(转)谈谈C++中的swap函数

(转)谈谈C++中的swap函数
转自：http://blog.csdn.net/ryfdizuo/article/details/6435847
1，最通用的模板交换函数模式：创建临时对象，调用对象的赋值操作符。
[cpp] view plain copy
print?
1. template <class T> void swap ( T& a, T& b )
2. {
3. T c(a); a=b; b=c;
4. }
需要构建临时对象，一个拷贝构造，两次赋值操作。
2，针对int型优化：
[cpp] view plain copy
print?
1. void swap(int & __restrict a, int & __restrict b)
2. {
3. a ^= b;
4. b ^= a;
5. a ^= b;
6. }
无需构造临时对象，异或
因为指针是int，所以基于这个思路可以优化1：
[cpp] view plain copy
print?
1. template <typename T> void Swap(T & obj1,T & obj2)
2. {
3. unsigned char * pObj1 = reinterpret_cast<unsigned char *>(&obj1);
4. unsigned char * pObj2 = reinterpret_cast<unsigned char *>(&obj2);
5. for (unsigned long x = 0; x < sizeof(T); ++x)
6. {
7. pObj1[x] ^= pObj2[x];
8. pObj2[x] ^= pObj1[x];
9. pObj1[x] ^= pObj2[x];
10. }
11. }
3，针对内建类型的优化： int, flaot, double 等，甚至重载运算符的用户自定义类型：向量，矩阵，图像等。。。
type a; -- e.g 10
type b; -- e.g 5

a = a+b ; -- a=15,b=5
b = a-b ; -- a=15,b=10
a= a -b ; -- a= 5,b=10
// 无需构造临时变量。使用基本运算操作符。
[cpp] view plain copy
print?
1. Ok, let's see.
2. a = a + b;
3. b = a - b;
4. a = a - b;
5. Let's introduce new names
6. c = a + b;
7. d = c - b;
8. e = c - d;
9. And we want to prove that d == a and e == b.
10. d = (a + b) - b = a, proved.
11. e = (a + b) - ((a + b) - b) = (a + b) - a = b, proved.
12. For all real numbers.
4，swap的一些特化：
std::string, std::vector各自实现了swap函数，
string中
[cpp] view plain copy
print?
1. template<class _Elem,
2. class _Traits,
3. class _Alloc> inline
4. void __CLRCALL_OR_CDECL swap(basic_string<_Elem, _Traits, _Alloc>& _Left,
5. basic_string<_Elem, _Traits, _Alloc>& _Right)
6. { // swap _Left and _Right strings
7. _Left.swap(_Right);
8. }
9. void __CLR_OR_THIS_CALL swap(_Myt& _Right)
10. { // exchange contents with _Right
11. if (this == &_Right)
12. ; // same object, do nothing
13. else if (_Mybase::_Alval == _Right._Alval)
14. { // same allocator, swap control information
15. #if _HAS_ITERATOR_DEBUGGING
16. this->_Swap_all(_Right);
17. #endif /* _HAS_ITERATOR_DEBUGGING */
18. _Bxty _Tbx = _Bx;
19. _Bx = _Right._Bx, _Right._Bx = _Tbx;
20. size_type _Tlen = _Mysize;
21. _Mysize = _Right._Mysize, _Right._Mysize = _Tlen;
22. size_type _Tres = _Myres;
23. _Myres = _Right._Myres, _Right._Myres = _Tres;
24. }
25. else
26. { // different allocator, do multiple assigns
27. _Myt _Tmp = *this;
28. *this = _Right;
29. _Right = _Tmp;
30. }
31. }
第二个swap(Right)进行判断，如果使用了相同的分配器，则直接交换控制信息，否则调用string::operator=进行拷贝赋值。。。所以建议优先使用swap函数，而不是赋值操作符。

vector中
[cpp] view plain copy
print?
1. template<class _Ty,
2. class _Alloc> inline
3. void swap(vector<_Ty, _Alloc>& _Left, vector<_Ty, _Alloc>& _Right)
4. { // swap _Left and _Right vectors
5. _Left.swap(_Right);
6. }
7. void swap(_Myt& _Right)
8. { // exchange contents with _Right
9. if (this == &_Right)
10. ; // same object, do nothing
11. else if (this->_Alval == _Right._Alval)
12. { // same allocator, swap control information
13. #if _HAS_ITERATOR_DEBUGGING
14. this->_Swap_all(_Right);
15. #endif /* _HAS_ITERATOR_DEBUGGING */
16. this->_Swap_aux(_Right);
17. _STD swap(_Myfirst, _Right._Myfirst);
18. _STD swap(_Mylast, _Right._Mylast);
19. _STD swap(_Myend, _Right._Myend);
20. }
21. else
22. { // different allocator, do multiple assigns
23. this->_Swap_aux(_Right);
24. _Myt _Ts = *this;
25. *this = _Right;
26. _Right = _Ts;
27. }
28. }
vector的swap原理跟string完全一致，只有当当使用了不同分配器才进行字节拷贝。其余情况直接交换控制信息。

