＜读书笔记＞ Thinking in python （Python 设计模式) 1. Singlton的c++与python的实现
1.由于python的天然的泛型特征，python 中没有接口继承，所有的继承都是实现继承，子类为了继承父类的实现。
2.Singlton 单件模式
- singlton in c++
下面看一个最简单的C++实现，它做到的是通过
类static变量，以及将默认构造函数私有化，
从而使得只能有一个GlobalClass的实体存在，
1 #include <iostream>
2 using namespace std;
3
4 class GlobalClass {
5 public:
6 static GlobalClass* instance();
7 static void create();
8 static void destroy();
9 void print() {
10 cout << "haha" << endl;
11 }
12 private:
13 static GlobalClass* s_instance;
14 GlobalClass();
15 ~GlobalClass(){
16 //destroy();
17 cout << "destruct" << endl;
18
19 }
20 };
21
22 GlobalClass* GlobalClass::s_instance = NULL;
23
24 GlobalClass::GlobalClass() {
25 cout << "construct global class" << endl;
26 }
27 //GloblaClass::~GlobalClass() {
28 // destroy();
29 // cout << "destruct" << endl;
30 //}
31 GlobalClass* GlobalClass::instance() {
32 return s_instance;
33 }
34
35 void GlobalClass::create() {
36 if (!s_instance)
37 s_instance = new GlobalClass();
38 }
39
40 void GlobalClass::destroy() {
41 delete s_instance;
42 s_instance = NULL;
43 cout << "delete ok" << endl;
44 }
45
46 int main(int argc, char *argv[])
47 {
48 //GlobalClass my_global_class; //can't create by this
49 GlobalClass::create();
50 GlobalClass* p_global_class = GlobalClass::instance();
51 p_global_class->print();
52 GlobalClass::destroy();
53 return 0;
54 }
3
4 class GlobalClass {
5 public:
6 static GlobalClass* instance();
7 static void create();
8 static void destroy();
9 void print() {
10 cout << "haha" << endl;
11 }
12 private:
13 static GlobalClass* s_instance;
14 GlobalClass();
15 ~GlobalClass(){
16 //destroy();
17 cout << "destruct" << endl;
18
19 }
20 };
21
22 GlobalClass* GlobalClass::s_instance = NULL;
23
24 GlobalClass::GlobalClass() {
25 cout << "construct global class" << endl;
26 }
27 //GloblaClass::~GlobalClass() {
28 // destroy();
29 // cout << "destruct" << endl;
30 //}
31 GlobalClass* GlobalClass::instance() {
32 return s_instance;
33 }
34
35 void GlobalClass::create() {
36 if (!s_instance)
37 s_instance = new GlobalClass();
38 }
39
40 void GlobalClass::destroy() {
41 delete s_instance;
42 s_instance = NULL;
43 cout << "delete ok" << endl;
44 }
45
46 int main(int argc, char *argv[])
47 {
48 //GlobalClass my_global_class; //can't create by this
49 GlobalClass::create();
50 GlobalClass* p_global_class = GlobalClass::instance();
51 p_global_class->print();
52 GlobalClass::destroy();
53 return 0;
54 }
//result
construct global class
haha
destruct
delete ok
刚刚想到effective c++第四款，提到的的利用函数内部local static变量的方法。这样也可以给出一种Siglton的实现
1 //file singlton.h用local static的这个方法也保证了只有一个实例的存在，不过仅供学习吧，不如第一种方案，毕竟有一个类类型的static变量的存在，
2
3 #ifndef _SINGLTON_H_
4 #define _SINGLTON_H_
5 #include <iostream>
6 using namespace std;
7
8 class GlobalClass {
9 public:
