(原創) 若class中的data member有container，而且內含pointer時，也一定要big three!! (C/C++)
Abstract
當class的data member含pointer時，我們知道此時一定要big three(copy constructor，assignment operator，destructor)，若是container內含pointer時呢?答案是也需big three。
Introduction
首先做個實驗，有一個vector為v1，內含Foo*，我們希望clone出一個vector v2，使用vector所提供的copy constructor進行clone。
1
#include <iostream>
2#include <vector>
3
4using namespace std;
5
6
class Foo {
7
private:
8 int _value;
9
10public:
11
Foo(int n = 0) : _value(n) {}
12
13 int getValue() {
14 return _value;
15
}
16
};
17
18int main() {
19 vector<Foo*> v1;
20 v1.push_back(new Foo(1));
21 v1.push_back(new Foo(2));
22 v1.push_back(new Foo(3));
23
24 cout << "vector v1:" << endl;
25 for(vector<Foo*>::iterator iter = v1.begin(); iter != v1.end(); ++iter)
26 cout << (*iter)->getValue() << " ";
27
28 vector<Foo*>v2(v1);
29
30 cout << endl << "vector v2:" << endl;
31 for(vector<Foo*>::iterator iter = v2.begin(); iter != v2.end(); ++iter)
32 cout << (*iter)->getValue() << " ";
33
34 cout << endl;
35}
#include <iostream>2
#include <vector>3
4
using namespace std;5
6
class Foo {
7
private:8
int _value;9
10
public:11
Foo(int n = 0) : _value(n) {}
Foo(12
13
int getValue() {14
return _value;15
}16
};17
18
int main() {19
vector<Foo*> v1;20
v1.push_back(new Foo(1));21
v1.push_back(new Foo(2));22
v1.push_back(new Foo(3));23
24
cout << "vector v1:" << endl;25
for(vector<Foo*>::iterator iter = v1.begin(); iter != v1.end(); ++iter)26
cout << (*iter)->getValue() << " ";27
28
vector<Foo*>v2(v1);29
30
cout << endl << "vector v2:" << endl;31
for(vector<Foo*>::iterator iter = v2.begin(); iter != v2.end(); ++iter)32
cout << (*iter)->getValue() << " ";33
34
cout << endl;35
}
執行結果
vector v1:1 2 3vector v2:1 2 3
28行
vector<Foo*>v2(v1);
使用vector的copy constructor進行clone。
現在我們將v1內的pointer刪除，若v2能正常顯示，則表示v2 clone成功。
1#include <iostream>
2#include <vector>
3
4using namespace std;
5
6class Foo {
7private:
8 int _value;
9
10public:
11 Foo(int n = 0) : _value(n) {}
12
13 int getValue() {
14 return _value;
15 }
16};
17
18int main() {
19 vector<Foo*> v1;
20 v1.push_back(new Foo(1));
21 v1.push_back(new Foo(2));
22 v1.push_back(new Foo(3));
23
24 cout << "vector v1:" << endl;
25 for(vector<Foo*>::iterator iter = v1.begin(); iter != v1.end(); ++iter)
26 cout << (*iter)->getValue() << " ";
27
28 vector<Foo*>v2(v1);
29
30 for(vector<Foo*>::iterator iter = v1.begin(); iter != v1.end(); ++iter)
31 delete *iter;
32
33 cout << endl << "vector v2:" << endl;
34 for(vector<Foo*>::iterator iter = v2.begin(); iter != v2.end(); ++iter)
35 cout << (*iter)->getValue() << " ";
36
37 cout << endl;
38}
#include <iostream>2
#include <vector>3
4
using namespace std;5
6
class Foo {7
private:8
int _value;9
10
public:11
Foo(int n = 0) : _value(n) {}12
13
int getValue() {14
return _value;15
}16
};17
18
int main() {19
vector<Foo*> v1;20
v1.push_back(new Foo(1));21
v1.push_back(new Foo(2));22
v1.push_back(new Foo(3));23
24
cout << "vector v1:" << endl;25
for(vector<Foo*>::iterator iter = v1.begin(); iter != v1.end(); ++iter)26
cout << (*iter)->getValue() << " ";27
28
vector<Foo*>v2(v1);29
30
for(vector<Foo*>::iterator iter = v1.begin(); iter != v1.end(); ++iter)31
delete *iter;32
33
