原文来自：http://www.cnblogs.com/xloogson/p/3360847.html
1.C++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 = ; x < sizeof(T); ++x)
{
pObj1[x] ^= pObj2[x];
pObj2[x] ^= pObj1[x];
pObj1[x] ^= pObj2[x];
}
}
可以省下拷贝构造的过程。
3.针对内建类型的优化： int, float, double 等，甚至重载运算符的用户自定义类型：向量，矩阵，图像等。。。
template <class T> void swap ( T& a, T& b )
{
a = a + b;
b = a - b;
a = a - b;
}
// 无需构造临时变量。使用基本运算操作符。
4.swap的一些特化：std::string, std::vector各自实现了swap函数
string:
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:
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。
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~
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。
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);
}
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.