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

[Athul Shaji]

Hello, if you are using the PostProcessor subclass within output_results(), then is not required to use FracturePhaseFieldProblem<2>::Postprocessor as the class and the function are in the same main class FracturePhaseFieldProblem.

I think this can be the problem.

Regards,

Athul

On Wed, Jan 31, 2024 at 7:16 AM Tiny Y <yuyi...@163.com> wrote:

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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[Wolfgang Bangerth]

Yu:

> classFracturePhaseFieldProblem
>
> {

>
>     public:
>
> …
>
>     private:
>
> …
>
> void output_results () const;
>
> class Postprocessor;

This is what's called a "forward declaration". The line really just says
"There is a class Postprocessor" but because you don't provide the
details in a {...} block, the compiler knows nothing about it.

>
> …
>
> };

>
> However when I want to declare a variable as the type
> FracturePhaseFieldProblem<2>::Postprocessor in functionoutput_results
> (),I got the error:
>
> error: cannot declare variable ‘postprocessor’ to be of abstract type
> ‘FracturePhaseFieldProblem<2>::Postprocessor’

And here the compiler is telling you "You're trying to create a variable
of this type. I know is that the class *exists*, but in order to create
a variable of this type, I need to know *what the class does* and what
its member variables are".

Best
W.