cout << endl << "vector v2:" << endl;34
for(vector<Foo*>::iterator iter = v2.begin(); iter != v2.end(); ++iter)35
cout << (*iter)->getValue() << " ";36
37
cout << endl;38
}
執行結果
vector v1:1 2 3vector v2: -572662307 -572662307 -572662307
30~31行
for(vector<Foo*>::iterator iter = v1.begin(); iter != v1.end(); ++iter) delete *iter;
只要將v1中的pointer delete，v2就無法顯示，證明vector的copy constructor並非真正的clone，只是將pointer copy過去，這樣會造成v1和v2中的pointer指的是同一個object，只要v1一delete的，v2也無法正常了。
正確的寫法如下
1/*
2(C) OOMusou 2007 http://oomusou.cnblogs.com
3
4Filename : ContainerWithPointer_Clone.cpp
5Compiler : Visual C++ 8.0 / BCB 6.0 / gcc 3.4.2 / ISO C++
6Description : Demo how to clone container with pointer
7Release : 05/18/2007 1.0
8*/
9#include <iostream>
10#include <vector>
11#include <algorithm>
12#include <functional>
13
14using namespace std;
15
16struct printElem {
17 template <typename T>
18 void operator()(T*& iter) const {
19 cout << iter->getValue() << " ";
20 }
21};
22
23class Foo {
24private:
25 int _value;
26
27public:
28 Foo(int n = 0) : _value(n) {}
29
30 int getValue() {
31 return _value;
32 }
33};
34
35int main() {
36 vector<Foo*> v1;
37 v1.push_back(new Foo(1));
38 v1.push_back(new Foo(2));
39 v1.push_back(new Foo(3));
40
41 cout << "vector v1:" << endl;
42 for_each(v1.begin(), v1.end(), printElem());
43
44 vector<Foo*> v2;
45 for(vector<Foo*>::iterator iter = v1.begin(); iter != v1.end(); ++iter) {
46 v2.push_back(new Foo((*iter)->getValue()));
47 }
48
49 for(vector<Foo*>::iterator iter = v1.begin(); iter != v1.end(); ++iter)
50 delete *iter;
51
52 cout << endl << "vector v2:" << endl;
53 for_each(v2.begin(), v2.end(), printElem());
54
55 cout << endl;
56}
/* 2
(C) OOMusou 2007 http://oomusou.cnblogs.com3
4
Filename : ContainerWithPointer_Clone.cpp5
Compiler : Visual C++ 8.0 / BCB 6.0 / gcc 3.4.2 / ISO C++6
Description : Demo how to clone container with pointer7
Release : 05/18/2007 1.08
*/9
#include <iostream>10
#include <vector>11
#include <algorithm>12
#include <functional>13
14
using namespace std;15
16
struct printElem {17
template <typename T>18
void operator()(T*& iter) const {19
cout << iter->getValue() << " ";20
}21
};22
23
class Foo {24
private:25
int _value;26
27
public:28
Foo(int n = 0) : _value(n) {}29
30
int getValue() {31
return _value;32
}33
};34
35
int main() {36
vector<Foo*> v1;37
v1.push_back(new Foo(1));38
v1.push_back(new Foo(2));39
v1.push_back(new Foo(3));40
41
cout << "vector v1:" << endl;42
for_each(v1.begin(), v1.end(), printElem());43
44
vector<Foo*> v2; 45
for(vector<Foo*>::iterator iter = v1.begin(); iter != v1.end(); ++iter) {46
v2.push_back(new Foo((*iter)->getValue()));47
}48
49
for(vector<Foo*>::iterator iter = v1.begin(); iter != v1.end(); ++iter)50
delete *iter;51
52
cout << endl << "vector v2:" << endl;53
for_each(v2.begin(), v2.end(), printElem());54
55
cout << endl;56
}
執行結果
vector v1:1 2 3vector v2:1 2 3
44行
vector<Foo*> v2; for(vector<Foo*>::iterator iter = v1.begin(); iter != v1.end(); ++iter) { v2.push_back(new Foo((*iter)->getValue())); }
唯有重新new過，這樣才叫做真正的clone。
以下介紹兩種方法，達成big three的要求。
1.方法一：使用一般big three的方式。
1/*
2(C) OOMusou 2007 http://oomusou.cnblogs.com
3
4Filename : ContainerWithPointer_BigThree.cpp
