c++ swap 函数

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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。

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

需要构建临时对象，一个拷贝构造，两次赋值操作。

，针对int型优化：

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

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：

    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 = ; x < sizeof(T); ++x)
        {
            pObj1[x] ^= pObj2[x];
            pObj2[x] ^= pObj1[x];
            pObj1[x] ^= pObj2[x];
        }
    }  

，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
type  b; -- e.g 

a = a+b ; -- a=,b=
b = a-b ; -- a=,b=
a= a -b ; -- a= ,b=

// 无需构造临时变量。使用基本运算操作符。

    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.  

，swap的一些特化：

std::string, std::vector各自实现了swap函数，

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

    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：

，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~

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

boost::share_ptr，share_ptr定义了自己的swap函数。

    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);
    }  

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。

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

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) .

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

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：

    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;
    }  

优化3，以最原始的传值方式传参，避免临时对象创建：

    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 

附上网络版的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[]; //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.
            itsString[] = '/0';
            itsLen=;
    }
    String::String(unsigned short len)
    {
            itsString = new char[len+];
            for (unsigned short i =;i<=len;i++)
                    itsString[i] = '/0';
            itsLen=len;
    }
    String::String(const char * const cString)
    {
            itsLen = strlen(cString);
            itsString = new char[itsLen+];
            for (unsigned short i=;i<itsLen;i++)
                    itsString[i] = cString[i];
            itsString[itsLen] = '/0';
    }
    String::String(const String & rhs)
    {
            itsLen = rhs.GetLen();
            itsString = new char[itsLen+];
            for (unsigned short i = ;i<itsLen;i++)
                    itsString[i] = rhs[i];
            itsString[itsLen] = '/0';
    }
    String::~String()
    {
            delete [] itsString;
            itsLen = ;
    }
    String& String::operator =(const String & rhs)
    {
            if (this == &rhs)
                    return *this;
            delete [] itsString;
            itsLen=rhs.GetLen();
            itsString = new char[itsLen+];
            for (unsigned short i = ;i<itsLen;i++)
                    itsString[i] = rhs[i];
            itsString[itsLen] = '/0';
            return *this;
    }
    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？
    {
            if (offset > itsLen)
                    return itsString[itsLen-]; //这个返回itslen-1到底是什么意思？为什么要减去1 ？？
            else
                    return itsString[offset];
    }
    char String::operator [](unsigned short offset)const
    {
            if (offset > itsLen)
                    itsString[itsLen-];
            else
                    return itsString[offset];
    }
    String String::operator +(const String& rhs)
    {
            unsigned short totalLen = itsLen + rhs.GetLen();
            String temp(totalLen);
            unsigned short i;
            for (i=;i<itsLen;i++)
                    temp[i] = itsString[i];
            for (unsigned short j = ;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 = ;i<itsLen;i++)
                    temp[i] = itsString[i];
            for (unsigned short j = ;j<rhs.GetLen();j++,i++)
                    temp[i] = rhs[i-itsLen];
            temp[totalLen] = '/0';
    }
    int main()
    {  

            String s1("initial test"); //调用了什么函数？
            cout<<"S1:/t"<<s1.GetString()<<endl;
            char *temp ="Hello World";
            s1 = temp;//调用了什么函数？
            cout<<"S1:/t"<<s1.GetString()<<endl;
            char tempTwo[];
            strcpy(tempTwo,"; nice to be here!");
            s1 += tempTwo;
            cout<<"tempTwo:/t"<<tempTwo<<endl;
            cout<<"S1:/t"<<s1.GetString()<<endl;
            cout<<"S1[4]:/t"<<s1[]<<endl;
            cout<<"S1[999]:/t"<<s1[]<<endl;//调用了什么函数？
            String s2(" Anoter string");//调用了什么函数？
            String s3;
            s3 = s1+s2;
            cout<<"S3:/t" <<s3.GetString()<<endl;
            String s4;
            s4 = "Why does this work?";//调用了什么函数？
            cout<<"S4:/t"<<s4.GetString()<<endl;
            return ;
    }  

参考引用：

，http://www.vbforums.com/showthread.php?t=245517

，http://www.cplusplus.com/reference/algorithm/swap/

，http://codeguru.earthweb.com/forum/showthread.php?t=485643

，http://stackoverflow.com/questions/1998744/benefits-of-a-swap-function

，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