C++ Boost递归变量重载apply_visitor

Question

摘要:我希望在一个包含递归boost::variant对象的类中重载apply_visitor()方法。

在下面的代码中有这样的方法：

template <typename T>
ostream& apply_visitor(const T& fn) const

我想为不同的访问者重载这个方法。如下所示：

ostream& apply_visitor(const PrintData& fn) const

但问题是PrintData类还不完整(参见下面代码中的注释)。它是在Node类之后定义的。所以我有两个问题(除了其他问题--我欢迎对这段代码的一般性评论，这段代码是我想要投入生产的建模)。

1)有没有办法让apply_visitor(PrintData&)正常工作？

2)我可以重新排列(递归)变量，这样所有的访问者方法都在PrintData中，而不需要将apply_visitor添加到Node类中吗？

/**
 * compiled with gcc (tested with 4.7.2 on linux)
 * requires the boost development headers to be discoverable
 * by the compiler.
 *
 * g++ -otest-recursive-visit -std=c++11 test-recursive-visit.cpp
 * ./test-recursive-visit
 **/

#include <iostream>
#include <map>
#include <string>
#include <vector>

#include "boost/variant.hpp"
#include "boost/variant/recursive_wrapper.hpp"
#include "boost/variant/static_visitor.hpp"

using namespace std;

/// type name demangler (as implemented in g++). For other compilers,
/// we could add more #elif statements, but for now, just return
/// the mangled name.
#ifdef __GNUG__ /// compiler is g++
#include <cxxabi.h>
string type_demangle(const string& name)
{
    int status;
    char* res = abi::__cxa_demangle(
        name.c_str(), NULL, NULL, &status);
    string demangled_name((status==0) ? res : name);
    free(res);
    return demangled_name;
}
#else /// compiler is not g++
string type_demangle(const string& name) { return name; }
#endif

/// forward declaration of the Node class
/// (needed for recursive variant type)
class Node;

/// the actual recursive variant type
/// (typically hidden from the users)
typedef boost::variant<
    boost::recursive_wrapper<Node>,
    vector<int>,
    vector<float>
> data_t;

// forward declaration for PrintData. See note below concerning
// Node::apply_visitor()
class PrintData;

/// this is the object users will see
/// for prototyping, the tree object is public
class Node
{
  public:
    /// sub-nodes are identified by unique strings
    /// which point to one of the objects that data_t
    /// can hold
    map<string,data_t> tree;

    /// constructor for a std::map object, passed to tree
    Node(const initializer_list<pair<const string,data_t>>& l)
    : tree(l)
    {}

    //
    // INTERESTING PART OF THIS EXAMPLE IS HERE
    //
    // I tried to replace T& with PrintData& adding
    // a forward declaration at the top but I get the
    // errors:
    //
    //  line 86:
    //      invalid use of incomplete type ‘const class PrintData’
    //  line 53:
    //      forward declaration of ‘const class PrintData’
    //

    /// This is called by boost::apply_visitor(Visitor(),Node)
    //ostream& apply_visitor(const PrintData& fn) const
    template <typename T>
    ostream& apply_visitor(const T& fn) const
    {
        for (auto i : tree)
        {
            *fn.os << fn.prefix << i.first;
            i.second.apply_visitor(fn);
        }
        return *fn.os;
    }
};

/// the printing visitor to ostream object
class PrintData : public boost::static_visitor<ostream&>
{
  public:
    ostream* os;
    string prefix;

    /// keep a pointer to the ostream and keep
    /// a prefix string which will hold and "indent"
    /// which is something like "  " for every level
    /// of recursion
    PrintData(ostream& out_stream, const string& prefix_str="")
    : os(&out_stream)
    , prefix(prefix_str)
    {}

    /// recurse into the tree, adding indent characters to prefix
    ostream& operator()(Node& n) const
    {
        *os << endl;
        n.apply_visitor(PrintData(*os, prefix+"  "));
        *os;
    }

    /// actual data types that we want to print out
    template <typename T>
    ostream& operator()(const vector<T>& d) const
    {
        *os << " (vector<" << type_demangle(typeid(T).name()) << ">):";
        for (T i : d)
        {
            *os << " " << i;
        }
        *os << endl;
        return *os;
    }
};

/// convenience operator to allow: cout << node;
ostream& operator<<(ostream& os, const Node& n)
{
    return boost::apply_visitor(PrintData(os), n);
}


int main()
{
    /// hooray for initialization lists!!!
    Node n {
        {"X", Node{
            {"a", vector<int>{1,2,3}},
            {"b", vector<float>{2,3,4}}
        }},
        {"Y", vector<int>{3,4,5}},
        {"Z", Node{
            {"A", Node{
                {"c", vector<float>{4,5,6}}
            }}
        }}
    };

    /**
    applying PrintData to n prints out the following:

    X
      a (vector<int>): 1 2 3
      b (vector<float>): 2 3 4
    Y (vector<int>): 3 4 5
    Z
      A
        c (vector<float>): 4 5 6

    **/
    cout << n;
}

Answer 1

声明函数，但不要在类中定义它。相反，应该等到定义了PrintData之后再定义它，这时它将是一个完整的类型。

class PrintData;

class Node
{
  public:
  ...
    ostream& apply_visitor(const PrintData& fn) const;
  ...
};

class PrintData : public boost::static_visitor<ostream&>
{ ... };

inline ostream& Node::apply_visitor(const PrintData& fn) const
{
  ...
}

Answer 2

我遇到了类似的问题，我使用了make_recursive_variant，然后我使用map<Key,Value>代替了Node类。最后，我的节点类变成了包装std::map引用的实用函数。例如：

typedef boost::make_recursive_variant<
    std::map<std::string, boost::recursive_variant_>,
    vector<int>,
    vector<float>
>::type data_t;

// the printing visitor to ostream object
class PrintData : public boost::static_visitor<ostream&>
{
  public:
    ostream* os;
    string prefix;

    /// keep a pointer to the ostream and keep
    /// a prefix string which will hold and "indent"
    /// which is something like "  " for every level
    /// of recursion
    PrintData(ostream& out_stream, const string& prefix_str="")
    : os(&out_stream)
    , prefix(prefix_str)
    {}

    /// recurse into the tree, adding indent characters to prefix
    ostream& operator()(std::map<std::string, data_t>& tree) const
    {
        *os << endl;
        for (auto& i : tree)
        {
            *os << prefix << i.first;
            //you may want to change prefix and use new PrintData instead of *this
            boost::apply_visitor(*this, i.second);
        }
        return *os;
    }

    /// actual data types that we want to print out
    template <typename T>
    ostream& operator()(const vector<T>& d) const
    {
        *os << " (vector<" << type_demangle(typeid(T).name()) << ">):";
        for (T i : d)
        {
            *os << " " << i;
        }
        *os << endl;
        return *os;
    }
};


class Node
{
public:
    //implicit ctor for data_t conversion
    Node(data_t& data) : data(data) { }
    //...
private:
    data_t& data;
};

编辑:我忘记提到在Node类中不再需要apply_visitor了，但是我对initializer_list ctor不太确定。它可能不适用于data_t类型。