BGL Adding an edge with multiple properties

Question

I want to have all edges have to properties, weight and capacity. I found that BGL has these both already defined. So I define Edge and Vertex properties for the Graph ``` typedef property VertexProperty; typedef property > EdgeProperty; typedef adjacency_list Graph; ``` Here is where I am trying to add the edges to the graph: ``` 172: EdgeProperty prop = (weight, capacity); 173: add_edge(vertex1,vertex2, prop, g); ``` If I had just 1 property I know it would be prop = 5; However, with two I am confused about the formatting. Here is the error I am receiving: ``` graph.cc: In function ‘void con_graph()’: graph.cc:172: warning: left-hand operand of comma has no effect ```

Accepted Answer

If you look at the implementation of boost::property you'll see that a property value cannot be initialized this way. And even then, the syntax you have `(weight, capacity)` is not valid anyways, so, if it was possible to initialize the property like that, it would be written `EdgeProperty prop = EdgeProperty(weight, capacity);` or just `EdgeProperty prop(weight, capacity);`. But, again, that won't work. Technically, this is the way you would need to initialize the property value:

```
EdgeProperty prop = EdgeProperty(weight, property<edge_capacity_t, int>(capacity));
```

But this is kind of ugly as the number of properties increase. So, it would be cleaner to default-construct the edge-property and then manually set each individual property:

```
EdgeProperty prop;
get_property_value(prop, edge_weight_t) = weight;
get_property_value(prop, edge_capacity_t) = capacity;
```

Of course, the better alternative is to use bundled properties instead of the older boost::property chains.

BGL Adding an edge with multiple properties

Solution

Problem