5Compiler : Visual C++ 8.0 / BCB 6.0 / gcc 3.4.2 / ISO C++
6Description : Demo how to use big three for pointer in container
7Release : 05/18/2007 1.0
8*/
9#include <iostream>
10#include <vector>
11#include <algorithm>
12#include <functional>
13
14using namespace std;
15
16struct DeletePointer {
17 template<typename T>
18 void operator()(const T* ptr) const {
19 if (ptr) {
20 delete ptr;
21 ptr = 0;
22 }
23 }
24};
25
26class Foo {
27private:
28 int _value;
29
30public:
31 Foo(int n = 0) : _value(n) {}
32
33 int getValue() const {
34 return _value;
35 }
36};
37
38class Moo {
39private:
40 vector<Foo*> _foos;
41
42public:
43 Moo() {}
44 // copy constructor
45 Moo(Moo& otherMoo) {
46 for(vector<Foo*>::iterator iter = otherMoo._foos.begin(); iter != otherMoo._foos.end(); ++iter) {
47 _foos.push_back(new Foo((*iter)->getValue()));
48 }
49 }
50
51 // Assignment operator
52 Moo& operator=(Moo& otherMoo) {
53 for(vector<Foo*>::iterator iter = otherMoo._foos.begin(); iter != otherMoo._foos.end(); ++iter) {
54 _foos.push_back(new Foo((*iter)->getValue()));
55 }
56
57 return *this;
58 }
59
60 // Destructor
61 ~Moo() {
62 for_each(_foos.begin(), _foos.end(),DeletePointer());
63 }
64
65 void insert(Foo* foo) {
66 _foos.push_back(foo);
67 }
68
69 void printElem() const {
70 transform(_foos.begin(), _foos.end(), ostream_iterator<int>(cout, " "), mem_fun(&Foo::getValue));
71
72 cout << endl;
73 }
74};
75
76int main() {
77 Moo moo1;
78 moo1.insert(new Foo(1));
79 moo1.insert(new Foo(2));
80 moo1.insert(new Foo(3));
81 cout << "moo1:";
82 moo1.printElem();
83
84 // use copy constructor
85 Moo moo2(moo1);
86 Moo moo3 = moo1;
87
88 // use assign operator
89 Moo moo4;
90 moo4 = moo1;
91
92 // call moo1's destuctor to delete all pointer
93 moo1.~Moo();
94
95 cout << "moo2:";
96 moo2.printElem();
97 cout << "moo3:";
98 moo3.printElem();
99 cout << "moo4:";
100 moo4.printElem();
101}
/* 2
(C) OOMusou 2007 http://oomusou.cnblogs.com3
4
Filename : ContainerWithPointer_BigThree.cpp5
Compiler : Visual C++ 8.0 / BCB 6.0 / gcc 3.4.2 / ISO C++6
Description : Demo how to use big three for pointer in container7
Release : 05/18/2007 1.08
*/9
#include <iostream>10
#include <vector>11
#include <algorithm>12
#include <functional>13
14
using namespace std;15
16
struct DeletePointer {17
template<typename T>18
void operator()(const T* ptr) const {19
if (ptr) {20
delete ptr;21
ptr = 0;22
}23
}24
};25
26
class Foo {27
private:28
int _value;29
30
public:31
Foo(int n = 0) : _value(n) {}32
33
int getValue() const {34
return _value;35
}36
};37
38
class Moo {39
private:40
vector<Foo*> _foos;41
42
public: 43
Moo() {}44
// copy constructor45
Moo(Moo& otherMoo) {46
for(vector<Foo*>::iterator iter = otherMoo._foos.begin(); iter != otherMoo._foos.end(); ++iter) {47
_foos.push_back(new Foo((*iter)->getValue()));48
}49
}50
51
// Assignment operator52
Moo& operator=(Moo& otherMoo) {53
for(vector<Foo*>::iterator iter = otherMoo._foos.begin(); iter != otherMoo._foos.end(); ++iter) {54
_foos.push_back(new Foo((*iter)->getValue()));55
}56
57
return *this;58
}59
60
// Destructor61
~Moo() {62
for_each(_foos.begin(), _foos.end(),DeletePointer());63
}64
65
void insert(Foo* foo) {66
_foos.push_back(foo);67
