談談C++中的swap函式

1，最通用的模板交換函式模式：建立臨時物件，呼叫物件的賦值操作符。

template <class T> void swap ( T& a, T& b ) { T c(a); a=b; b=c; }

需要構建臨時物件，一個拷貝構造，兩次賦值操作。

2，針對int型優化：

void swap(int & __restrict a, int & __restrict b) { a ^= b; b ^= a; a ^= b; }

無需構造臨時物件，異或

因為指標是int，所以基於這個思路可以優化1：

template <typename T> void Swap(T & obj1,T & obj2) { unsigned char * pObj1 = reinterpret_cast<unsigned char *>(&obj1); unsigned char * pObj2 = reinterpret_cast<unsigned char *>(&obj2); for (unsigned long x = 0; x < sizeof(T); ++x) { pObj1[x] ^= pObj2[x]; pObj2[x] ^= pObj1[x]; pObj1[x] ^= pObj2[x]; } }

3，針對內建型別的優化： int, flaot, double 等，甚至過載運算子的使用者自定義型別：向量，矩陣，影像等。。。

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

// 無需構造臨時變數。使用基本運算操作符。

Ok, let's see. a = a + b; b = a - b; a = a - b; Let's introduce new names c = a + b; d = c - b; e = c - d; And we want to prove that d == a and e == b. d = (a + b) - b = a, proved. e = (a + b) - ((a + b) - b) = (a + b) - a = b, proved. For all real numbers.

4，swap的一些特化：

std::string, std::vector各自實現了swap函式，

string中

template<class _Elem, class _Traits, class _Alloc> inline void __CLRCALL_OR_CDECL swap(basic_string<_Elem, _Traits, _Alloc>& _Left, basic_string<_Elem, _Traits, _Alloc>& _Right) { // swap _Left and _Right strings _Left.swap(_Right); } void __CLR_OR_THIS_CALL swap(_Myt& _Right) { // exchange contents with _Right if (this == &_Right) ; // same object, do nothing else if (_Mybase::_Alval == _Right._Alval) { // same allocator, swap control information #if _HAS_ITERATOR_DEBUGGING this->_Swap_all(_Right); #endif /* _HAS_ITERATOR_DEBUGGING */ _Bxty _Tbx = _Bx; _Bx = _Right._Bx, _Right._Bx = _Tbx; size_type _Tlen = _Mysize; _Mysize = _Right._Mysize, _Right._Mysize = _Tlen; size_type _Tres = _Myres; _Myres = _Right._Myres, _Right._Myres = _Tres; } else { // different allocator, do multiple assigns _Myt _Tmp = *this; *this = _Right; _Right = _Tmp; } }

第二個swap(Right)進行判斷，如果使用了相同的分配器，則直接交換控制資訊，否則呼叫string::operator=進行拷貝賦值。。。所以建議優先使用swap函式，而不是賦值操作符。

vector中

template<class _Ty, class _Alloc> inline void swap(vector<_Ty, _Alloc>& _Left, vector<_Ty, _Alloc>& _Right) { // swap _Left and _Right vectors _Left.swap(_Right); } void swap(_Myt& _Right) { // exchange contents with _Right if (this == &_Right) ; // same object, do nothing else if (this->_Alval == _Right._Alval) { // same allocator, swap control information #if _HAS_ITERATOR_DEBUGGING this->_Swap_all(_Right); #endif /* _HAS_ITERATOR_DEBUGGING */ this->_Swap_aux(_Right); _STD swap(_Myfirst, _Right._Myfirst); _STD swap(_Mylast, _Right._Mylast); _STD swap(_Myend, _Right._Myend); } else { // different allocator, do multiple assigns this->_Swap_aux(_Right); _Myt _Ts = *this; *this = _Right; _Right = _Ts; } }

vector的swap原理跟string完全一致，只有噹噹使用了不同分配器才進行位元組拷貝。其餘情況直接交換控制資訊。

測試用例：

5，Copy and Swap idiom

目的：C++異常有三個級別：基本，強，沒有異常。通過建立臨時物件然後交換，能夠實現過載賦值操作符的強異常安全的執行。

Loki中智慧指標臨時變數跟this交換，臨時變數自動銷燬~

SmartPtr& operator=(SmartPtr<T1, OP1, CP1, KP1, SP1, CNP1 >& rhs) { SmartPtr temp(rhs); temp.Swap(*this); return *this; }

boost::share_ptr，share_ptr定義了自己的swap函式。

shared_ptr & operator=( shared_ptr const & r ) // never throws { this_type(r).swap(*this); return *this; } void swap(shared_ptr<T> & other) // never throws { std::swap(px, other.px); pn.swap(other.pn); }

記得本科上C++課，老師特別喜歡拿String來舉例子，面試題也特別喜歡String。。。下面說說String::opreator=函式的優化：

最一般的寫法，特點：使用const string& 傳參防止臨時物件。

String& String::operator =(const String & rhs) { if (itsString) delete [] itsString; itsLen = rhs.GetLen(); itsString = new char[itsLen+1]; for (unsigned short i = 0;i<itsLen;i++) itsString[i] = rhs[i]; itsString[itsLen] = '/0'; return *this; }

優化1，防止自我間接賦值，a = b; c = b; a = c; 如果沒有第一個if判斷，當把c賦給a的時候，刪除了a.itsString，後面的拷貝就會出錯。注意是if(this==&rhs), 而不是if(*this==rhs) .

