条款7：预先准备好内存不够的情况

1. operator new在无法完成内存分配请求时会抛出异常（在以前或者某些特定情况（声明nothrow）下new会返回0）

2. 使用C语言的宏定义处理会是这样

#define NEW(PTR, TYPE)							\
try {(PTR) = new TYPE; }					\
catch (std::bad_alloc&) { assert(0); }

3. 用C++的异常处理机制，处理方法是这样

#include <iostream>
#include <new>

using namespace std;

/*
#define NEW(PTR, TYPE)							\
try {(PTR) = new TYPE; }					\
catch (std::bad_alloc&) { assert(0); }
*/

//new异常处理模版类
template <class T>
class NewHandlerSupport
{
public:
static new_handler set_new_handler(new_handler handler);
static void* operator new(size_t size);
private:
static new_handler current_handler;
};

//设置异常处理函数
template <class T>
new_handler NewHandlerSupport<T>::set_new_handler(new_handler handler)
{
new_handler old_handler = current_handler;
current_handler = handler;
return old_handler;
}

//重写new
template<class T>
void* NewHandlerSupport<T>::operator new(size_t size)
{
void* memory = NULL;
new_handler old_handler = ::set_new_handler(current_handler);

try
{
memory = ::operator new(size);
}
catch (std::bad_alloc&)
{
set_new_handler(old_handler);
throw;
}

set_new_handler(old_handler);
return memory;
}

//初始化静态数据
template<class T>
new_handler NewHandlerSupport<T>::current_handler;

//具体类继承模版实例
class Example : public NewHandlerSupport<Example>
{
public:
//...
private:
//...
};

int main()
{
Example* e = new Example;
return 0;
}

4. 如果想让new分配失败不抛出异常而是返回0，可以像下面这样

int* i = new (nothrow) int;