Understanding slightly cryptic warnings from modified Step-52 code

Krishnakumar Gopalakrishnan

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Jun 24, 2020, 5:14:10 PM6/24/20

to deal.II User Group

Hi,

I am solving the basic diffusion equation using the direct method (based on Step-52).

I have made the following changes from Step-52 tutorial

1. The domain is now 1D (as opposed to 2D in step-52)

2. We have Neumann BCs on both ends (left NeumannBC = 0.5, right NeumannBC= 0)

3. Removes the code for MMS validation (i.e. no source term S(t)) so that we solve for the unknown field variable, instead of verifying whether the pre-formulated analytical solution is retrieved)

4. Sets the absorpotion coefficient, Sigma_a = 0

5. Sets a non-zero initial condition (but a spatially constant value).

The C++ source code & Cmakelists.txt files are attached herewith. The solution is indeed correct and is as expected, and I have visualised this in Paraview. However, I'd like to understand the compilation warnings.

Some of them have to do with "unused variables", which are somewhat straightforward to get rid of. But the other warnings which all have the string "required from here" is not so clear to me.

Can someone please explain what these warnings mean, and what is the best practice for refactoring it to avoid such warnings.

Thanks!

Krishna

solid_diffusion.cc

CMakeLists.txt

Wolfgang Bangerth

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Jun 24, 2020, 6:54:23 PM6/24/20

to dea...@googlegroups.com, Krishnakumar Gopalakrishnan

On 6/24/20 3:14 PM, Krishnakumar Gopalakrishnan wrote:
> Hi,
>
> I am solving the basic diffusion equation using the direct method (based on
> Step-52).
>
> I have made the following changes from Step-52 tutorial
>
> 1. The domain is now 1D (as opposed to 2D in step-52)
> 2. We have Neumann BCs on both ends (left NeumannBC = 0.5, right NeumannBC= 0)
> 3. Removes the code for MMS validation (i.e. no source term S(t)) so that we
> solve for the unknown field variable, instead of verifying whether the
> pre-formulated analytical solution is retrieved)
> 4. Sets the absorpotion coefficient, Sigma_a = 0
> 5. Sets a non-zero initial condition (but a spatially constant value).
>
> The C++ source code & Cmakelists.txt files are attached herewith. The solution
> is indeed correct and is as expected, and I have visualised this in Paraview.
> However, I'd like to understand the compilation warnings.
>
> Some of them have to do with "unused variables", which are somewhat
> straightforward to get rid of. But the other warnings which all have the
> string "required from here" is not so clear to me.

Read the error/warning message in its entirety. For example, the first one says

/home/bangerth/p/deal.II/1/install/examples/step-1/step-1.cc: In instantiation
of ‘void DiffusionEqn::SolidDiffusion<dim>::run() [with int dim = 1]’:
/home/bangerth/p/deal.II/1/install/examples/step-1/step-1.cc:441:26:
required from here
/home/bangerth/p/deal.II/1/install/examples/step-1/step-1.cc:420:24: warning:
variable ‘n_steps’ set but not used [-Wunused-but-set-variable]
unsigned int n_steps = 0;
^~~~~~~

If you strip the details, it says:

In instantiation of function b() // i.e., while compiling a()
required from here // i.e., we got to a() while compiling b()
warning: variable unused

In other words, the compiler isn't just telling you where (in which function
and line of code) the problem happened, but also *why* it is compiling this
function (because it's called from some other place) and with which template
arguments.

Best
W.

>
> Can someone please explain what these warnings mean, and what is the best
> practice for refactoring it to avoid such warnings.
>
> Thanks!

> Krishna
>
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www: http://www.math.colostate.edu/~bangerth/

Krishnakumar Gopalakrishnan

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Jun 24, 2020, 7:04:40 PM6/24/20

to Wolfgang Bangerth, Deal. II Googlegroup

Dear Prof Bangerth,

Ah. That now was helpful. Thank you! A scrolling screenful of warnings which wrap across multiple lines is pretty intimidating for those not so comfortable with C++.

Hi all,

Is there a tutorial or general online resource that folks here would recommend for debugging?