10 static GlobalClass& instance(int weight = 3){
11 static GlobalClass global_object(weight);
12 return global_object;
13 }
14 void print() {
15 cout << "haha" << this << endl;
16 cout << m_weight << endl << endl;
17 }
18 ~GlobalClass(){
19 cout << "destruct" << this << endl;
20
21 }
22 void addOne() {
23 m_weight += 1;
24 }
25
26 private:
27 GlobalClass() {
28 cout << "construct global class" << this << endl;
29
30 }
31 GlobalClass(int weight):m_weight(weight){ cout << "construct global class with weight" << this << endl;}
32 GlobalClass& operator=( const GlobalClass&);
33 GlobalClass(const GlobalClass&);
34 int m_weight;
35 };
36 #endif
37
38
39 //file use_singlton.h
40 #ifndef USE_SIGLTON_H
41 #define USE_SIGLTON_H
42
43 #include "singlton.h"
44 void useSinglton();
45
46 #endif
47
48 //file use_singlton.cc
49 #include <iostream>
50 #include "use_singlton.h"
51 using namespace std;
52 void useSinglton() {
53 GlobalClass& p_global_class = GlobalClass::instance(16);
54 p_global_class.print();
55 p_global_class.addOne();
56 p_global_class.print();
57
58 GlobalClass& p_global_class2 = GlobalClass::instance(46);
59 p_global_class2.print();
60 p_global_class2.addOne();
61 p_global_class2.print();
62 }
63
64 //file test_singlton.cc
65 #include <iostream>
66 #include "singlton.h"
67 #include "use_singlton.h"
68 using namespace std;
69
70 int main(int argc, char *argv[])
71 {
72 cout << "use singlton" << endl;
73 useSinglton();
74
75 cout << "wa ha ha " << endl;
76 GlobalClass& p_global_class = GlobalClass::instance(4);
77 p_global_class.print();
78 p_global_class.addOne();
79 p_global_class.print();
80
81 GlobalClass& p_global_class2 = GlobalClass::instance(8);
82 p_global_class2.print();
83 p_global_class2.addOne();
84 p_global_class2.print();
85
86 cout << "ha ha wa" << endl;
87 useSinglton();
88
89 return 0;
90 }
91
92 //allen:~/study/design_pattern/singlton_test$ g++ -g -o test_singlton test_singlton.cc use_singlton.cc
93 allen:~/study/design_pattern/singlton_test$ ./test_singlton
94 use singlton
95 construct global class with weight0x804a5c0
96 haha0x804a5c0
97 16
98
99 haha0x804a5c0
100 17
101
102 haha0x804a5c0
103 17
104
105 haha0x804a5c0
106 18
107
108 wa ha ha
109 haha0x804a5c0
110 18
111
112 haha0x804a5c0
113 19
114
115 haha0x804a5c0
116 19
117
118 haha0x804a5c0
119 20
120
121 ha ha wa
122 haha0x804a5c0
123 20
124
125 haha0x804a5c0
126 21
127
128 haha0x804a5c0
129 21
130
131 haha0x804a5c0
132 22
133
134 destruct0x804a5c0
135
而且它只能在main介绍的时候才会被析构，上面的我们可以随时主动去析构对象。
Singlton的基本思想很简单，但是用C++实现，关于资源的释放，何时释放还是感觉很别扭的。
下面是网上找到的一个基于模版的singlton实现，可以方便的复用该框架，同时利用了shared_ptr，无需操心释放动态内存。
另外给出一个OPEMESH中的singlton模版的实现，它也是使用了产生一个类类型的static变量（用户需要的时候才会产生出来）。
这两种方式都会在main结束后析构掉siglton对象资源，如shared_ptr自动释放 new的资源，而OPEMESH的方法static的类对象会销毁（调用起析构函数）.
