C++避免常量和非常量访问的代码重复

发布于 2024-10-21 09:27:57 字数 603 浏览 3 评论 0原文

我有一个类应该为每个成员变量调用访问者方法。像这样的事情：

class A{
    int a, b, c;

public:
    void accept(Visitor &visitor){
        visitor.visit(a);
        visitor.visit(b);
        visitor.visit(c);
    }
};

如何在不重复代码的情况下使用相同的代码获得 void accept() const 方法？

重复的明显解决方案是添加一个方法：

void accept(Visitor &visitor) const {
    visitor.visit(a);
    visitor.visit(b);
    visitor.visit(c);
}

该方法具有我想要的确切含义，但我想避免代码重复。使用这两种方法的原因是能够通过“读取”访问者读取变量，并很好地使用 accept 方法 const。然后非常量 accept 将可以用于“写入/更新”访问者。

原文

I have a class that should call a visitor method for every member variable. Something like this:

class A{
    int a, b, c;

public:
    void accept(Visitor &visitor){
        visitor.visit(a);
        visitor.visit(b);
        visitor.visit(c);
    }
};

How can I get void accept() const method with the same code without code duplication?

The obvious solution with the duplication is to add a method:

void accept(Visitor &visitor) const {
    visitor.visit(a);
    visitor.visit(b);
    visitor.visit(c);
}

That method has exactly the meaning I want, but I would like to avoid the code duplication. The reason to have both methods is to be able to read the variables by a 'reading' visitor and having the accept method nicely const. Then the non-const accept would be possible to use for 'writing/updating' visitors.

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左秋 2024-10-28 09:27:58

您可以创建一个类静态模板辅助函数，该函数将根据您提供给它的 this 指针的类型推断出常量性。像这样：

class A{
    int a, b, c;

public:

    void accept(Visitor &visitor){
        acceptImpl(*this, visitor);
    }
    void accept(Visitor &visitor) const{
        acceptImpl(*this, visitor);
    }

private:
    template<typename t_A>
    static void acceptImpl(t_A& aObj, Visitor &visitor)
    {
        visitor.visit(aObj.a);
        visitor.visit(aObj.b);
        visitor.visit(aObj.c);
    }
};

You could create a class static template helper function that will deduce the constness based on the type of the this pointer you provide to it. Like this:

class A{
    int a, b, c;

public:

    void accept(Visitor &visitor){
        acceptImpl(*this, visitor);
    }
    void accept(Visitor &visitor) const{
        acceptImpl(*this, visitor);
    }

private:
    template<typename t_A>
    static void acceptImpl(t_A& aObj, Visitor &visitor)
    {
        visitor.visit(aObj.a);
        visitor.visit(aObj.b);
        visitor.visit(aObj.c);
    }
};

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欢烬 2024-10-28 09:27:58

模板助手：

class A{
    int a, b, c;

private:
    template <typename T>
    static void do_visiting(T &self, Visitor &visitor) {
        visitor.visit(self.a);
        visitor.visit(self.b);
        visitor.visit(self.c);
    }
public:
    void accept(Visitor &visitor) {
        do_visiting(*this, visitor); // calls do_visiting<A>
    }
    void accept(Visitor &visitor) const {
        do_visiting(*this, visitor); // calls do_visiting<const A>
    }
};

Template helper:

class A{
    int a, b, c;

private:
    template <typename T>
    static void do_visiting(T &self, Visitor &visitor) {
        visitor.visit(self.a);
        visitor.visit(self.b);
        visitor.visit(self.c);
    }
public:
    void accept(Visitor &visitor) {
        do_visiting(*this, visitor); // calls do_visiting<A>
    }
    void accept(Visitor &visitor) const {
        do_visiting(*this, visitor); // calls do_visiting<const A>
    }
};

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