[转]谈谈C++中的swap函数

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。

2，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 = 0; x < sizeof(T); ++x)
    {
        pObj1[x] ^= pObj2[x];
        pObj2[x] ^= pObj1[x];
        pObj1[x] ^= pObj2[x];
    }
}

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。

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：

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~

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+1];
    for (unsigned short i = 0;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+1];
    for (unsigned short i = 0;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[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.
        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) //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-1]; //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-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"); //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[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;//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 0;
}

 

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