//使用shared_ptr实现的singlton模版类
1 #include <iostream>
2 #include <tr1/memory>
3 using namespace std;
4 using namespace std::tr1;
5 template <typename T>
6 class Singlton {
7 public:
8 static T* instance();
9 static void create();
10 void print() {
11 cout << "haha" << endl;
12 }
13 ~Singlton() {
14 cout << "destruct singlton" << endl;
15 }
16 protected:
17 Singlton();
18 //private:
19 protected:
20 static shared_ptr<T> s_instance;
21 //Singlton();
22 };
23 template <typename T>
24 shared_ptr<T> Singlton<T>::s_instance;
25
26 template <typename T>
27 Singlton<T>::Singlton() {
28 cout << "construct singlton" << endl;
29 }
30
31 template <typename T>
32 T* Singlton<T>::instance() {
33 return s_instance.get();
34 }
35
36 template <typename T>
37 void Singlton<T>::create() {
38 if (!s_instance.get())
39 s_instance.reset(new T);
40
41 }
42
43 // well 这里注意，我理解为Singlton<T>::create() 应该可以调用 MySinglton的私有函数，但事实上不行
44 // 因为理论上 还是调用基类的函数 Singlton<MySinglton>::create()
45 //class MySinglton : public Singlton<MySinglton> {
46 // //private:
47 // public:
48 // MySinglton(){
49 // cout << "construct my singlton" << endl;
50 // }
51 //
52 //};
53 class MySinglton : public Singlton<MySinglton> {
54 friend class Singlton<MySinglton>;
55 private:
56 MySinglton(){
57 cout << "construct my singlton" << endl;
58 }
59 MySinglton * MyInstance() {
60 return s_instance.get();
61 }
62 };
63
64 //or can directyly define one class like MyClass1, and to not consider siglton part,
65 //than use Singlton<MyClass1> is OK. May be typedef Singlton<MyClass1> MyClass1Singlton
66 //and use MyClass1Siglton
67 int main(int argc, char *argv[])
68 {
69
70 MySinglton::create();
71 MySinglton* p_my_singlton = MySinglton::instance();
72 p_my_singlton->print();
73 return 0;
74 }
75 /*result
76 construct singlton
77 construct my singlton
78 haha
79 destruct singlton
80 */
OPMESH的singlton模版类的实现，这个似乎更专业些：）
1 //SingltonT.hh使用的时候如
2 //=============================================================================
3 //
4 // Implements a simple singleton template
5 //
6 //=============================================================================
7
8
9 #ifndef __SINGLETON_HH__
10 #define __SINGLETON_HH__
11
12
13 //=== INCLUDES ================================================================
14
15 // OpenMesh
16 #include <OpenMesh/Core/System/config.h>
17
18 // STL
19 #include <stdexcept>
20 #include <iostream>
21
22
23 //== NAMESPACES ===============================================================
24
25
26 namespace OpenMesh {
27
28
29 //=== IMPLEMENTATION ==========================================================
30
31
32 /** A simple singleton template.
33 Encapsulates an arbitrary class and enforces its uniqueness.
34 */
35
36 template <typename T>
37 class SingletonT
38 {
39 public:
40
41 /** Singleton access function.
42 Use this function to obtain a reference to the instance of the
43 encapsulated class. Note that this instance is unique and created
44 on the first call to Instance().
45 */
46
47 static T& Instance()
48 {
49 if (!pInstance__)
50 {
51 // check if singleton alive
52 if (destroyed__)
53 {
54 OnDeadReference();
55 }
56 // first time request -> initialize
57 else
58 {
59 Create();
60 }
61 }
62 return *pInstance__;
63 }
64
65
66 private:
67
68 // Disable constructors/assignment to enforce uniqueness
69 SingletonT();
70 SingletonT(const SingletonT&);
71 SingletonT& operator=(const SingletonT&);
72
73 // Create a new singleton and store its pointer
74 static void Create()
75 {
76 static T theInstance;
77 pInstance__ = &theInstance;
78 }
79
80 // Will be called if instance is accessed after its lifetime has expired
81 static void OnDeadReference()
82 {
83 throw std::runtime_error("[Singelton error] - Dead reference detected!\n");
84 }
85
86 virtual ~SingletonT()
87 {
88 pInstance__ = 0;
89 destroyed__ = true;
90 }
91
92 static T* pInstance__;
93 static bool destroyed__;
94 };
95
96
97
