[Boost-users] [BGL] Encountering "bad lexical cast" exception when reading a class from graphviz file

21 Jul 2012

      Hi all,

I am trying to read a graphviz file into my graph implementation. The
minimum working code as well as the graphviz file are at the end of
this e-mail. The node of the graph is the Vertex struct, which
contains a point object (Point p). For Point class, I overload two
operators, >> and <<, and operator << was previously used to generate
the graphviz file.  The operator overloads work well with std::cin and
std::cout, but with the code below, I receive an exception:

bad lexical cast: source type value could not be interpreted as target .

The culprit might be in the << overload. In particular, when reading
from a file, the value of iToBool1 is true, whereas iToBool2 is false.
That is to say, the variable i might change when inserting the stream
data to the point. This does not happen when reading from std::cin, as
both variables are true.

I convert the i variable, originally an istream, to bool because when
the program returns to the calling function, which is
lexical_stream_limited_src::shr_using_base_class on line 1616 in
lexical_cast.hpp, this functions uses the return value of << operator
in a boolean comparison. The value of iToBool2 being false eventually
leads to calling BOOST_LCAST_THROW_BAD_CAST on line 1975.

I wonder if somebody can tell me what is wrong with my operator overload?

I use Boost 1.50 and Visual Studio 2008 on Windows 7.

Thanks!

Best regards,
Nicholas

//--------------------Graphviz--------------------

graph G {
1 [Point="1 2 3"];
}

//----------------------code----------------------

#include 
#include 
#include 

#include <iostream>
#include <fstream>
#include <string>

struct Point
{
    float x;
    float y;
    float z;

    inline friend std::istream& operator >>
        ( std::istream& i, Point& p )
    {
        bool iToBool1 = i;
        i >> p.x >> p.y >> p.z;
        bool iToBool2 = i;
        return i;
    }

    inline friend std::ostream& operator <<
        ( std::ostream& o, const Point& p )
    {
        o << p.x << " " << p.y << " " << p.z;
        return o;
    }
};

struct Vertex
{
    int ID;
    Point p;
};

struct Edge
{
    int ID;
};

typedef boost::adjacency_list<
            boost::listS,
            boost::vecS,
            boost::undirectedS,
            Vertex,                       // bundled data
            Edge,                       // bundled data
            boost::no_property,         // not needed
            boost::listS> Graph;

int main()
{
    // Part 1: input via std::cin
    Point p;
    std::cin >> p;
    Graph graph;

    // Part 2: input from graphviz
    boost::dynamic_properties dynamicProperties;
    dynamicProperties.property("NodeID", boost::get(&Vertex::ID, graph));
    dynamicProperties.property("Point", boost::get(&Vertex::p, graph));
    dynamicProperties.property("EdgeID", boost::get(&Edge::ID, graph));

    std::ifstream file("graph.txt");
    try
    {
        boost::read_graphviz(file, graph, dynamicProperties, "NodeID");
    }
    catch (std::exception& e)
    {
        std::cout << e.what() << std::endl;
    }

    return 0;
}

[Boost-users] [BGL] Encountering "bad lexical cast" exception when reading a class from graphviz file

Nicholas Mario Wardhana