Compiler warining message related to define block_operator

[Tao Jin]

Dear all,

In my dealii code hosted on the GitHub:

https://github.com/taojinllnl/Phasefield_gradient_projection_monolithic_solver

I have the following line of code defining a block_operator in line 5378:

const auto op_zT_wMwT_z = 

      block_operator<2, 2, BlockVector<double>>({op_uMuT, op_uMdT,
     op_dMuT, op_dMdT});

My code could compile and run as expected. However, I always get the following warning message:

"""

/home/taojin/Dropbox/dealII_Code/PhaseField/L-BFGS-B/main.cc:5378:70: warning: missing braces around initializer for ‘std::__array_traits<std::array<dealii::LinearOperator<dealii::Vector<double>, dealii::Vector<double>, dealii::internal::LinearOperatorImplementation::EmptyPayload>, 2>, 2>::_Type’ {aka ‘std::array<dealii::LinearOperator<dealii::Vector<double>, dealii::Vector<double>, dealii::internal::LinearOperatorImplementation::EmptyPayload>, 2> [2]’} [-Wmissing-braces]

 5378 |   const auto op_zT_wMwT_z = block_operator<2, 2, BlockVector<double>>({op_uMuT, op_uMdT,
      |                             ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~
 5379 |                op_dMuT, op_dMdT});

/home/taojin/Dropbox/dealII_Code/PhaseField/L-BFGS-B/main.cc:5378:70: warning: missing braces around initializer for ‘std::array<dealii::LinearOperator<dealii::Vector<double>, dealii::Vector<double>, dealii::internal::LinearOperatorImplementation::EmptyPayload>, 2>’ [-Wmissing-braces]

"""

I tried to place some extra curly braces but the warining message is still there.

Does anyone have any idea how to get rid of the warning message?

Thanks,

Tao

[Wolfgang Bangerth]

On 3/24/26 19:27, Tao Jin wrote:
>
> I tried to place some extra curly braces but the warining message is still there.
>
> Does anyone have any idea how to get rid of the warning message?

Sometimes it takes two more levels of braces. Alternatively, look up what the
type of the constructor argument is and construct that explicitly, rather than
in-place.

Best
Wolfgang

[Tao Jin]

Hi Wolfgang,

Thanks for your reply. I dug into the dealii source code and here is what I found.

Below is the instruction for the constructor

/**
 * @relatesalso BlockLinearOperator
 *
 * A variant of above function that encapsulates a given collection @p ops of
 * LinearOperators into a block structure. Here, it is assumed that Range and
 * Domain are block vectors, i.e., derived from
 * @ref BlockVectorBase.
 * The individual linear operators in @p ops must act on the underlying vector
 * type of the block vectors, i.e., on Domain::BlockType yielding a result in
 * Range::BlockType.
 *
 * The list @p ops is best passed as an initializer list. Consider for example
 * a linear operator block (acting on Vector<double>)
 * @code
 *  op_a00 | op_a01
 *         |
 *  ---------------
 *         |
 *  op_a10 | op_a11
 * @endcode
 * The corresponding block_operator invocation takes the form
 * @code
 * block_operator<2, 2, BlockVector<double>>({op_a00, op_a01, op_a10, op_a11});
 * @endcode
 *
 * @ingroup LAOperators
 */

I followed the above instruction but still got the same warning message during compilation.

Also, in the source code (block_linear_operator.h)
https://dealii.org/current/doxygen/deal.II/block__linear__operator_8h_source.html

On line 665 there is a TODO tag:

// TODO: Create block payload so that this can be initialized correctly

BlockLinearOperator<Range, Domain, BlockPayload> return_op{BlockPayload()};

I wonder whether this is the reason for the warning message. (Sorry that my C++ skill is not good enough to fix this myself since the block_linear_operator seems to be a quite complex class).

Best,

Tao

[Tao Jin]

I found the following way that makes the warning message [-Wmissing-braces] disappear.

Instead of using the instruction provided in the block_linear_operator.h

 block_operator<2, 2, BlockVector<double>>({op_a00, op_a01, op_a10, op_a11});

We can declare the block_operator the way below:

const std::array<std::array<dealii::LinearOperator<dealii::Vector<double>,

                                                   dealii::Vector<double>,
   dealii::internal::LinearOperatorImplementation::EmptyPayload>,
   2>, 2>

ops = {{
                    {{ op_uMuT, op_uMdT }},
    {{ op_dMuT, op_dMdT }}
   }};

const auto op_zT_wMwT_z = block_operator<2, 2, BlockVector<double>>(ops);

This way, the warning message will disappear during the compilation.

Best,

Tao