98 //=============================================================================
99 } // namespace OpenMesh
100 //=============================================================================
101 #if defined(OM_INCLUDE_TEMPLATES) && !defined(OPENMESH_SINGLETON_C)
102 # define OPENMESH_SINGLETON_TEMPLATES
103 # include "SingletonT.cc"
104 #endif
105 //=============================================================================
106 #endif // __SINGLETON_HH__
107 //=============================================================================
108
109
110 //SingltonT.cc
111 //=============================================================================
112 //
113 // Implements a simple singleton template
114 //
115 //=============================================================================
116
117
118 #define OPENMESH_SINGLETON_C
119
120
121 //== INCLUDES =================================================================
122
123
124 // header
125 #include <OpenMesh/Core/Utils/SingletonT.hh>
126
127
128 //== NAMESPACES ===============================================================
129
130
131 namespace OpenMesh {
132
133
134 //== SINGLETON'S DATA =========================================================
135
136
137 template <class T>
138 T* SingletonT<T>::pInstance__ = 0;
139
140 template <class T>
141 bool SingletonT<T>::destroyed__ = false;
142
143
144 //=============================================================================
145 } // namespace OpenMesh
146 //=============================================================================
147
typedef SingletonT<LoopSchemeMaskDouble> LoopSchemeMaskDoubleSingleton;
- singlton in python
那么在python中，作者提到的singlton的概念得到了放宽，
Alex Martelli makes the observation that what we really want with a
Singleton is to have a single set of state data for all objects. That is, you
could create as many objects as you want and as long as they all refer to
the same state information then you achieve the effect of Singleton.
你可以有任意多的对象，但是它们都指向相同的状态信息，即为singlton。Borg利用__dict__属性巧妙的实现了一个
singlton 模式。
1 class Borg():
2 shared_dict = {}
3 def __init__(self):
4 self.__dict__ = self.shared_dict
5
6 class Singleton(Borg):
7 def __init__(self, arg):
8 Borg.__init__(self)
9 self.val = arg
10 def __str__(self):
11 print(self.__dict__)
12 return self.val
13
14
15 if __name__ == '__main__':
16 x = Singleton('abc')
17 y = Singleton('def')
18 print(x)
19 print(y)
20
21 output = '''
22 {'val': 'def'}
23 def
24 {'val': 'def'}
25 def
26 '''
这种方案，简洁清楚，并且很容易通过继承而复用。当然作者还提到许多其它的实现方法，对比下面的方法。
1 class OnlyOne:
2
3 class __OnlyOne:
4 def __init__(self, arg):
5 self.val = arg
6 def __str__(self):
7 return 'self' + self.val
8 #return `self` + self.val
9 instance = None
10 def __init__(self, arg):
11 if not OnlyOne.instance:
12 OnlyOne.instance = OnlyOne.__OnlyOne(arg)
13 else:
14 OnlyOne.instance.val = arg
15 def __getattr__(self, name):
16 return getattr(self.instance, name)
17
18
19 x = OnlyOne('sausage')
20 print(x)
21 y = OnlyOne('eggs')
22 print(y)
23 z = OnlyOne('spam')
24 print(z)
25
26 print(x)
27 print(y)
28 print(z)
29 print('x')
30 print('y')
31 print('z')
32
33 print(x.instance)
34 output = '''
35 selfsausage
36 selfeggs
37 selfspam
38 selfspam
39 selfspam
40 selfspam
41 x
42 y
43 z
44 selfspam
45 '''
3 class __OnlyOne:
4 def __init__(self, arg):
5 self.val = arg
6 def __str__(self):
7 return 'self' + self.val
8 #return `self` + self.val
9 instance = None
10 def __init__(self, arg):
11 if not OnlyOne.instance:
12 OnlyOne.instance = OnlyOne.__OnlyOne(arg)
13 else:
14 OnlyOne.instance.val = arg
15 def __getattr__(self, name):
16 return getattr(self.instance, name)
17
18
19 x = OnlyOne('sausage')
20 print(x)
21 y = OnlyOne('eggs')
22 print(y)
23 z = OnlyOne('spam')
24 print(z)
25
26 print(x)
27 print(y)
28 print(z)
29 print('x')
30 print('y')
31 print('z')
32
33 print(x.instance)
34 output = '''
35 selfsausage
36 selfeggs
37 selfspam
38 selfspam
39 selfspam
40 selfspam
41 x
42 y
43 z
44 selfspam
45 '''