测试用例：

5，Copy and Swap idiom
目的：C++异常有三个级别：基本，强，没有异常。通过创建临时对象然后交换，能够实现重载赋值操作符的强异常安全的执行。
Loki中智能指针临时变量跟this交换，临时变量自动销毁~
[cpp] view plain copy
print?
1. SmartPtr& operator=(SmartPtr<T1, OP1, CP1, KP1, SP1, CNP1 >& rhs)
2. {
3. SmartPtr temp(rhs);
4. temp.Swap(*this);
5. return *this;
6. }
boost::share_ptr，share_ptr定义了自己的swap函数。
[cpp] view plain copy
print?
1. shared_ptr & operator=( shared_ptr const & r ) // never throws
2. {
3. this_type(r).swap(*this);
4. return *this;
5. }
6. void swap(shared_ptr<T> & other) // never throws
7. {
8. std::swap(px, other.px);
9. pn.swap(other.pn);
10. }
记得本科上C++课，老师特别喜欢拿String来举例子，面试题也特别喜欢String。。。下面说说String::opreator=函数的优化：
最一般的写法，特点：使用const string& 传参防止临时对象。
[cpp] view plain copy
print?
1. String& String::operator =(const String & rhs)
2. {
3. if (itsString)
4. delete [] itsString;
5. itsLen = rhs.GetLen();
6. itsString = new char[itsLen+1];
7. for (unsigned short i = 0;i<itsLen;i++)
8. itsString[i] = rhs[i];
9. itsString[itsLen] = '/0';
10. return *this;
11. }
优化1，防止自我间接赋值，a = b; c = b; a = c; 如果没有第一个if判断，当把c赋给a的时候，删除了a.itsString，后面的拷贝就会出错。注意是if(this==&rhs), 而不是if(*this==rhs) .
[cpp] view plain copy
print?
1. String& String::operator =(const String & rhs)
2. {
3. if (this == &rhs)
4. return *this;
5. if (itsString)
6. delete [] itsString;
7. itsLen=rhs.GetLen();
8. itsString = new char[itsLen+1];
9. for (unsigned short i = 0;i<itsLen;i++)
10. itsString[i] = rhs[i];
11. itsString[itsLen] = '/0';
12. return *this;
13. }
优化2，不进行拷贝赋值，只是交换控制信息，而且是强异常安全：
[cpp] view plain copy
print?
1. String & String::operator = (String const &rhs)
2. {
3. if (this != &rhs)
4. String(rhs).swap (*this); // Copy-constructor and non-throwing swap
5. // Old resources are released with the destruction of the temporary above
6. return *this;
7. }
优化3，以最原始的传值方式传参，避免临时对象创建：
[cpp] view plain copy
print?
1. String & operator = (String s) // the pass-by-value parameter serves as a temporary
2. {
3. s.swap (*this); // Non-throwing swap
4. return *this;
5. }// Old resources released when destructor of s is called.
最后这张方式主要是对C++新特性rvalue的优化，具体参见：http://en.wikibooks.org/wiki/More_C++_Idioms/Copy-and-swap
6. vector clear and swap trick
vector.clear并只是将size变量置为0，并没有及时归还OS，STL仍然持有内存，以便后续push_back。实测如下：
[cpp] view plain copy
print?
1. vector<int> temp;
此时打开资源管理器，内存如下：

增长vector然后清空：
[cpp] view plain copy
print?
1. temp.resize( 1024*1024*20 ); // 80M
2. temp.clear();
此时资源管理器内存：

clear以后进程兵没有及时将内存归还OS。。。通过swap方法：
[cpp] view plain copy
print?
1. tmp.resize(1024*1024*20); // 80M
2. // tmp.clear();
3. {
4. std::vector<int>().swap(tmp); // 将内存归还OS
5. }
退出作用域，临时对象销毁。内存归还OS。此时资源管理器中进程内存回到1,864K。

