这种语法合法吗?

Is this syntax legal?

本文关键字：语法      更新时间：2023-10-16

当我取消下面main()中的注释行时，Visual Studio 2015将无法编译(代码将以其他方式编译)。

#include <iostream>
#include <type_traits>
template <typename Output, std::size_t... Input> struct RemoveLastHelper;
template <template <std::size_t...> class Z, std::size_t... Accumulated, std::size_t First, std::size_t... Rest>
struct RemoveLastHelper<Z<Accumulated...>, First, Rest...> :
    RemoveLastHelper<Z<Accumulated..., First>, Rest...> {};
template <template <std::size_t...> class Z, std::size_t... Accumulated, std::size_t Last>
struct RemoveLastHelper<Z<Accumulated...>, Last> {
    using type = Z<Accumulated...>;
};
template <template <std::size_t...> class Z, std::size_t... Is>
using RemoveLast = RemoveLastHelper<Z<>, Is...>;
struct Base {
    virtual ~Base() = default;
    virtual void foo() const = 0;
};
template <std::size_t... Is> struct Derived;
template <std::size_t R>
struct Derived<R> : public Base {
    virtual void foo() const override {}
};
// For example, Derived<R,D1,D2,D3> inherits from Derived<R,D1,D2>, which inherits from
// Derived<R,D1>, which inherits from Derived<R>, which inherits from Base (by the above).
template <std::size_t R, std::size_t... Is>
struct Derived<R, Is...> : public RemoveLast<Derived, R, Is...>::type {
    virtual void foo() const override {RemoveLast<Derived, R, Is...>::type::foo();}
};
int main() {
//  Derived<0,2,1> r;  // This line won't compile.
}

给出如下错误:

'Derived<0,2>': invalid template argument for template parameter 'Z', expected a class template
'RemoveLast': left of '::' must be a class/struct/union
'type': is not a class or namespace name

那么这里有什么问题呢?GCC 5.1编译它，但我想知道这是否只是一个侥幸。语法virtual void foo() const override {RemoveLast<Derived, R, Is...>::type::foo();}合法吗?如果没有，我如何实现它(我只希望它调用其基类的foo())。如果它是合法的，我如何重写它，使Visual Studio将接受它(我需要它在Visual Studio上工作)?

这是一个简单的修复。将Derived替换为Derived::template Derived。

template <std::size_t R, std::size_t... Is>
struct Derived<R, Is...> : public RemoveLast<Derived, R, Is...>::type {
    virtual void foo() const override {RemoveLast<Derived::template Derived, R, Is...>::type::foo();}
};

::Derived也可以。这似乎是Clang和Visual Studio中存在的一个关于注入类名的错误。

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