Should ParameterHandler::add_parameter() execute the action?

[peterrum]

As pointed out by @mschreter, the following program has an odd behavior:

#include <deal.II/base/parameter_handler.h>

using namespace dealii;

int
main()
{
  double a = std::numeric_limits<double>::lowest();

  std::cout << (a == std::numeric_limits<double>::lowest())
            << std::endl; // true

  ParameterHandler prm;
  prm.add_parameter("test", a);

  std::cout << (a == std::numeric_limits<double>::lowest())
            << std::endl; // false
}

The function ParameterHandler::add_parameter() overrides the old value with a new values plus minus a small rounding error.

The problem is that the created action (which serializes the double to a string and than back o double)

dealii/include/deal.II/base/parameter_handler.h

Lines 2336 to 2340 in 9c96714

	auto action = [&, pattern_index](const std::string &val) {
	parameter = Patterns::Tools::Convert<ParameterType>::to_value(
	val, *patterns[pattern_index]);
	};
	add_action(entry, action);

is executed during ParameterHandler::add_parameter()/ParameterHandler::add_action()

dealii/source/base/parameter_handler.cc

Lines 902 to 905 in 9c96714

	// as documented, run the action on the default value at the very end
	const std::string default_value = entries->get<std::string>(
	get_current_full_path(entry) + path_separator + "default_value");
	action(default_value);

We noticed that something is odd when we saw that parameters have been modified that should have not, since they were not listed in the parameter file.

Our suggestion would be to not executed the action when calling ParameterHandler::add_parameter(), i.e., to introduce an additional parameter to ParameterHandler::add_action(), which allows to disable the execution of the action.