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

From: Douglas Gregor (dgregor_at_[hidden])
Date: 2005-02-11 08:22:37

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