如何在C ++中推导出一个内部类型的外部类型？

问题描述

我有很多类暴露一个名为 Binding 的内部类型。例如，其中一个可以是：

I have many classes exposing an inner type named Binding. For instance, one of them could be:

struct Message
{
    struct Binding
    {
    };
};

我调用函数 apply

apply< Message >([](Message::Binding& x)
{
    // setup binding fields
});

template <class TMessage, class TBindingExpression>
void apply(const TBindingExpression& expr)
{
    typedef typename TMessage::Binding BindingType;

    BindingType binding;
    expr(binding);

    apply(MessageUtil::typeId< TMessage >(), binding);
}

由于讯息在我调用 apply 的方式有点冗余，我想让编译器推导消息所以我可以写

Since Message is a bit redundant in the way I invoke apply, I would like to make the compiler deduce Message so I can write

apply([](Message::Binding x)
{
    //...
});

到目前为止，我被困在这里：

So far, I am stuck here:

template <class TBindingExpression>
void apply(const TBindingExpression& expr)
{
    // I get the type of the argument which is Message::Binding in this example
    typedef typename std::tuple_element
    <
        0,
        FunctionTraits< TBindingExpression >::ArgumentTypes
    >
    ::type BindingType;

    // so I can invoke my expression
    BindingType binding;
    expr(binding);

    // But now I need the type of the outer class, i.e. Message
    typedef typename MessageTypeFromBinding< BindingType >::Type MessageType;

    apply(MessageUtil::typeId< MessageType >(), binding);
}

有一种方法可以写/实现 MessageTypeFromBinding ？

Is there a way to write/achieve MessageTypeFromBinding?

显然，这是纯粹的好奇心和化妆品的问题。

Obviously, that's pure curiosity and cosmetic concerns.

推荐答案

template<class T>struct inner_class_of{using outer_class=T;};

struct Message {
  struct Binding:inner_class_of<Message> {
  };
};

template<class T>
inner_class_of<T> get_outer_helper(inner_class_of<T>const&);

template<class T>
using outer_class_of_t = typename decltype(get_outer_helper(std::declval<T>()))::outer_class;

现在 outer_class_of_t< Message :: Binding>

我有点工业实力，因为即使绑定隐藏 outer_class 。

I made it a bit industrial strength, as it works even if Binding hides outer_class.

您可以放置​​帮助程序并重写 outer_class_of_t = typename T :: outer_class （如果您愿意）。

You can drop helper and rewrite outer_class_of_t=typename T::outer_class if you prefer.

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