附上网络版的String：
[cpp] view plain copy
print?
1. #include <iostream>
2. #include <cstring>
3. using namespace std;
4. class String
5. {
6. public:
7. String();
8. String(const char *const);
9. String(const String &);
10. ~String();
11. char & operator[] (unsigned short offset);
12. char operator[] (unsigned short offset)const;
13. String operator+(const String&);
14. void operator+=(const String&);
15. String & operator= (const String &);
16. unsigned short GetLen()const {return itsLen;}
17. const char * GetString()const {return itsString;}
18. private:
19. String (unsigned short);
20. char * itsString;
21. unsigned short itsLen;
22. };
23. String::String()
24. {
25. itsString = new char[1]; //为什么设置成1，这样会导致内存1bytes无法释放吗？我觉得和itsString = new char没区别，那他为什么要设置成1，这样有什么用？21天学会C++那本书，我也有，书上也确实是设置成1.
26. itsString[0] = '/0';
27. itsLen=0;
28. }
29. String::String(unsigned short len)
30. {
31. itsString = new char[len+1];
32. for (unsigned short i =0;i<=len;i++)
33. itsString[i] = '/0';
34. itsLen=len;
35. }
36. String::String(const char * const cString)
37. {
38. itsLen = strlen(cString);
39. itsString = new char[itsLen+1];
40. for (unsigned short i=0;i<itsLen;i++)
41. itsString[i] = cString[i];
42. itsString[itsLen] = '/0';
43. }
44. String::String(const String & rhs)
45. {
46. itsLen = rhs.GetLen();
47. itsString = new char[itsLen+1];
48. for (unsigned short i = 0;i<itsLen;i++)
49. itsString[i] = rhs[i];
50. itsString[itsLen] = '/0';
51. }
52. String::~String()
53. {
54. delete [] itsString;
55. itsLen = 0;
56. }
57. String& String::operator =(const String & rhs)
58. {
59. if (this == &rhs)
60. return *this;
61. delete [] itsString;
62. itsLen=rhs.GetLen();
63. itsString = new char[itsLen+1];
64. for (unsigned short i = 0;i<itsLen;i++)
65. itsString[i] = rhs[i];
66. itsString[itsLen] = '/0';
67. return *this;
68. }
69. char & String::operator [](unsigned short offset) //这个程序这样写，起到了什么用处？？和main中的那一个对应？
70. {
71. if (offset > itsLen)
72. return itsString[itsLen-1]; //这个返回itslen-1到底是什么意思？为什么要减去1 ？？
73. else
74. return itsString[offset];
75. }
76. char String::operator [](unsigned short offset)const
77. {
78. if (offset > itsLen)
79. itsString[itsLen-1];
80. else
81. return itsString[offset];
82. }
83. String String::operator +(const String& rhs)
84. {
85. unsigned short totalLen = itsLen + rhs.GetLen();
86. String temp(totalLen);
87. unsigned short i;
88. for (i=0;i<itsLen;i++)
89. temp[i] = itsString[i];
90. for (unsigned short j = 0;j<rhs.GetLen();j++,i++)
91. temp[i] = rhs[j];
92. temp[totalLen] = '/0';
93. return temp;
94. }
95. void String::operator +=(const String& rhs)
96. {
97. unsigned short rhsLen = rhs.GetLen();
98. unsigned short totalLen = itsLen + rhsLen;
99. String temp(totalLen);
100. unsigned short i;
101. for (i = 0;i<itsLen;i++)
102. temp[i] = itsString[i];
103. for (unsigned short j = 0;j<rhs.GetLen();j++,i++)
104. temp[i] = rhs[i-itsLen];
105. temp[totalLen] = '/0';
106. }
107. int main()
108. {
110. String s1("initial test"); //调用了什么函数？
111. cout<<"S1:/t"<<s1.GetString()<<endl;
112. char *temp ="Hello World";
113. s1 = temp;//调用了什么函数？
114. cout<<"S1:/t"<<s1.GetString()<<endl;
115. char tempTwo[20];
116. strcpy(tempTwo,"; nice to be here!");
117. s1 += tempTwo;
118. cout<<"tempTwo:/t"<<tempTwo<<endl;
119. cout<<"S1:/t"<<s1.GetString()<<endl;
120. cout<<"S1[4]:/t"<<s1[4]<<endl;
121. cout<<"S1[999]:/t"<<s1[999]<<endl;//调用了什么函数？
122. String s2(" Anoter string");//调用了什么函数？
123. String s3;
124. s3 = s1+s2;
125. cout<<"S3:/t" <<s3.GetString()<<endl;
126. String s4;
127. s4 = "Why does this work?";//调用了什么函数？
128. cout<<"S4:/t"<<s4.GetString()<<endl;
129. return 0;
130. }
参考引用：
1，http://www.vbforums.com/showthread.php?t=245517
2，http://www.cplusplus.com/reference/algorithm/swap/
3，http://codeguru.earthweb.com/forum/showthread.php?t=485643
4，http://stackoverflow.com/questions/1998744/benefits-of-a-swap-function
5，http://answers.google.com/answers/threadview/id/251027.html

C++ idioms
http://en.wikibooks.org/wiki/Category:More_C%2B%2B_Idioms

Copy and Swap idiom
http://stackoverflow.com/questions/3279543/what-is-the-copy-and-swap-idiom

History：
20140401 - add 6 vector clear and swap trick！
相关阅读:
Spring AOP详解、 JDK动态代理、CGLib动态代理
 mysql 日期字符串时间戳转换
 图文：通过sql server 连接mysql
c# 数据绑定之 DataFormatString 格式
 sql 截取字符串与截取字符串最长的字符串
 oracle 清除表空间
 sql 遍历结果print和表格形式
 国家与城市的sql
sql2005 将一列的多行内容拼接成一行
 oracle和mssql中复制表的比较
原文地址：https://www.cnblogs.com/xj2015/p/7383234.html