c++ - 通过指向虚基类的指针访问模板类的模板成员函数？

Question

我正在尝试实现一系列模板化对象，这些对象将通过非模板虚拟基类指针访问。简化后，基类如下所示:

class Base
{
public:
  virtual void printThing(const int &thing) = 0;
  virtual void printThing(const double &thing) = 0;
  virtual void printThing(const bool &thing) = 0;
};

我想做的是在下面的派生类实现中勾画出来的:

#include <iostream>

template <typename T>
class Derived : public Base
{
public:
  void printThing(const T &thing);

  template <typename U>
  void printThing(const U &thing);
};

template <typename T>
void Derived<T>::printThing(const T &thing)
{
  std::cout << "Derived same type " << thing << std::endl;
}

template <typename T>
template <typename U>
void Derived<T>::printThing(const U &thing)
{
  std::cout << "Derived diff type " << thing << std::endl;
}

template <>
template <>
void Derived<double>::printThing(const int &thing)
{
  std::cout << "Derived<double> specialized for int " << thing << std::endl;
}

这适用于任何类型的 U - 只要代码直接在 Derived 的实例上调用成员函数，并且 U 在编译时已知。

但是，当我尝试通过指向 Base 的指针访问 Derived 时，出现编译器错误，如下面的测试程序所示:

int main(int argc, char* argv[])
{
  Derived<int> dint;
  Derived<double> ddouble;
  Base * bint = &dint;
  Base * bdouble = &ddouble;
  double d = 3.14;
  int i = 42;
  bint->printThing(i);
  bint->printThing(d);
  bdouble->printThing(i);
  bdouble->printThing(d);

  return 0;
}

Mac OS 10.8.5 上的 clang++ 给出了这样的反馈:

razor:playpen cfry$ clang++ template-specialization.cc 
template-specialization.cc:43:16: error: variable type 'Derived<int>' is an abstract class
  Derived<int> dint;
               ^
template-specialization.cc:7:16: note: unimplemented pure virtual method 'printThing' in 'Derived'
  virtual void printThing(const double &thing) = 0;
               ^
template-specialization.cc:8:16: note: unimplemented pure virtual method 'printThing' in 'Derived'
  virtual void printThing(const bool &thing) = 0;
               ^
template-specialization.cc:44:19: error: variable type 'Derived<double>' is an abstract class
  Derived<double> ddouble;
                  ^
template-specialization.cc:6:16: note: unimplemented pure virtual method 'printThing' in 'Derived'
  virtual void printThing(const int &thing) = 0;
               ^
template-specialization.cc:8:16: note: unimplemented pure virtual method 'printThing' in 'Derived'
  virtual void printThing(const bool &thing) = 0;
               ^
2 errors generated.
razor:playpen cfry$ 

请注意，我已经明确实现了一个 Derived<double>::printThing(const int &) ，编译器声称不存在。 和广义 Derived<T>::printThing(const U &)成员函数似乎没有被实例化。

是否有任何可移植的方式告诉编译器我打算为每个“未实现”的虚拟方法实例化通用模板成员函数？

我已经尝试了很多替代方案，但到目前为止，唯一可行的是为基类成员函数提供默认实现，并编写 Derived 的包装器，为所需的 U 类型显式实现 printThing()。

Answer 1

我使用 Curiously Recurring Template Pattern (CRTP) 找到了答案，explained here by Eli Bendersky .

需要再增加一层模板类:

template <class Child>
class Adapter : public Base
{
public:
  void printThing(const int &thing)
  {
    static_cast<Child *>(this)->printThingInternal(thing);
  }
  void printThing(const double &thing)
  {
    static_cast<Child *>(this)->printThingInternal(thing);
  }
  void printThing(const bool &thing)
  {
    static_cast<Child *>(this)->printThingInternal(thing);
  }
};

template <typename T>
class Derived : public Adapter<Derived <T> >
{
public:
  void printThingInternal(const T &thing);

  template <typename U>
  void printThingInternal(const U &thing);
};

有了这个添加，以及对 Derived 的简单更改以从 Adapter 继承，程序安抚了编译器，更好的是，生成了我正在寻找的结果:

razor:playpen cfry$ clang++ template-specialization.cc 
razor:playpen cfry$ ./a.out
Derived same type 42
Derived diff type 3.14
Derived<double> specialized for int 42
Derived same type 3.14
razor:playpen cfry$