C++ 继承向下转型 [英] C++ inheritance downcasting

问题描述

我的基类如下:

class point    //concrete class
{
 ...    //implementation
}

class subpoint : public point  //concrete class
{
...     //implementation
}

如何从点对象转换为子点对象?我已经尝试了以下所有三个:

How do I cast from a point object to a subpoint object? I have tried all three of the following:

point a;
subpoint* b = dynamic_cast<subpoint*>(&a);
subpoint* b = (subpoint*)a;
subpoint b = (subpoint)a;

这些演员表有什么问题?

What is wrong with these casts?

推荐答案

如何从点对象转换为子点对象?

How do I cast from a point object to a subpoint object?

你不能；除非 point 有一个转换操作符，或者 subpoint 有一个转换构造函数，在这种情况下，对象类型可以在不需要强制转换的情况下进行转换.

You can't; unless either point has a conversion operator, or subpoint has a conversion constructor, in which case the object types can be converted with no need for a cast.

您可以从 point reference(或指针)转换为 subpoint reference(或指针), 如果引用的对象实际上是 subpoint 类型:

You could cast from a point reference (or pointer) to a subpoint reference (or pointer), if the referred object were actually of type subpoint:

subpoint s;

point & a = s;
subpoint & b1 = static_cast<subpoint&>(a);
subpoint & b2 = dynamic_cast<subpoint&>(a);

第一个 (static_cast) 更危险；没有检查转换是否有效，因此如果 a 不引用 subpoint，则使用 b1 将具有未定义的行为.

The first (static_cast) is more dangerous; there is no check that the conversion is valid, so if a doesn't refer to a subpoint, then using b1 will have undefined behaviour.

第二个 (dynamic_cast) 更安全，但只有在 point 是多态的(也就是说，如果它有一个虚函数)时才有效.如果 a 引用了一个不兼容类型的对象，那么它会抛出异常.

The second (dynamic_cast) is safer, but will only work if point is polymorphic (that is, if it has a virtual function). If a refers to an object of incompatible type, then it will throw an exception.

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