c++ - 带有模板的基于枚举的工厂无法转换类型

Question

我遇到了一个对我来说有点太大的问题。搜索它是另一个复杂的问题，因为我无法想象到底要搜索什么。所以我希望有人能给我指明正确的方向。

当使用枚举实现工厂以选择作为模板类的变体时，问题就出现了。问题是即使对于不可能的情况，编译器也会执行类型检查。最小示例如下:

template <class T1, class T2>
class Base {
 public:
  Base() {}
};

template <class T1>
class A : public Base<T1, T1> {
 public:
  A(): Base<T1, T1>() {}
};

template <class T1, class T2>
class B : public Base<T1, T2> {
 public:
  B(): Base<T1, T2>() {}
};

enum class Type {
  TYPE_A, TYPE_B
};

class Factory {
 public:
  template<class T1, class T2>
  static Base<T1, T2>* Create(Type type) {
    switch (type) {
      case Type::TYPE_A:
        return new A<T1>();
      case Type::TYPE_B:
        return new B<T1, T2>();
    }
    return nullptr;
  }
};

int main(int argc, char const * argv[]) {
  Base<int, double>* ptr = Factory::Create<int, double>(Type::TYPE_B);
  delete ptr;
  return 0;
}

对于那些你不想运行它的人的错误信息:

test_template_factory.cpp: In instantiation of ‘static Base<T1, T2>* Factory::Create(Type) [with T1 = int; T2 = double]’:
test_template_factory.cpp:38:69:   required from here
test_template_factory.cpp:29:26: error: cannot convert ‘A<int>*’ to ‘Base<int, double>*’ in return
         return new A<T1>();
                          ^

有解决办法吗？有没有一种方法可以在保持灵 active 的同时避免它？提前致谢!

Answer 1

你应该在编译时选择做什么，而不是在运行时。

这样的事情可以帮助你。

template <class T1, class T2>
class Base {
 public:
  Base() {}
};

template <class T1>
class A : public Base<T1, T1> {
 public:
  A(): Base<T1, T1>() {}
};

template <class T1, class T2>
class B : public Base<T1, T2> {
 public:
  B(): Base<T1, T2>() {}
};

enum class Type {
  TYPE_A, TYPE_B
};

class Factory {
 public:
  template<Type type, class T1, class T2>
  static Base<T1, T2>* Create() {
    return create_helper<type, T1, T2>::apply();
  }
private:
  template<Type type, class T1, class T2>
  struct create_helper
  {
     static Base<T1, T2>* apply() { return nullptr; }
  };
};

template<class T1, class T2>
struct Factory::create_helper<Type::TYPE_A, T1, T2>
{
   static Base<T1, T2>* apply() { return new A<T1>(); }
};

template<class T1, class T2>
struct Factory::create_helper<Type::TYPE_B, T1, T2>
{
   static Base<T1, T2>* apply() { return new B<T1, T2>(); }
};


int main(int argc, char const * argv[]) {
  Base<int, double>* ptr = Factory::Create<Type::TYPE_B, int, double>();
  delete ptr;
  return 0;
}