c++ - 模板参数子类型的使用(进阶案例)

Question

我有几个具有以下继承关系的类:

template <typename ID_t>
class Identifiable {
public:
    virtual ID_t unique_ID() const {
        return 0;
    }
};
template <typename DerivedName>
class SomeBase : public Identifiable<typename DerivedName::ID_t> {
public:
    virtual DerivedName & instance() = 0;
};
template <uint16_t SomeParam = 5>
class SomeDerived : public SomeBase<SomeDerived<SomeParam>> {
public:
    using ID_t = uint16_t;
    SomeDerived & instance() override {
        return *this;
    }
    ID_t unique_ID() const override {
        return SomeParam;
    }
};
int main() {    
    std::cout << "Hello, World!\n";
    SomeDerived<> instance;
    std::cout << std::to_string(instance.unique_ID()) << std::endl;
}

但这不会在 SomeBase 声明行中使用“No type named ID_t in class SomeDerived”进行编译。尽管接缝的原因很明显，有没有办法在没有这个检查的情况下强制编译？或者在不将 ID_t 作为另一个模板参数传递的情况下修复它？

这是沙箱的链接:cpp.sh/57ox6

Answer 1

您缺少来自 Identifiable 的类型别名:

template <typename ID>
class Identifiable {
public:
    using ID_t = ID;
    virtual ID_t unique_ID() const {
        return 0;
    }
};