Question

关于树的访问者模式，我遇到了代码重复问题。目前的情况如下：我有一棵树，由两个不同的节点类组成，即叶子和非叶子。另外，我有两个看起来非常相似的访客基类，除了一个访问const树和其他非const树。具体访问者必须执行的实际操作与节点的具体类型无关。我举一个简短的例子：

class Visitor;
class ConstVisitor;

class Node {
public:
  virtual void accept(Visitor&) = 0;
  virtual void accept(ConstVisitor&) const = 0;
};

class Leaf : public Node {
  virtual void accept(Visitor& v)        {v.visitLeaf(*this);}
  virtual void accept(ConstVisitor& cv)  {cv.visitLeaf(*this);}
};

class CompoundNode : public Node {
public:
  vector<Node*> getChildren() const;
  virtual void accept(Visitor& v)        {v.visitCompoundNode(*this);}
  virtual void accept(ConstVisitor& cv)  {cv.visitCompoundNode(*this);}
};

class Visitor {
protected:
  virtual void processNode(Node& node) = 0;
public:
  void visitLeaf(Leaf& leaf) {
    processNode(leaf);
  }
  void visitCompoundNode(CompoundNode& cNode) {
    processNode(cNode);
    auto children = cNode.getChildren();
    for (auto child : children)
      child->accept(this);
  }
};

class ConstVisitor {
protected:
  virtual void processNode(Node const& node) = 0;
public:
  void visitLeaf(Leaf const& leaf) {
    processNode(leaf);
  }
  void visitCompoundNode(CompoundNode const& cNode) {
    processNode(cNode);
    auto children = cNode.getChildren();
    for (auto child : children)
      child->accept(this);
  }
};

具体的访问者类继承自Visitor或ConstVisitor，具体取决于他们的processNode方法是否必须更改已访问的节点。

你知道，两个访问者之间存在大量代码重复，因为我必须实现另一个遍历策略，对于const和nonconst节点，我想避免重复。是否有可能提取重复的代码，最好不要在整个地方使用const_cast？

Answer 1

您可以定义TVisitor类模板，如下所示：

#include <type_traits>

class Node;
class CompoundNode;
class Leaf;

template<bool isNonConstVisitor>
class TVisitor
{
    typedef typename std::conditional<isNonConstVisitor, 
        Node, Node const>::type node_type;

    typedef typename std::conditional<isNonConstVisitor, 
        CompoundNode, CompoundNode const>::type compound_node_type;

    typedef typename std::conditional<isNonConstVisitor, 
        Leaf, Leaf const>::type leaf_node_type;

protected:

    virtual void processNode(node_type& node) = 0;

public:

    void visitLeaf(leaf_node_type& leaf) { processNode(leaf); }

    void visitCompoundNode(compound_node_type& cNode) {
        processNode(cNode);
        auto children = cNode.getChildren();
        for (auto child : children) { child->accept(*this); }
    }
};

然后使用Visitor和ConstVisitor作为该类模板的相应实例化的类型别名：

typedef TVisitor<true> Visitor;
typedef TVisitor<false> ConstVisitor;

Answer 2

您可以使用模板：

template<typename NodeType,
         typename CompoundNodeType,
         typename LeafType>
class BaseVisitor {
protected:
    virtual void processNode(NodeType& node) = 0;
public:
    void visitLeaf(LeafType& leaf) {
        processNode(leaf);
    }
    void visitCompoundNode(CompoundNodeType& cNode) {
        processNode(cNode);
        auto children = cNode.getChildren();
        for (auto child : children)
        child->accept(this);
    }
};

class Visitor: public BaseVisitor<Node, CompoundNode, Leaf> {
};

class ConstVisitor: public BaseVisitor<const Node, const CompoundNode, const Leaf> {
};

树的const和非const版本的访问者模式

2 个答案: