参数包扩展失败 [英] Parameter pack expansion fails

问题描述

考虑以下简化的C ++代码：

Consider the following simplified C++ code:

template <typename ... TEventArgs>
struct Event
{
    // ...
};

template <typename T>
struct Parameter
{
    using Type = T;
    // ...
};

template <typename ... Parameters>
struct Command
{
    Event<typename Parameters::Type...> Invoked;
};

int main()
{
    Command<Parameter<int>, Parameter<float>> c;
}

Visual Studio C ++编译器（2013年11月CTP，Visual Studio 2013 Update 1）产生以下错误：
source.cpp（17）：错误C3546：'...'：没有参数包可扩展

The Visual Studio C++ compiler (November 2013 CTP, Visual Studio 2013 Update 1) produces the following error: source.cpp(17): error C3546: '...' : there are no parameter packs available to expand

Mingw 4.8 .1。另一方面，编译代码没有任何问题。显然，Visual Studio编译器存在一个错误，当表达式涉及访问可变参数的类型时，它会阻止其扩展参数包。不过，其他扩展也可以。例如， Event< std :: vector< Parameters> ...>调用； 成功编译，或者您甚至可以成功地访问静态成员以在 Command 的构造函数中调用可变参数函数，如下所示： SomeVariadicFunc（Parameters :: SomeStaticFunc（）...）; 。

Mingw 4.8.1. on the other hand compiles the code without any problems. Apparently, the Visual Studio compiler has a bug that prevents it from expanding the parameter pack when the expression involves accessing a type of the variadic parameters. Other expansions work, though. For instance, Event<std::vector<Parameters>...> Invoked; compiles successfully or you could even successfully access static members to call a variadic function like this in Command's constructor: SomeVariadicFunc(Parameters::SomeStaticFunc()...);.

因此，问题是：

1）哪个编译器错误：Visual Studio或mingw？尽管我看不到任何会阻止 typename Parameters :: Type 参数包扩展正常工作的东西，但我不是100％地确定它是有效的C ++。

1) Which compiler is wrong: Visual Studio or mingw? Although I don't see anything that would prevent the typename Parameters::Type parameter pack expansion from working, I'm not 100% sure it's valid C++.

2）是否有解决方法？基本上，我必须执行从 Parameters 的序列到 Parameters :: Type 。那可能吗？我试图使用递归结构构造该列表，但我只能想到类似 myStruct< type1，mystruct< type2，mystruct< type3，...>>> ，这不是我所需要的。

2) Is there a work around? Basically, I would have to perform a projection from a "sequence" of Parameters to a "sequence" of Parameters::Type. Is that possible? I tried to construct that list using a recursive struct but I could only come up with something like myStruct<type1, mystruct<type2, mystruct<type3, ...>>>, which is not what I need.

谢谢您的帮助。

推荐答案

Yakk在上面的评论中提出了解决该问题的方法。可以与Visual Studio和mingw完美编译的最终版本如下：

Yakk was able to come up with a workaround for the problem in the comments above. The final version that compiles perfectly with both Visual Studio an mingw is the following:

template <typename ... TEventArgs>
struct Event
{
    // ...
};

template <typename T>
struct Parameter
{
    using Type = T;
    // ...
};

template <typename ... Parameters>
struct Command
{
private:
    // Workaround for the Visual Studio bug
    template<typename T> struct ExpandArgs
    {
        typedef typename T::Type Type;
    };

public:
    Event<typename ExpandArgs<Parameters>::Type...> Invoked;
};

int main()
{
    Command<Parameter<int>, Parameter<float>> c;
}

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