《More Effective C++》------ ( 2 )
条款6:区别increment / decrement操作符的前置和后置形式
(1):prefix和postfix语法上有区别，因为他们的函数形式表面上一样的，于是实际上 postfix 是多了一个参数的，值始终为0而已。
并且，prefix返回引用，postfix返回const对象(代表旧值，const是避免连加)。
int& operator++() ............. ++i;
const int operator++( int )............. i++(0);
(2):从效率上说，prefix比postfix高；从维护上说，一般的实现版本中,postfix代码调用prefix函数来完成累加动作，这样维护 加动作 只需要维护prefix的函数。
条款7:千万不要重载&&, ||和，操作符
(1): &&, || 有一定的提前判断， ，操作符表示顺序执行，返回最后一个操作的值
(2):该条款告诉你，某些operator不要重载，会破坏C++的语言特性。
不可重载的有:
. .* :: ?:
new delete sizeof typeid
4个cast
条款8:了解不同意义的new 和 delete
(1):new operator, 即new expression, CA *p = new CA; 展开后为： void *buffer = operator new ( size ), 在buffer上构建CA, 调用CA的构造函数
这里的operator new 确实存在, #include <new> ( stl ), void* operator new( size_t t );
(2):new 和 delete 对应， 分配内存并调用构造函数， 析构再删除内存。
如果采用operator new 取得内存，在此内存上产生对象，则不要调用delete, 只需要调用析构，再free内存。 称为 placement new。
void *pBuffer = operator new( sizeof( CA ) );
if ( pBuffer != NULL )
{
CA *pA = new( pBuffer ) CA(); //pA正常
//释放方式
//delete pA; //错误
pA->~CA();
free( pBuffer );
}
...
(3):数组, new[ ] 和 delete[ ] 要对应， 调用的constructor 和 destructor数量跟数组的元素个数相关。
(4):new operator和delete operator属于内建操作符，不可修改其行为；但是 operator new 和 operator delete 则可修改其行为。
-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
异常(Exception)
Exception的好处和理由：
(1):返回错误码的方式，需要函数调用者检查错误，可以被跳过或者忽略掉。而抛出exception,若未被catch，则程序会马上停止；
(2):exception发生时，栈上对象可以析构掉，其他方法无法保证。
条款9:利用Destructor避免资源泄露
(1):本条款的思想就是: 利用exception发生时，析构可以自动调用，避免内存泄露。
容易出错的代码:
... CA *pA = new pA; pA->dosomething(); delete pA; ...
若dosomething()发生异常,则delete无法执行到；
正确的代码:
... CAObject obj( new CA ); obj.GetPtr()->dosomething(); ...
用一种类似于智能指针的思想，以局部变量出现，在析构中清理资源。
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