[deal.II] error: cannot declare variable ‘postprocessor’ to be of abstract type ‘FracturePhaseFieldProblem<2>::Postprocessor’

[Tiny Y]

Hi,
I built a class called  FracturePhaseFieldProblem<dim>   , which contains a 
subclass called  class Postprocessor，

template <int dim>

class FracturePhaseFieldProblem

{

    public:

…

    private:

  …

void output_results () const;

class Postprocessor;

…

};

However when I want to declare a variable as the type 
FracturePhaseFieldProblem<2>::Postprocessor in function output_results (),I 
got the error: 

error: cannot declare variable ‘postprocessor’ to be of abstract type 
‘FracturePhaseFieldProblem<2>::Postprocessor’

Thank you very much.

Best,

Yu


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