[Ipopt] invalid use of incomplete type ‘struct Ipopt::SolveStatistics’

[cocobollo at gmx.net]

Hello,

i wrote an example program and everything worked just fine.
But when i try to access the statistics with

Number final_objective = app->Statistics()-> FinalObjective();

or for example

Index iter_count = app->Statistics()->IterationCount();

i get the following error:

error: invalid use of incomplete type ‘struct Ipopt::SolveStatistics’

What exactly ist the problem here and how can i solve it?

Thank you in advance

peter