declarations in metafunctions

[MaxSagebaum]

New:
This merge request adds the new function 'add_declaration_to_parent_namespacein thecompiler_services`. This allows metafunctions to add declarations to the outer namespace of the current declaration.

One example of such a use case would be to create a make function:

//-----------------------------------------------------------------------
//
//  maker - add a make_<class name> function
//
maker: (inout t: meta::type_declaration) =
{
    t.add_declaration_to_parent_namespace("make_(t.name())$: (args...: _) -> _ = { return (t.name())$(args...); }");
}

The merge request depends on #506. One of the unique pointer declarations caused one of the corner cases in UFCS.

Old:
I wanted to play around a little bit with my own metafunctions. For my application I need to declare something outside of the structure, that "calls" the metafunction.

Patch 5a04b1d adds this functionality and 3ea209b provides an example implementation. It creates a make_<name> function like the ones for make_tuple.

I wanted to post this here so I can get some feedback on the implementation. It probably needs to be polished a little bit. One question would be if the member metafunction_declarations is the right approach?

[MaxSagebaum]

An example for my usecase is implemented in dfb1113. What we do is to implement some operators and add them to an expression framework. In order to that, we need to overload the original operator for the expression types. E.g. you have double * double and you want to have Expr * Expr. The overloads are always the same and we currently create them with super macros. That is we have *.tpp file which has no include guards and expects some preprocessor macros definitions. This could be:

#ifndef OPERATION_LOGIC
  #error Please define a name for the binary expression.
#endif
#ifndef FUNCTION
  #error Please define the primal function representation.
#endif

template<typename Real, typename ArgA, typename ArgB>
auto FUNCTION(
    ExpressionInterface<Real, ArgA> const& argA, ExpressionInterface<Real, ArgB> const& argB) {
  return BinaryExpression<Real, ArgA, ArgB, OPERATION_LOGIC>(argA, argB);
}

template<typename Real, typename ArgA>
auto FUNCTION(
    ExpressionInterface<Real, ArgA> const& argA, RealTraits::PassiveReal<Real> const& argB) {
  return BinaryExpression<Real, ArgA, ConstantExpression<RealTraits::PassiveReal<Real>>, OPERATION_LOGIC>(
      argA, ConstantExpression<RealTraits::PassiveReal<Real>>(argB));
}

template<typename Real, typename ArgB>
auto FUNCTION(
    RealTraits::PassiveReal<Real> const& argA, ExpressionInterface<Real, ArgB> const& argB) {
  return BinaryExpression<Real, ConstantExpression<RealTraits::PassiveReal<Real>>, ArgB, OPERATION_LOGIC>(
      ConstantExpression<RealTraits::PassiveReal<Real>>(argA), argB);
}

#undef FUNCTION
#undef OPERATION_LOGIC

For functions with more than 2 arguments this can get quite large, since we need 2^n - 1 combinations with n the number of arguments.

dfb1113 also just adds the variants for the binary expression. But it would be quite simple to write a general generate_expressions metafunction which creates the 2^n - 1 possibilities. One could even extend the functionality so that for certain arguments no expression version is generated or that the constant version is not generated.

[MaxSagebaum]

Thanks for the comments. If there is nothing else, I would remove my examples and squash the commits.

[MaxSagebaum]

Cleaned up the merge request and updated the first comment.

Removed WIP status.

[hsutter]

Hi! Sorry it took me so long to get to this one. I'm going to close it as probably-dated now, but if you want to pursue it please reopen and refresh. Thanks again, and sorry again for the lag.

[MaxSagebaum]

@hsutter This MR was integrated by @DyXel in #1261.