I should have looked at the code more carefully.
I appears that:

## Advertising

1. The original graph is destroyed and replaced with the new one.
2. vertex_comb is checked for being NULL. In that case the attribute
handling code is skipped.
On 19 September 2016 at 15:23, Szabolcs Horvát <szhor...@gmail.com> wrote:
> Actually I don't fully understand this function.
>
> The signature is:
>
> int igraph_contract_vertices(igraph_t *graph,
> const igraph_vector_t *mapping,
> const igraph_attribute_combination_t *vertex_comb);
>
> 'graph' is the input graph. Where is the new graph returned?
>
> On 19 September 2016 at 15:17, Szabolcs Horvát <szhor...@gmail.com> wrote:
>> Hello everyone,
>>
>> Quick question about contract_vertices():
>>
>> http://igraph.org/c/doc/igraph-Structural.html#igraph_contract_vertices
>>
>> I don't have attributes. Can I just pass vertex_comb=NULL?
>>
>> Szabolcs
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