}68
69
void printElem() const {70
transform(_foos.begin(), _foos.end(), ostream_iterator<int>(cout, " "), mem_fun(&Foo::getValue));71
72
cout << endl;73
}74
};75
76
int main() {77
Moo moo1;78
moo1.insert(new Foo(1));79
moo1.insert(new Foo(2));80
moo1.insert(new Foo(3));81
cout << "moo1:";82
moo1.printElem();83
84
// use copy constructor85
Moo moo2(moo1);86
Moo moo3 = moo1;87
88
// use assign operator89
Moo moo4;90
moo4 = moo1;91
92
// call moo1's destuctor to delete all pointer93
moo1.~Moo();94
95
cout << "moo2:";96
moo2.printElem();97
cout << "moo3:";98
moo3.printElem();99
cout << "moo4:";100
moo4.printElem();101
}
執行結果
moo1:1 2 3moo2:1 2 3moo3:1 2 3moo4:1 2 3
16行
struct DeletePointer { template<typename T> void operator()(const T* ptr) const { if (ptr) { delete ptr; ptr = 0; } }};
是effective STL item 7所介紹刪除pointer的方法，花了二頁的篇幅介紹為什麼不用class template而用function object的function template。
44行
// copy constructorMoo(Moo& otherMoo) { for(vector<Foo*>::iterator iter = otherMoo._foos.begin(); iter != otherMoo._foos.end(); ++iter) { _foos.push_back(new Foo((*iter)->getValue())); }}
重新改寫copy constructor，避免compiler只是做pointer的copy，而非值的copy。
51行
// Assignment operatorMoo& operator=(Moo& otherMoo) { for(vector<Foo*>::iterator iter = otherMoo._foos.begin(); iter != otherMoo._foos.end(); ++iter) { _foos.push_back(new Foo((*iter)->getValue())); } return *this;}
重新改寫assign operator，避免compiler只是做pointer的assign，而非值的assign。
62行
for_each(_foos.begin(), _foos.end(),DeletePointer());
手動刪除所有pointer。
93行
// call moo1's destuctor to delete all pointermoo1.~Moo();
呼叫了moo1的destructor，故意刪除了moo1中所有的pointer，以測試copy constructor和assignment operator是否正常。
方法二：另外加上clone() member function。
1/*
2(C) OOMusou 2007 http://oomusou.cnblogs.com
3
4Filename : ContainerWithPointer_BigThree2.cpp
5Compiler : Visual C++ 8.0 / BCB 6.0 / gcc 3.4.2 / ISO C++
6Description : Demo how to use big three for pointer in container by clone()
7Release : 05/18/2007 1.0
8*/
9#include <iostream>
10#include <vector>
11#include <algorithm>
12#include <functional>
13
14using namespace std;
15
16struct DeletePointer {
17 template<typename T>
18 void operator()(const T* ptr) const {
19 if (ptr) {
20 delete ptr;
21 ptr = 0;
22 }
23 }
24};
25
26class Foo {
27private:
28 int _value;
29
30public:
31 Foo(int n = 0) : _value(n) {}
32
33 virtual Foo* clone() const {
34 return new Foo(*this);
35 }
36
37 int getValue() const {
38 return _value;
39 }
40};
41
42class Moo {
43private:
44 vector<Foo*> _foos;
45
46public:
47 Moo() {}
48 // copy constructor
49 Moo(Moo& otherMoo) {
50 transform(otherMoo._foos.begin(), otherMoo._foos.end(), back_inserter(_foos), mem_fun(&Foo::clone));
51 }
52
53 // Assignment operator
54 Moo& operator=(Moo& otherMoo) {
55 transform(otherMoo._foos.begin(), otherMoo._foos.end(), back_inserter(_foos), mem_fun(&Foo::clone));
56
57 return *this;
58 }
59
60 // Destructor
61 ~Moo() {
62 for_each(_foos.begin(), _foos.end(),DeletePointer());
63 }
64
65 void insert(Foo* foo) {
66 _foos.push_back(foo);
67 }
68
69 void printElem() const {
70 transform(_foos.begin(), _foos.end(), ostream_iterator<int>(cout, " "), mem_fun(&Foo::getValue));
71
72 cout << endl;
73 }
74};
75
76int main() {
77 Moo moo1;
