从非模板基 class 指针调用模板派生 class 方法的方法

Question

I'm trying to implement a Property system in my project similar to Property system in Qt. We just started with some ideas and are in prototyping stage. Basically, what I understood from Qt is, client should be able to pass the get function, set function and property type through some macro in the.h file. So I tried to mimic the same.

Following is my sample code:

Abstract getter class. This type of getter class is a member in Property Class

class AbstractFunc
{
public:
  template < typename R > 
  R Invoke ()
  {
    return (this)->Invoke ();
  }
};

Get Function template: Return values can be T, T&, const T&, T* etc..

template < typename R, class T > class GetterFunction : public AbstractFunc
{
  typedef R (T::*GetterFunc) ();

public:
  GetterFunction (T * obj, GetterFunc func):m_Obj (obj), m_Func (func)
  {
  }

  R Invoke ()
  {
    return m_Obj->*(m_Func) ();
  }

public:
  T * m_Obj;
  GetterFunc m_Func;
};

Property Class:

class Property
{
public:
  Property (string name, AbstractFunc* getter):m_name (name), m_getter (getter)
  {

  }

  template < typename R > R GetValue ()
  {
    return m_getter->Invoke < R > ();
  }

private:
  string m_name;
  AbstractFunc* m_getter;
}; 

Some Window Class:

class Window
{
public:

};

Example window class

class CheckBox :public Window
{
public:
  int GetChecked ()
  {
    return m_checked;
  }
  void SetChecked (int nChecked)
  {
    m_checked = nChecked;
  }

  void AddProperty (string name)
  {
    m_prop = new Property (name, new GetterFunction< int, Checked >(this, &Checked::GetChecked));
  }

  int m_checked;
  Property *m_prop;
};

main function:

int main ()
{

  CheckBox cc;
  cc.AddProperty ("Hello");

  cout<<"value:"<< cc.m_prop->GetValue<int>();

  return 0;
}

PROBLEM: Getter function is remembered as AbstractFunc in Property Class. I want to call 'Invoke' on AbstractFunc* instance and it should invoke the member function and return correct return type . The above code throws error at AbstractFunc::Invoke.

see live

Answer 1

Your AbstractFunc isn't abstract at all: its Invoke isn't virtual. So even though GetterFunction also has a method named Invoke , that method doesn't actually override AbstractFunc::Invoke ; it just hides it. When you try to call Invoke through the AbstractFunc* , it calls AbstractFunc::Invoke , which goes into infinite recursion and thus produces UB.

I would follow @nm's suggestion to make a class hierarchy like so:

class AbstractFunc {
  // lock down construction
  AbstractFunc() = default;
public:
  template<typename R>
  R Invoke();
  template<typename R>
  bool HasType() const noexcept;
  virtual ~AbstractFunc() = default; // need to have SOME virtual method so that we have runtime type info; also a virtual destructor is required anyway

  template<typename R>
  friend class TypedFunc;
};
template<typename R>
struct TypedFunc : AbstractFunc { // the ONLY instances of AbstractFunc are also instances of specializations of TypedFunc
  virtual R InvokeTyped() = 0;
};
// one kind of TypedFunc applies a getter on an object
template<typename R, typename T>
struct GetterFunc : TypedFunc<R> {
  // you never see a GetterFunc in the interface anyway... don't see a need to hide these
  T *obj; // have you considered std::shared_ptr?
  R (T::*getter)();
  GetterFunc(T *obj, R (T::*getter)()) : obj(obj), getter(getter) { }
  R InvokeTyped() override { return (obj->*getter)(); }
};
template<typename R, typename T>
std::unique_ptr<GetterFunc<R, T>> MakeGetterFunc(T *obj, R (T::*getter)()) {
  return std::make_unique<GetterFunc<R, T>>(obj, getter);
}
// another kind applies a functor, etc.
template<typename R, typename F>
struct FunctorFunc : TypedFunc<R> {
  F func;
  template<typename... G>
  FunctorFunc(G&&... args) : func(std::forward<G>(args)...) { }
  R InvokeTyped() override { return func(); }
};

This is already usable: if you have an AbstractFunc* or an AbstractFunc& , you can dynamic_cast it down to a TypedFunc of the expected type (eg TypedFunc<int> ). If that succeeds (you get a nonnull pointer or there is no std::bad_cast exception), then you just call InvokeTyped without having to know what kind of GetterFunc / FunctorFunc /whatever you are actually dealing with. The functions Invoke and HasType declared in AbstractFunc are just sugar to help do this.

template<typename R>
bool AbstractFunc::HasType() const noexcept {
  return dynamic_cast<TypedFunc<R> const*>(this);
}
template<typename R>
R AbstractFunc::Invoke() {
  return dynamic_cast<TypedFunc<R>&>(*this).InvokeTyped();
  // throws std::bad_cast if cast fails
}

Done.

class Property {
  std::string name;
  std::unique_ptr<AbstractFunc> getter;
public:
  Property(std::string name, std::unique_ptr<AbstractFunc> getter) : name(std::move(name)), getter(std::move(getter)) { }
  template<typename R>
  bool HasType() const noexcept { return getter->HasType<R>(); }
  template<typename R>
  R GetValue() const { return getter->Invoke<R>(); }
  std::string const &GetName() const noexcept { return name; }
};
struct Window {
  virtual ~Window() = default;
  // doesn't really make sense to add/remove these from outside...
  virtual std::vector<Property> GetProperties() = 0;
};
class CheckBox : public Window {
  int checked = 0;
public:
  int GetChecked() /*const*/ noexcept { return checked; }
  void SetChecked(int checked) noexcept { this->checked = checked; }
  std::vector<Property> GetProperties() override {
    std::vector<Property> ret;
    ret.emplace_back("Boxes Checked", MakeGetterFunc(this, &CheckBox::GetChecked));
    return ret;
  }
};

int main() {
  CheckBox cb;
  cb.SetChecked(5);
  for(auto const &prop : cb.GetProperties()) std::cout << prop.GetName() << ": " << prop.GetValue<int>() << "\n";
}

You could then add eg a virtual std::type_info const& GetType() const or similar to AbstractFunc if you want to be able to directly get at the type, etc.