[Lemon-user] is there a way to write a subgraph?

[Kovács Péter]

Hi All,

Thanks for the corrections. I'm terribly sorry for the stupid mistake. 
So I suggest passing references, but of course, not for local variables. :)

Péter



>     FilterNodes<ListDigraph>& get_subgraph(void)
>     {
>         FilterNodes<ListDigraph> subgraph;
>         ...
>         return g;
>     }
>
> This is a bad idea as it creates a reference to a temporary variable. I
> think the real problem is that the FilterNodes is initiated with a local
> map, which is destroyed when the execution leaves the function scope.
>
> Balazs
>
> On Fri, Dec 30, 2016 at 9:17 PM, Kovács Péter <kpeter at inf.elte.hu
> <mailto:kpeter at inf.elte.hu>> wrote:
>
>     Hi John,
>
>     You should avoid passing graphs (and other large data structures) as
>     values between functions. Use references or pointers instead. For
>     example:
>
>     FilterNodes<ListDigraph>& get_subgraph(void)
>     {
>         FilterNodes<ListDigraph> subgraph;
>         ...
>         return g;
>     }
>
>     Note the '&' sign in the return type of the function. (If its
>     meaning is not clear, read some C++ tutorials about references and
>     pointers.)
>
>     Regards,
>     Péter
>
>
>
>         Péter,
>
>         It turns out that the issue I am having is related to trying to
>         return
>         a FilterNodes<ListDigraph> issue from a function.  If
>         the FilterNodes<ListDigraph> object is local I can iterate
>         through the
>         nodes as expected.  If the object is returned from a function i.e.
>
>         FilterNodes<ListDigraph> get_subgraph(void)
>         {
>         }
>
>         void another_function()
>         {
>              auto x = get_subgraph();
>
>              for (FilterNodes<ListDigraph>::NodeIt n(x); n != INVALID; ++n)
>              ...
>         }
>
>         I get an exception when trying to iterate through the nodes.
>
>         John
>
>
>
>
>         On Thu, Dec 29, 2016 at 5:28 PM, Kovács Péter
>         <kpeter at inf.elte.hu <mailto:kpeter at inf.elte.hu>
>         <mailto:kpeter at inf.elte.hu <mailto:kpeter at inf.elte.hu>>> wrote:
>
>             Hi John,
>
>             What kind of write method are you looking for? Could you
>         send a code
>             snippet that works for the graph, but does not apply to the
>         subgraph?
>
>             Regards,
>             Péter
>
>
>                 I have a subgraph as follows:
>
>                 FilterNodes<ListDigraph> sub(*graph, filter);
>
>                 There doesn't appear to be a write method for the
>         subgraph as
>                 there is
>                 for the graph.
>
>                 Suggestions?
>
>
>                 --
>                 John Lagerquist
>                 Chief Engineer
>                 RallyTronics LLC
>                 801-866-5981
>
>
>
>
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>
>
>
>
>
>         --
>         John Lagerquist
>         Chief Engineer
>         RallyTronics LLC
>         801-866-5981
>
>
>
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