String& String::operator =(const String & rhs) { if (this == &rhs) return *this; if (itsString) delete [] itsString; itsLen=rhs.GetLen(); itsString = new char[itsLen+1]; for (unsigned short i = 0;i<itsLen;i++) itsString[i] = rhs[i]; itsString[itsLen] = '/0'; return *this; }

優化2，不進行拷貝賦值，只是交換控制資訊，而且是強異常安全：

String & String::operator = (String const &rhs) { if (this != &rhs) String(rhs).swap (*this); // Copy-constructor and non-throwing swap // Old resources are released with the destruction of the temporary above return *this; }

優化3，以最原始的傳值方式傳參，避免臨時物件建立：

String & operator = (String s) // the pass-by-value parameter serves as a temporary { s.swap (*this); // Non-throwing swap return *this; }// Old resources released when destructor of s is called.

最後這張方式主要是對C++新特性rvalue的優化，具體參見：http://en.wikibooks.org/wiki/More_C++_Idioms/Copy-and-swap

6. vector clear and swap trick

vector.clear並只是將size變數置為0，並沒有及時歸還OS，STL仍然持有記憶體，以便後續push_back。實測如下：

vector<int> temp;

此時開啟資源管理器，記憶體如下：

增長vector然後清空：

temp.resize( 1024*1024*20 );    // 80M
temp.clear();

此時資源管理器記憶體：

clear以後程式兵沒有及時將記憶體歸還OS。。。通過swap方法：

 tmp.resize(1024*1024*20);	 // 80M
 // tmp.clear();
 {
      std::vector<int>().swap(tmp);        // 將記憶體歸還OS
 }

退出作用域，臨時物件銷燬。記憶體歸還OS。此時資源管理器中程式記憶體回到1,864K。

附上網路版的String：

#include <iostream> #include <cstring> using namespace std; class String { public: String(); String(const char *const); String(const String &); ~String(); char & operator[] (unsigned short offset); char operator[] (unsigned short offset)const; String operator+(const String&); void operator+=(const String&); String & operator= (const String &); unsigned short GetLen()const {return itsLen;} const char * GetString()const {return itsString;} private: String (unsigned short); char * itsString; unsigned short itsLen; }; String::String() { itsString = new char[1]; //為什麼設定成1，這樣會導致記憶體1bytes無法釋放嗎？我覺得和itsString = new char沒區別，那他為什麼要設定成1，這樣有什麼用？21天學會C++那本書，我也有，書上也確實是設定成1. itsString[0] = '/0'; itsLen=0; } String::String(unsigned short len) { itsString = new char[len+1]; for (unsigned short i =0;i<=len;i++) itsString[i] = '/0'; itsLen=len; } String::String(const char * const cString) { itsLen = strlen(cString); itsString = new char[itsLen+1]; for (unsigned short i=0;i<itsLen;i++) itsString[i] = cString[i]; itsString[itsLen] = '/0'; } String::String(const String & rhs) { itsLen = rhs.GetLen(); itsString = new char[itsLen+1]; for (unsigned short i = 0;i<itsLen;i++) itsString[i] = rhs[i]; itsString[itsLen] = '/0'; } String::~String() { delete [] itsString; itsLen = 0; } String& String::operator =(const String & rhs) { if (this == &rhs) return *this; delete [] itsString; itsLen=rhs.GetLen(); itsString = new char[itsLen+1]; for (unsigned short i = 0;i<itsLen;i++) itsString[i] = rhs[i]; itsString[itsLen] = '/0'; return *this; } char & String::operator [](unsigned short offset) //這個程式這樣寫，起到了什麼用處？？和main中的那一個對應？ { if (offset > itsLen) return itsString[itsLen-1]; //這個返回itslen-1到底是什麼意思？為什麼要減去1 ？？ else return itsString[offset]; } char String::operator [](unsigned short offset)const { if (offset > itsLen) itsString[itsLen-1]; else return itsString[offset]; } String String::operator +(const String& rhs) { unsigned short totalLen = itsLen + rhs.GetLen(); String temp(totalLen); unsigned short i; for (i=0;i<itsLen;i++) temp[i] = itsString[i]; for (unsigned short j = 0;j<rhs.GetLen();j++,i++) temp[i] = rhs[j]; temp[totalLen] = '/0'; return temp; } void String::operator +=(const String& rhs) { unsigned short rhsLen = rhs.GetLen(); unsigned short totalLen = itsLen + rhsLen; String temp(totalLen); unsigned short i; for (i = 0;i<itsLen;i++) temp[i] = itsString[i]; for (unsigned short j = 0;j<rhs.GetLen();j++,i++) temp[i] = rhs[i-itsLen]; temp[totalLen] = '/0'; } int main() { String s1("initial test"); //呼叫了什麼函式？ cout<<"S1:/t"<<s1.GetString()<<endl; char *temp ="Hello World"; s1 = temp;//呼叫了什麼函式？ cout<<"S1:/t"<<s1.GetString()<<endl; char tempTwo[20]; strcpy(tempTwo,"; nice to be here!"); s1 += tempTwo; cout<<"tempTwo:/t"<<tempTwo<<endl; cout<<"S1:/t"<<s1.GetString()<<endl; cout<<"S1[4]:/t"<<s1[4]<<endl; cout<<"S1[999]:/t"<<s1[999]<<endl;//呼叫了什麼函式？ String s2(" Anoter string");//呼叫了什麼函式？ String s3; s3 = s1+s2; cout<<"S3:/t" <<s3.GetString()<<endl; String s4; s4 = "Why does this work?";//呼叫了什麼函式？ cout<<"S4:/t"<<s4.GetString()<<endl; return 0; }

參考引用：

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！

談談C++中的swap函式

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