模板函数中的获取/推导类型以获取返回值

Question

I am using the Boost Graph Library and have build my own classes upon that.

Now, I have the following function:

template< class VERTEXPROPERTIES >
VERTEXPROPERTIES& properties(const Vertex& v) const
{   
    typename property_map<FilteredGraphContainer, vertex_properties_t>::const_type param = get(vertex_properties, graph_);

    return param[v];
}

When I use this function in a class template like this:

template< class VERTEXPROPERTIES, class EDGEPROPERTIES >
class Graph
{
// all the code
};

it works. However, when I want to use it in a class like this:

template < class GRAPH, class EDGE_PREDICATE, class VERTEX_PREDICATE >
class FilteredGraph
{
// all the code
};

and apply the properties() function to a FilteredGraph object, the compiler complains that it can't find the function.
As far as I have found out so far this is probably due to the missing template-parameter in the arguments list.
For the Graph-class, this template-parameter is easy to find out, as it gets defined when creating a Graph-object.
But for the FilteredGraph, it should be able to deduce it from the GRAPH-template parameter.
Then again, I think that the compiler nevertheless will have problems getting the right return-value type as this might probably be necessary to find out beforehand...
I would really like to keep that concept with the properties() function as it allows to keep the other functions generic.
Do you have any ideas how to solve that issue, possibly keeping the concept?

Answer 1

Again, my apologies for not providing more information.
It seemed to me that the compiler was not able to deduce the dependant type there. I encountered this issue some times before when working with templates.
I will provide the fix here in case somebody might encounter similar issues.

The FilteredGraph uses an instance of Graph (basically an adjacency_list with internal custom properties) as its input. The Graph-class has two template parameters, one for the properties of the vertices and one for the properties of the edges.
As a little sidenote, using bundled properties (not supported by older version of boost graph library and also not by some compilers), I think, this problem could be circumvented as you would not need an extra function returning a value of a(possibly) unknown type. Because the Graph-object passed to the FilteredGraph needs to be specified for some particular types of vertex/edge properties, this information can actually be extracted.
I achieved this via:

/// Type of the internal properties of the edges
typedef typename property_traits< typename property_map<FilteredGraphContainer, vertex_properties_t>::type >::value_type VProps;
/// Type of the internal properties of the edges
typedef typename property_traits< typename property_map<FilteredGraphContainer, edge_properties_t>::type >::value_type EProps;

and then defining:

VProps& properties(const Vertex& v)
{
        typename property_map<FilteredGraphContainer, vertex_properties_t>::type param = get(vertex_properties, graph_);
        return (param[v]);
}

Thanks again for your interest in this question and I am sorry not having provided more information.

Best regards.