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POC++ Boost recursive variant overloading apply_visitor
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2013-04-04T21:26:29.330
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abstract: I am looking to overload the apply_visitor() method in a class that contains a recursive boost::variant object. In the code included below there is the method: <pre><code>template <typename T> ostream& apply_visitor(const T& fn) const </code></pre> I would like to overload this method for different visitors. Something like this: <pre><code>ostream& apply_visitor(const PrintData& fn) const </code></pre> But the problem is that the class PrintData is not yet complete (see comments in the code below). It is defined after the Node class. So I have two questions (among others -- I would welcome a general critique on this code which is modeling something I'd like to put into production). 1) Is there a way to get apply_visitor(PrintData&) to work? 2) Can I rearrange the (recursive) variant so all visitor methods are in PrintData and I wouldn't have to add apply_visitor to the Node class? <pre class="lang-cpp prettyprint-override"><code>/** * 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; } </code></pre>
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<c++><recursion><g++><boost-variant>
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C++ Boost recursive variant overloading apply_visitor
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