C/C++深入讲解内存管理

int main()
{
	int* p1 = (int*)malloc(sizeof(int));
	int* p2 = (int*)calloc(4, sizeof(int));
	int* p3 = (int*)realloc(p2, sizeof(int) * 10);
	free(p1);
	free(p2);
	free(p3);
	return 0;
}

int main()
{
	// new/delete和malloc/free 针对内置类型没有任何差别，只是用法不同
	//动态申请一个int类型的空间
	int* p1 = new int;
	delete p1;
	//动态申请一个int类型的空间并初始化为10
	int* p2 = new int(10);
	delete p2;
	//动态申请10个int类型的空间
	int* p3 = new int[10];
	delete[] p3;
	return 0;
}

/*
operator new：该函数实际通过malloc来申请空间，当malloc申请空间成功时直接返回；申请空间失败，
尝试执行空间不足应对措施，如果改应对措施用户设置了，则继续申请，否则抛异常。
*/
void* __CRTDECL operator new(size_t size) _THROW1(_STD bad_alloc)
{
	// try to allocate size bytes
	void* p;
	while ((p = malloc(size)) == 0)
		if (_callnewh(size) == 0)
		{
			// report no memory
			// 如果申请内存失败了，这里会抛出bad_alloc 类型异常
			static const std::bad_alloc nomem;
			_RAISE(nomem);
		}
	return (p);
}
/*
operator delete: 该函数最终是通过free来释放空间的
*/
void operator delete(void* pUserData)
{
	_CrtMemBlockHeader* pHead;
	RTCCALLBACK(_RTC_Free_hook, (pUserData, 0));
	if (pUserData == NULL)
		return;
	_mlock(_HEAP_LOCK); /* block other threads */
	__TRY
		/* get a pointer to memory block header */
		pHead = pHdr(pUserData);
	/* verify block type */
	_ASSERTE(_BLOCK_TYPE_IS_VALID(pHead->nBlockUse));
	_free_dbg(pUserData, pHead->nBlockUse);
	__FINALLY
		_munlock(_HEAP_LOCK); /* release other threads */
	__END_TRY_FINALLY
		return;
}
/*
free的实现
*/
#define free(p) _free_dbg(p, _NORMAL_BLOCK)

struct ListNode
{
	ListNode(int data = 0)
		:_next(nullptr)
		,_prev(nullptr)
		,_data(data)
	{}
	ListNode* _next;
	ListNode* _prev;
	int _data;
};
//operator new的用法跟malloc和free是一样的，都是在堆上申请空间
//只是申请空间失败后的处理方式不一样，malloc失败返回NULL,operator new失败以后抛异常
int main()
{
	//C语言
	ListNode* p1 = (ListNode*)malloc(sizeof(ListNode));
	free(p1);
	//c++
	ListNode* p2 = (ListNode*)operator new(sizeof(ListNode));
	operator delete(p2);
	int* p3 = (int*)malloc(100000000000000000);
	if (p3 == NULL)
	{
		cout << "malloc fail" << endl;
	}
	try
	{
		int* p4 = (int*)operator new(100000000000000000);
	}
	//开辟空间失败，捕获异常信息
	catch (exception& e)
	{
		cout << e.what() << endl;
	}
	return 0;
}

class A
{
public:
	A(int a = 0)
	{
		cout << "A()" << this << endl;
	}
	~A()
	{
		cout << "~A()" << this << endl;
	}
private:
	int _a;
};
int main()
{
	//c语言  ->  A* p = (A*)malloc(sizeof(A));
	//等价于直接用 A* p = new A;
	A* p = (A*)operator new(sizeof(A));
	new(p)A; // new(p)A(2);  定位new，placement-new,显示调用构造函数初始化这块空间对象
	//等价于 delete p
	p->~A();  //析构函数可以显示调用
	operator delete(p);
	return 0;
}

struct ListNode
{
	void* operator new(size_t n)
	{
		void* p = nullptr;
		p = allocator<ListNode>().allocate(1);
		cout << "memory pool allocate" << endl;
		return p;
	}
	void operator delete(void* p)
	{
		allocator<ListNode>().deallocate((ListNode*)p, 1);
		cout << "memory pool deallocate" << endl;
	}
	ListNode* _next;
	ListNode* _prev;
	int _data;
};
class List
{
public:
	List()
	{
		_head = new ListNode();
		_head->_next = _head;
		_head->_prev = _head;
	}
	~List()
	{
		ListNode* cur = _head->_next;
		while (cur != _head)
		{
			ListNode* next = cur->_next;
			delete cur;
			cur = next;
		}
		delete _head;
		_head = nullptr;
	}
private:
	ListNode* _head;
};
int main()
{
	List h;
	return 0;
}

//如果申请失败，new会返回一个空指针（NULL）
int* p1 = new int;            //如果分配空间失败，new会抛出std::bad_alloc
int* p2 = new (nothrow) int;  //如果分配空间失败，new返回一个空指针