}
58
};
59![](https://www.cnblogs.com/Images/OutliningIndicators/None.gif)
60![](https://www.cnblogs.com/Images/OutliningIndicators/ExpandedBlockStart.gif)
int 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 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![](https://www.cnblogs.com/Images/OutliningIndicators/ExpandedBlockStart.gif)
/**//*
2
(C) OOMusou 2007 http://oomusou.cnblogs.com
3
Filename : GenericAlgo_for_each_FunctionTemplate.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 function template
6
Release : 05/11/2007 1.0
7
*/
8
#include <iostream>
9
#include <vector>
10
#include <iostream>
11
#include <algorithm>
12![](https://www.cnblogs.com/Images/OutliningIndicators/None.gif)
13
using namespace std;
14![](https://www.cnblogs.com/Images/OutliningIndicators/None.gif)
15
template<typename T>
16![](https://www.cnblogs.com/Images/OutliningIndicators/ExpandedBlockStart.gif)
void printElem(T elem)
{
17
cout << elem << endl;
18
}
19![](https://www.cnblogs.com/Images/OutliningIndicators/None.gif)
20![](https://www.cnblogs.com/Images/OutliningIndicators/ExpandedBlockStart.gif)
int main()
{
21![](https://www.cnblogs.com/Images/OutliningIndicators/ExpandedSubBlockStart.gif)
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
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![](https://www.cnblogs.com/Images/OutliningIndicators/ExpandedBlockStart.gif)
/**//*
2
(C) OOMusou 2007 http://oomusou.cnblogs.com
3
Filename : GenericAlgo_for_each_FunctionTemplateWithNontypeParameter.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 function template with nontype parameter
6
Release : 05/11/2007 1.0
7
*/
8
#include <iostream>
9
#include <vector>
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#include <iostream>
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#include <algorithm>
12![](https://www.cnblogs.com/Images/OutliningIndicators/None.gif)
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using namespace std;
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template<typename T, int i>
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void printElem(T elem)
{
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cout << i << ":" << 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(), printElem<int, 5>);
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}
執行結果
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![](https://www.cnblogs.com/Images/OutliningIndicators/ExpandedBlockStart.gif)
/**//*
2
(C) OOMusou 2007 http://oomusou.cnblogs.com
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Filename : GenericAlgo_for_each_ClassTemplate.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 class template
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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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template<typename T>
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class printElem : public unary_function<T, void>
{
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public:
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void operator() (T elem)
{
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cout << elem << endl;
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}
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};
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int main()
{
25![](https://www.cnblogs.com/Images/OutliningIndicators/ExpandedSubBlockStart.gif)
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(), printElem<int>());
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}
執行結果
17行
![](https://www.cnblogs.com/Images/OutliningIndicators/ExpandedBlockStart.gif)
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![](https://www.cnblogs.com/Images/OutliningIndicators/ExpandedBlockStart.gif)
/**//*
2
(C) OOMusou 2007 http://oomusou.cnblogs.com
3
Filename : GenericAlgo_for_each_ClassTemplateWithParameter.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 class template & 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>
13![](https://www.cnblogs.com/Images/OutliningIndicators/None.gif)
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using namespace std;
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template<typename T, typename U>
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class printElem : public unary_function<T, void>
{
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private:
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U _prefix;
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public:
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printElem(U prefix) : _prefix(prefix)
{}
23
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void operator() (T elem)
{
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cout << _prefix << elem << endl;
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}
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};
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int main()
{
30![](https://www.cnblogs.com/Images/OutliningIndicators/ExpandedSubBlockStart.gif)
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(), printElem<int, const char*>("Element:"));
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}
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。
See Also
(原創) 如何使用Function Object? (C/C++) (STL)
(原創) 如何使用for_each() algorithm? (C/C++) (STL)
(原創) Function Pointer、Delegate和Function Object (C/C++) (C) (STL) (template) (C#)
(原創) 如何使for_each()傳回值? (C/C++) (STL)
(原創) 如何為程式碼加上行號? (C/C++) (STL)
Reference
Stanley B. Lippman,C++ Primer 3/e,1998
Scott Meyers, Effective STL, Addison Wesley,2001
David Vandevoorde / Nicolai M. Josuttis, C++ Templates The Complete Guide, Addison Wesley,2002
Nicolai M. Josuttis,The C++ Standard Library : A Tutorial and Referencd,Addison Wesley,1999
鄭家瑜,C++物件導向程式設計進階與應用,博碩文化,2004