使用模板实现“访问者模式”(Implement the “visitor pattern” with templates)

 我试图以参数方式实现访问者模式的修改版本，以这种方式避免每个具体元素都有重载的“通用访问者”，但是，由于我在模板编程方面没有很多经验我不知道如何完成“模式”。 
 
 码： 
 
// test.cpp
#include <iostream>
#include <vector>

using namespace std;

struct Base
{
    virtual ~Base() {}
    virtual void visit() = 0;
};

template<typename Visitor>
struct ElementBase : public Base
{
    // No virtual.
    void visit()
    {
        _e.visit(this);
    }

private:
    Visitor _e;
};

// Atoms.
template<typename Visitor>
struct ElementA : public ElementBase<Visitor>
{
    ElementA() : a(5) {}

    int a;
};

// Visitors.
struct VisitorA
{
    void visit(ElementBase<VisitorA> *a)
    {
        ElementA<VisitorA>* elto = dynamic_cast<ElementA<VisitorA>*>(a);

        cout << elto->a << endl;
    }

    /*
    void visit(ElementA<VisitorA> *a)
    {
        cout << a->a << endl;
    }
    */
};

std::vector<Base*> v;

int main()
{
    v.push_back(new ElementA<VisitorA>());

    for (auto i : v)
        i->visit();
}
 
 这工作正常，其输出为5（如预期的那样）。 但是，我假装的是直接使用VisitorA中“访问”的第二个（注释）版本。 
 
 显然，这不起作用，因为“this”具有ElementBase <...> *类型。 
 
 如何将指针“this”向下转换为ElementBase中的实际派生类？ 

I'm trying to implement a modified version of the visitor pattern in a parametric manner, avoiding in this way a "universal visitor" with a overload for each concrete element, but, due to I haven't a lot of experience in template programming I don't know how I can complete the "pattern".
 
Code:
 
// test.cpp
#include <iostream>
#include <vector>

using namespace std;

struct Base
{
    virtual ~Base() {}
    virtual void visit() = 0;
};

template<typename Visitor>
struct ElementBase : public Base
{
    // No virtual.
    void visit()
    {
        _e.visit(this);
    }

private:
    Visitor _e;
};

// Atoms.
template<typename Visitor>
struct ElementA : public ElementBase<Visitor>
{
    ElementA() : a(5) {}

    int a;
};

// Visitors.
struct VisitorA
{
    void visit(ElementBase<VisitorA> *a)
    {
        ElementA<VisitorA>* elto = dynamic_cast<ElementA<VisitorA>*>(a);

        cout << elto->a << endl;
    }

    /*
    void visit(ElementA<VisitorA> *a)
    {
        cout << a->a << endl;
    }
    */
};

std::vector<Base*> v;

int main()
{
    v.push_back(new ElementA<VisitorA>());

    for (auto i : v)
        i->visit();
}
 
This works fine and its output is 5 (as expected). But that I pretend is to make the same but directly with the second (commented) version of the "visit" in VisitorA.
 
Obviously, this doesn't work because "this" has the type ElementBase<...>*.
 
How can I downcast the pointer "this" to the actual derived class inside ElementBase?

原文：https://stackoverflow.com/questions/13865360