为什么要使用虚函数？ [英] Why use virtual functions?

问题描述

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设计原则。

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Can someone explain C++ Virtual Methods?

I have a question regarding to the C++ virtual functions.

Why and when do we use virtual functions? Can anyone give me a real time implementation or use of virtual functions?

解决方案

You use virtual functions when you want to override a certain behavior (read method) for your derived class rather than the one implemented for the base class and you want to do so at run-time through a pointer to the base class.

The classic example is when you have a base class called Shape and concrete shapes (classes) that derive from it. Each concrete class overrides (implements a virtual method) called Draw().

The class hierarchy is as follows:

The following snippet shows the usage of the example; it creates an array of Shape class pointers wherein each points to a distinct derived class object. At run-time, invoking the Draw() method results in the calling of the method overridden by that derived class and the particular Shape is drawn (or rendered).

Shape *basep[] = { &line_obj, &tri_obj,
                   &rect_obj, &cir_obj};
for (i = 0; i < NO_PICTURES; i++)
    basep[i] -> Draw ();

The above program just uses the pointer to the base class to store addresses of the derived class objects. This provides a loose coupling because the program does not have to change drastically if a new concrete derived class of shape is added anytime. The reason is that there are minimal code segments that actually use (depend) on the concrete Shape type.

The above is a good example of the Open Closed Principle of the famous SOLID design principles.

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