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Visitor.h
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142 lines (127 loc) · 3.12 KB
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#pragma once
#include "DesignPatterns.h"
inline namespace VisitorNewEd {
/*
改进
使用可变参数模板
*/
template <typename... Types>
class Visitor;
template <typename T, typename... Types>
class Visitor<T, Types...> : Visitor<Types...> {
public:
Visitor() = default;
virtual ~Visitor() = default;
// 通过 using 避免隐藏基类的 Visit 同名方法
using Visitor<Types...>::Visit;
virtual void Visit(const T&) = 0;
};
template <typename T>
class Visitor<T> {
public:
Visitor() = default;
virtual ~Visitor() = default;
virtual void Visit(const T&) = 0;
};
void TestVisitor() {
struct stA;
struct stB;
struct Base {
using MyVisitor = Visitor<stA, stB>;
virtual void Accept(MyVisitor&) = 0;
};
struct stA : Base {
double val = 0.0;
virtual void Accept(Base::MyVisitor& v) override {
v.Visit(*this);
}
};
struct stB : Base {
int val = 0;
virtual void Accept(Base::MyVisitor& v) override {
v.Visit(*this);
}
};
struct PrintVisitor : Base::MyVisitor {
virtual void Visit(const stA& a) override {
std::cout << "from stA: " << a.val << std::endl;
}
virtual void Visit(const stB& b) override {
std::cout << "from stB: " << b.val << std::endl;
}
};
PrintVisitor vis;
stA a;
a.val = 3.14159;
stB b;
b.val = 42;
Base* base = &a;
base->Accept(vis);
base = &b;
base->Accept(vis);
}
} // inline namespace VisitorNewEd
namespace VisitorOldEd {
/*
缺点
每增加一种被访问者类型就要在访问者基类 Visitor 中增加一个对应的纯虚函数，
若新增 ConcreteElement3 则需要在 Visitor 中增加一个纯虚函数：
virtual void Visit(ConcreteElement3* element) = 0;
所以说 Visitor 的接口不稳定，所以需要重新定义一个稳定的接口层，
即不会因为增加新的被访问者而修改接口层。
*/
class ConcreteElement1;
class ConcreteElement2;
// 访问者 基类
class Visitor {
public:
Visitor() = default;
virtual ~Visitor() = default;
virtual void Visit(ConcreteElement1* element) = 0;
virtual void Visit(ConcreteElement2* element) = 0;
};
// 被访问者 基类
class Element {
public:
Element() = default;
virtual ~Element() = default;
virtual void Accept(Visitor& visitor) = 0;
};
// 访问者 子类
class ConcreteVisitor : public Visitor {
public:
ConcreteVisitor() = default;
virtual ~ConcreteVisitor() = default;
virtual void Visit(ConcreteElement1* element) override {
std::cout << "Visit ConcreteElement1\n";
};
virtual void Visit(ConcreteElement2* element) override {
std::cout << "Visit ConcreteElement2\n";
};
};
// 被访问者 子类
class ConcreteElement1 : public Element {
public:
ConcreteElement1() = default;
virtual ~ConcreteElement1() = default;
virtual void Accept(Visitor& visitor) override {
visitor.Visit(this);
}
};
class ConcreteElement2 : public Element {
public:
ConcreteElement2() = default;
virtual ~ConcreteElement2() = default;
virtual void Accept(Visitor& visitor) override {
visitor.Visit(this);
}
};
// 测试代码
void TestVisitor() {
ConcreteVisitor v;
std::unique_ptr<Element> e1(new ConcreteElement1());
std::unique_ptr<Element> e2(new ConcreteElement2());
e1->Accept(v);
e2->Accept(v);
}
} // namespace VisitorOldEd