“There are three ways to make a living in this business: be first, be smarter, or cheat.”
“商业世界里只有三种生存模式,更快、更聪明,或者更会骗人。”
– 美国电影《商海通牒》
“There are three ways to make a living in this business: be first, be smarter, or cheat.”
“商业世界里只有三种生存模式,更快、更聪明,或者更会骗人。”
– 美国电影《商海通牒》
Consider
delete p;
// ...
delete p;
If the … part doesn’t touch p then the second “delete p;” is a serious error that a C++ implementation cannot effectively protect itself against (without unusual precautions). Since deleting a zero pointer is harmless by definition, a simple solution would be for “delete p;” to do a “p=0;” after it has done whatever else is required. However, C++ doesn’t guarantee that.One reason is that the operand of delete need not be an lvalue. Consider:
delete p+1;
delete f(x);
Here, the implementation of delete does not have a pointer to which it can assign zero. These examples may be rare, but they do imply that it is not possible to guarantee that “any pointer to a deleted object is 0.” A simpler way of bypassing that “rule” is to have two pointers to an object:
T* p = new T;
T* q = p;
delete p;
delete q; // ouch!
C++ explicitly allows an implementation of delete to zero out an lvalue operand, and I had hoped that implementations would do that, but that idea doesn’t seem to have become popular with implementers.If you consider zeroing out pointers important, consider using a destroy function:
template<class T> inline void destroy(T*& p) { delete p; p = 0; }
Consider this yet-another reason to minimize explicit use of new and delete by relying on standard library containers, handles, etc.
Note that passing the pointer as a reference (to allow the pointer to be zero’d out) has the added benefit of preventing destroy() from being called for an rvalue:
int* f();
int* p;
// ...
destroy(f()); // error: trying to pass an rvalue by non-const reference
destroy(p+1); // error: trying to pass an rvalue by non-const reference
“人生就好比航海,不断地向目的地出发,中间会在一些港口停留,而这些港口都是我们补给的地方。”
Qt 提供了 scene、view、item 框架,对于交互事件很多依赖于鼠标操作,本篇对此进行概括和总结。
参见:https://doc.qt.io/qt-5/graphicsview.html
Qt 的图形视图,提供了一个界面用来管理大量2D图形项目;一个视图部件用来可视化这些图像项目,支持缩放和旋转。这个框架包括一个事件传播体系结构,该体系结构对场景中的项目进行精度和双精度交互功能。项目可以处理按键事件。鼠标按下、移动、释放和双击事件,他们还可以耿总鼠标的移动。图形视图使用BSP树提供了非常快速的项目检索功能,因此它可以实时可视化大型场景,甚至包括数百万的项目。
材料准备
获取尺寸 -> 设计外壳数模 -> 3D打印 -> 装机验证 -> 修正 -> 成品。