78 moo1.insert(new Foo(1));
79 moo1.insert(new Foo(2));
80 moo1.insert(new Foo(3));
81 cout << "moo1:";
82 moo1.printElem();
83
84 // use copy constructor
85 Moo moo2(moo1);
86 Moo moo3 = moo1;
87
88 // use assign operator
89 Moo moo4;
90 moo4 = moo1;
91
92 // call moo1's destuctor to delete all pointer
93 moo1.~Moo();
94
95 cout << "moo2:";
96 moo2.printElem();
97 cout << "moo3:";
98 moo3.printElem();
99 cout << "moo4:";
100 moo4.printElem();
101}
/* 2
(C) OOMusou 2007 http://oomusou.cnblogs.com3
4
Filename : ContainerWithPointer_BigThree2.cpp5
Compiler : Visual C++ 8.0 / BCB 6.0 / gcc 3.4.2 / ISO C++6
Description : Demo how to use big three for pointer in container by clone()7
Release : 05/18/2007 1.08
*/9
#include <iostream>10
#include <vector>11
#include <algorithm>12
#include <functional>13
14
using namespace std;15
16
struct DeletePointer {17
template<typename T>18
void operator()(const T* ptr) const {19
if (ptr) {20
delete ptr;21
ptr = 0;22
}23
}24
};25
26
class Foo {27
private:28
int _value;29
30
public:31
Foo(int n = 0) : _value(n) {}32
33
virtual Foo* clone() const {34
return new Foo(*this);35
}36
37
int getValue() const {38
return _value;39
}40
};41
42
class Moo {43
private:44
vector<Foo*> _foos;45
46
public: 47
Moo() {}48
// copy constructor49
Moo(Moo& otherMoo) {50
transform(otherMoo._foos.begin(), otherMoo._foos.end(), back_inserter(_foos), mem_fun(&Foo::clone));51
}52
53
// Assignment operator54
Moo& operator=(Moo& otherMoo) {55
transform(otherMoo._foos.begin(), otherMoo._foos.end(), back_inserter(_foos), mem_fun(&Foo::clone));56
57
return *this;58
}59
60
// Destructor61
~Moo() {62
for_each(_foos.begin(), _foos.end(),DeletePointer());63
}64
65
void insert(Foo* foo) {66
_foos.push_back(foo);67
}68
69
void printElem() const {70
transform(_foos.begin(), _foos.end(), ostream_iterator<int>(cout, " "), mem_fun(&Foo::getValue));71
72
cout << endl;73
}74
};75
76
int main() {77
Moo moo1;78
moo1.insert(new Foo(1));79
moo1.insert(new Foo(2));80
moo1.insert(new Foo(3));81
cout << "moo1:";82
moo1.printElem();83
84
// use copy constructor85
Moo moo2(moo1);86
Moo moo3 = moo1;87
88
// use assign operator89
Moo moo4;90
moo4 = moo1;91
92
// call moo1's destuctor to delete all pointer93
moo1.~Moo();94
95
cout << "moo2:";96
moo2.printElem();97
cout << "moo3:";98
moo3.printElem();99
cout << "moo4:";100
moo4.printElem();101
}
執行結果
moo1:1 2 3moo2:1 2 3moo3:1 2 3moo4:1 2 3
33行
virtual Foo* clone() const { return new Foo(*this);}
自己加上一個clone() member function。
48行
Moo(Moo& otherMoo) { transform(otherMoo._foos.begin(), otherMoo._foos.end(), back_inserter(_foos), mem_fun(&Foo::clone));}
因為已經有了clone()，copy constructor變的非常精簡，只需用transform()複製vector即可，注意此處不能用copy()，因為copy()只能針對vector內的iterator做copy，而不能呼叫iterator的member function，所以在此可以發現trnasform()比copy()功能強大。
雖然使用clone()看似方便，程式非常精簡，但clone()有個缺點，就是得另外增加clone()，若使用component無法修改source code時，則無法達成。
若在inheritance hierarchy下，使用方法一將非常麻煩，方法二可獲得相當漂亮的解法。
Conclusion
當container內裝pointer時，也必須乖乖的big three，才能保證執行結果正確，我承認這是C++很麻煩的地方，但C++的style就是如此，class內不用pointer則已，一旦用了pointer就得big three。
See Also
(原創) 若class中data member的container，含的是polymorphism的pointer，該如何big three? (高級) (C++) (OO C++)
浙公网安备 33010602011771号