[英]Storing multiple-inheritance objects in container
A library I use has many types, all of which derive from the same 2 interfaces: 我使用的库有很多类型,所有类型都来自相同的2个接口:
class Huey : public IDuck, public ICartoonCharacter
{
...
};
class Dewey : public IDuck, public ICartoonCharacter
{
...
};
class Louie : public IDuck, public ICartoonCharacter
{
...
};
I'd like to store objects of all the above types in a wrapper class and stick objects of that wrapper class in a container. 我想将所有上述类型的对象存储在包装类中,并将该包装类的对象粘贴到容器中。 Of course I should be able to call methods belonging to both interfaces from my wrapper class. 当然,我应该能够从我的包装类中调用属于这两个接口的方法。
What are my options here? 我有什么选择? I could think of 我能想到
IDuck *
s in my wrapper and dynamic_cast-ing to ICartoonCharacter
, or 将IDuck *
存储在我的包装器中,并将dynamic_cast-ing存储到ICartoonCharacter
,或者 boost::any
while making my wrapper a class-template, with a couple of static_asserts
to ensure the template parameter inherits from IDuck
and ICartoonCharacter
. 使用boost::any
类的东西,同时使我的包装器成为一个类模板,带有几个static_asserts
以确保模板参数继承自IDuck
和ICartoonCharacter
。 but neither option particularly appeals. 但两种选择都没有特别吸引人。 Any ideas? 有任何想法吗?
two interfaces, multiple inheritance combine into one container? 两个接口,多个继承组合成一个容器? is a related question, but James Kanze's answer doesn't work for me, as I can't change the 3 classes. 是一个相关的问题,但詹姆斯坎泽的答案对我不起作用,因为我不能改变3个班级。
EDIT: Don't use multiple inheritance often, had forgotten syntax. 编辑:不要经常使用多重继承,忘记了语法。 Now inheriting public
ly from both interfaces. 现在从两个接口继承public
ly。
EDIT: Now using dynamic_cast instead of static_cast (which won't work). 编辑:现在使用dynamic_cast而不是static_cast(这将无法工作)。
EDIT: I found both Mike Seymour's and Matthieu M's answers promising. 编辑:我发现Mike Seymour和Matthieu M的答案都很有希望。 I'll accept one of their answers once I've coded it all up. 一旦我编码完毕,我会接受他们的一个答案。 Thanks! 谢谢!
A simple option is to store two pointers in the wrapper: 一个简单的选择是在包装器中存储两个指针:
struct CartoonDuckWrapper {
IDuck * duck;
ICartoonCharacter * toon;
template <class CartoonDuck>
CartoonDuckWrapper(CartoonDuck & cd) : duck(&cd), toon(&cd) {}
};
There's no particular need to use static_assert
to check that CartoonDuck
inherits from both base classes, although that might give slightly better diagnostics than simply letting the pointer conversions fail. 没有特别需要使用static_assert
来检查CartoonDuck
从两个基类继承,尽管这可能比仅仅让指针转换失败提供更好的诊断。
If the base classes are polymorphic (which, being interfaces, they probably are), you could save the space of one pointer, in exchange for a run-time cost, by using dynamic_cast
to convert one to the other. 如果基类是多态的(可能是接口,它们可能是接口),则可以通过使用dynamic_cast
将一个指针转换为另一个来节省一个指针的空间,以换取运行时成本。 static_cast
can't be used for such a "cross-cast" between base classes. static_cast
不能用于基类之间的这种“交叉投射”。
As all issues in programming, you can solve it by adding one more level of indirection. 作为编程中的所有问题,您可以通过添加一个更多级别的间接来解决它。
class ICartoonDuck: public IDuck, public ICartoonCharacter {};
template <typename T>
class CartoonDuck: public ICartoonDuck {
public:
explicit CartoonDuck(T t): _t(std::move(t)) {}
// IDuck interface
virtual void foo() override { t.foo(); }
// ICartoonCharacter interface
virtual void bar() override { t.bar(); }
private:
T _t; // or any ownership scheme that makes sense
}; // class CartoonDuck
template <typename T>
CartoonDuck<T> makeCartoonDuck(T t) { return CartoonDuck(std::move(t)); }
template <typename T, typename... Args>
std::unique_ptr<CartoonDuck<T>> makeUniqueCartoonDuck(Args&&...) {
return std::unique_ptr<CartoonDuck<T>>(new T(std::forward<Args>()...);
}
Now, you can happily store std::unique_ptr<ICartoonDuck>
in your container. 现在,您可以愉快地将std::unique_ptr<ICartoonDuck>
存储在容器中。
This can be used as: 这可以用作:
std::vector<std::unique_ptr<ICartoonDuck>> cartoonDucks;
cartoonDucks.push_back(makeUniqueCartoonDuck<Huey>());
cartoonDucks.push_back(makeUniqueCartoonDuck<Dewey>());
cartoonDucks.push_back(makeUniqueCartoonDuck<Louie>());
for (std::unique_ptr<ICartoonDuck> const& cd: cartoonDucks) {
cd->foo();
cd->bar();
}
Create an intermediate class: 创建一个中间类:
class ILuckyDuck: public IDuck, ICartoonCharacter //...
with: 有:
class Huey : public ILuckyDuck //...
etc , and store: 等 ,并存储:
std::vector<std:shared_ptr<ILuckyDuck>> donald;
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