将c ++接口实例化为子类 [英] Instantiating c++ interface as a child class

问题描述

我有一个界面，让我们称之为生物，他有虚拟函数，使它是抽象的。

I have an interface, let's call it Creature, who has virtual functions that cause it to be abstract.

我有这个界面的子类，例如 Dog ， Cat 和 Pig 。

I have child classes of this interface such as Dog, Cat, and Pig.

编译器似乎不喜欢下面的行，因为无法声明变量 thing 的抽象类型生物。

The compiler doesn't seem to like the following line due to not being able to declare variable thing to be of abstract type Creature.

Creature thing = Dog();

我知道我不能实例化接口等，但这只是一个 Dog 被声明为 Creature 。

I know I can't instantiate interfaces and the like, but this is just a Dog being declared as a Creature.

我需要一种方法让所有的孩子都有一个声明工作（例如，可以把 Dog（）， Cat（）或 Pig（）其中 Dog $ c>在上面的行）。

I need some way of having one declaration work for all the children (i.e., being able to put Dog(), Cat(), or Pig() where Dog() is in the line above).

这可以在c ++中完成，还是我完全错误地使用继承和接口？

Can this be done in c++ or am I misusing inheritance and interfaces completely?

推荐答案

对象类型本身在C ++中不是多态的。您给定的行声明了一个 Creature 对象，然后尝试使用 Dog 对象初始化它。如果 Creature 不是抽象的，这将导致切片 - thing 不会是 Dog ，它只是一个生物。因为它是抽象的，你根本不能有一个 Creature 对象。

Object types themselves are not polymorphic in C++. The line you've given declares a Creature object and then attempts to initialise it with a Dog object. If Creature weren't abstract, this would result in slicing - thing wouldn't be a Dog any more, it would just be a Creature. Since it is abstract, you simply can't have a Creature object anyway.

你需要使用指针或多态行为的参考。考虑例如：

You need to use pointers or references for polymorphic behaviour. Consider for example:

Creature* thing = new Dog();

现在可以取消引用 thing 作为 Creature ，即使它的动态类型是 Dog 。但是，通常不推荐使用这样的原始指针，因为您必须手动确保对象 delete d。所有权可能变得混乱。你最好的办法是把它放在一个聪明的指针，如：

You can now dereference thing and use it as a Creature, even though it's dynamic type is Dog. However, using raw pointers like this is usually not recommended, as you have to manually ensure that the object is deleted at some point. The ownership can become confusing. Your best bet is to put it in a smart pointer, such as:

std::unique_ptr<Creature> thing(new Dog()); // or std::make_unique when we have it

在这里，我演示了 std :: unique_ptr ，但是智能指针的选择将取决于该对象的所有权语义。一个常见的替代方案是 std :: shared_ptr 。

Here, I've demonstrated std::unique_ptr, but the choice of smart pointer will depend on the ownership semantics for that object. A common alternative is std::shared_ptr.

要显示引用的多态性：

Dog dog;
Creature& thing = dog;
// Can now use dog as a Creature

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