Search code examples
c++templatesc++11inheritancevariadic

Is this syntax legal?

When I uncomment the line in main() below, Visual Studio 2015 won't compile (the code compiles otherwise).

#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.
}

It gives the errors:

'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

So what is wrong here? GCC 5.1 compiles it but I wonder if that's just a fluke or not. Is the syntax virtual void foo() const override {RemoveLast<Derived, R, Is...>::type::foo();} legal? If not, how do I achieve it (I merely want it to call its base class's foo()). If it is legal, how do I rewrite this so that Visual Studio will accept it (I need it working on Visual Studio)?

Solution

  • This is a simple fix. Replace Derived with 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 works too. This appears to be a bug that exists in both Clang and Visual Studio regarding injected class names.