item51：定制new的注意点

Item 51: Adhere to convention when writing new and delete

scott这里给了一个new的大致流程伪代码，非常有用：

void * operator new(std::size_t size) throw(std::bad_alloc)
{                                      // your operator new might take additional params
  using namespace std;     
  if (size == 0) {                     // handle 0-byte requests by treating them as 1-byte requests
    size = 1;
  } 
  while (true) {
   attempt to allocate size bytes;
    if (the allocation was successful)
       return (a pointer to the memory);


    // allocation was unsuccessful; find out what the
    // current new-handling function is (see below)

    new_handler globalHandler = set_new_handler(0);
    set_new_handler(globalHandler);
    if (globalHandler) (*globalHandler)();
    else throw std::bad_alloc();
  }
}

空类或结构都是会占据1字节的，前面的item讲过，调用 set_new_handler，其返回值是当前handler，回调之，如果已经是NULL了，异常之。

E文参考资料是这么说的：get_new_handler 是c++0x新增的

Allocates count bytes from free store. Calls the function pointer returned by std::get_new_handler on failure and repeats allocation attempts until new handler does not return or becomes a null pointer, at which time throws std::bad_alloc. 

scott在这个item强调的是new内部的无限循环，和其终止条件；对0内存申请的特殊处理；delete要对空指针进行保护。