RE: [Boost-users] BGL problem,originally [Another C++ inheritence/polymorphism problem]

11 Feb 2005

      Yeah, if I change the vertex property writer to be:

  void operator()(std::ostream& out, Vertex& v) 

It works fine.

-----Original Message-----
From: boost-users-bounces@lists.boost.org
[mailto:boost-users-bounces@lists.boost.org] On Behalf Of Douglas Gregor
Sent: Friday, February 11, 2005 6:23 AM
To: boost-users@lists.boost.org
Subject: Re: [Boost-users] BGL problem,originally [Another C++
inheritence/polymorphism problem]

On Feb 10, 2005, at 10:25 PM, Welson Sun wrote:
...
Well,  I found out that this is not a problem related to inheritence 
nor polymorphism, it is a problem with the BGL
1. Even if I make BasicOperator not inherit from Operator, the problem 
is the same.
Here is the modified BasicOperator:
class BasicOperator /* : public Operator */{
public:
  BasicOperator() {};
  BasicOperator(OperatorType* type) : _type(type) {toString = 
_type->getMnemonic();};
  BasicOperator(OperatorType* type, string cust) : _type(type),
_cust(cust)
{toString = _type->getMnemonic() + "&" + _cust;};
  OperatorType* getOperatorType() { return _type; }
  string toString() { return toString; }
  string toString;
private:
  OperatorType* _type;
  string _cust;
};
And the vertex property writer is:
  void operator()(std::ostream& out, const Vertex& v) const {
    out << "[label=\""
        << "(" << g[v].toString() << ")"
        << "\"]";
  }
The error is:
main.cpp:52: error: passing `const BasicOperator' as `this' argument 
of `
   std::string BasicOperator::toString()' discards qualifiers
Looking at the code you previously posted and mapping that to what you've
written here, here's what happens:

schedule_vertex_writer stores a _copy_ of the graph (called g) inside it.
Since operator() is marked "const", g[v] returns a reference to a const
BasicOperator. But, when you call toString(), it is not marked const, so you
get the error above.

I have two suggestions:
   (1) Pass and store the graph g as a "const Graph&" in
schedule_vertex_writer. Copying graphs can be really expensive.
   (2) Make toString() const. If you absolutely cannot do this, then you'll
need to store a non-const reference to the graph instead of a const one.
...
So it looks like that the problem is related to the "const" 
BasicOperator I
passed into std::string BasicOperator::toString(). How can I fix this? 
Which
means if the property of a vertex is a Class, how can I call its 
member function in the vertex writer?
2. If I changes the vertex property writer as:
  void operator()(std::ostream& out, const Vertex& v) const {
    out << "[label=\""
        << "(" << g[v].toString << ")"
        << "\"]";
  }
Everything is OK. Which mean in the vertex property writer, I can 
access the data member without any problem, but I cannot access the 
member function.
Right, because data members can be accessed irrespective of the
cv-qualifiers on the object type. You'll just get a const std::string& back.

	Doug

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