我尝试使用模板化派生类
实现访问者模式这里是VisitorTemplate.hpp,我专门在类Visitor中派生,但我希望能够处理任何类型:
编辑:感谢interjay的建议,代码编译并运行时没有错误
#ifndef VISITORTEMPLATE_HPP_
#define VISITORTEMPLATE_HPP_
#include <iostream>
#include <string>
using namespace std;
template<class T> Derived;
class Visitor
{
public:
virtual void visit(Derived<string> *e) = 0;
};
class Base
{
public:
virtual void accept(class Visitor *v) = 0;
};
template<class T>
Derived: public Base
{
public:
virtual void accept(Visitor *v)
{
v->visit(this);
}
string display(T arg)
{
string s = "This is : " + to_string(arg);
return s;
}
};
class UpVisitor: public Visitor
{
virtual void visit(Derived<string> *e)
{
cout << "do Up on " + e->display("test") << '\n';
}
};
class DownVisitor: public Visitor
{
virtual void visit(Derived<string> *e)
{
cout << "do Down on " + e->display("test") << '\n';
}
};
#endif /* VISITORTEMPLATE_HPP_ */
的main.cpp
Base* base = new Derived<string>();
Visitor* up = new UpVisitor();
Visitor* down = new DownVisitor();
base->accept(up);
base->accept(down);
现在我的目标是在没有专业化的情况下使用Derived进行访问;不幸的是,访问是一种虚拟方法,所以我无法模板化
答案 0 :(得分:9)
来自Modern C ++ - 应用设计通用编程和设计模式 - Andrei Alexandrescu
#include <iostream>
class BaseVisitor
{
public:
virtual ~BaseVisitor() {};
};
template <class T, typename R = int>
class Visitor
{
public:
virtual R visit(T &) = 0;
};
template <typename R = int>
class BaseVisitable
{
public:
typedef R ReturnType;
virtual ~BaseVisitable() {};
virtual ReturnType accept(BaseVisitor & )
{
return ReturnType(0);
}
protected:
template <class T>
static ReturnType acceptVisitor(T &visited, BaseVisitor &visitor)
{
if (Visitor<T> *p = dynamic_cast< Visitor<T> *> (&visitor))
{
return p->visit(visited);
}
return ReturnType(-1);
}
#define VISITABLE() \
virtual ReturnType accept(BaseVisitor &v) \
{ return acceptVisitor(*this, v); }
};
/** example of use */
class Visitable1 : public BaseVisitable<int>
{
/* Visitable accept one BaseVisitor */
public:
VISITABLE();
};
class Visitable2 : public BaseVisitable<int>
{
/* Visitable accept one BaseVisitor */
public:
VISITABLE();
};
class VisitorDerived : public BaseVisitor,
public Visitor<Visitable1, int>,
public Visitor<Visitable2, int>
{
public:
int visit(Visitable1 & c)
{
std::cout << __PRETTY_FUNCTION__ << std::endl;
}
int visit(Visitable2 & c)
{
std::cout << __PRETTY_FUNCTION__ << std::endl;
}
};
int main(int argc, char **argv)
{
VisitorDerived visitor;
Visitable1 visitable1;
Visitable2 visitable2;
visitable1.accept(visitor);
visitable2.accept(visitor);
}
可以避免使用CRTP模式的dynamic_cast:
#include <iostream>
class BaseVisitor
{
public:
virtual ~BaseVisitor() {};
};
template <class T>
class Visitor
{
public:
virtual void visit(T &) = 0;
};
template <class Visitable>
class BaseVisitable
{
public:
template <typename T>
void accept(T & visitor)
{
visitor.visit(static_cast<Visitable &>(*this));
}
};
/** example of use */
class Visitable1 : public BaseVisitable<Visitable1>
{
};
class Visitable2 : public BaseVisitable<Visitable2>
{
};
class VisitorDerived : public BaseVisitor,
public Visitor<Visitable1>,
public Visitor<Visitable2>
{
public:
void visit(Visitable1 & c)
{
std::cout << __PRETTY_FUNCTION__ << std::endl;
}
void visit(Visitable2 & c)
{
std::cout << __PRETTY_FUNCTION__ << std::endl;
}
};
int main(int argc, char **argv)
{
VisitorDerived visitor;
Visitable1 visitable1;
Visitable2 visitable2;
visitable1.accept<VisitorDerived>(visitor);
visitable2.accept<VisitorDerived>(visitor);
}
答案 1 :(得分:1)
您的Derived
类无法使用Visitor
,因为它尚未定义(它只是前向声明,因此是不完整的类型)。
您可以通过在Visitor
之前添加Derived
定义来修复编译错误。在定义Derived
之前,您还需要转发声明Visitor
:
template <class T> class Derived;
class Visitor {
public:
virtual void visit(Derived<string> *e) = 0;
};
template <class T>
class Derived : public Base {
//.... can call Visitor methods here ...
};