Ichthyostega
ed7e3b4b32
Complete the investigation and turn the solution into a generic mix-in-template, which can be used in flexible ways to support this qualifier notation. Moreover, recapitulate requirements for the ElementBoxWidget
99 lines
4 KiB
C++
99 lines
4 KiB
C++
/*
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BUILDER-QUALIFIER-SUPPORT.hpp - accept arbitrary qualifier terms for builder functions
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Copyright (C) Lumiera.org
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2022, Hermann Vosseler <Ichthyostega@web.de>
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This program is free software; you can redistribute it and/or
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modify it under the terms of the GNU General Public License as
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published by the Free Software Foundation; either version 2 of
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the License, or (at your option) any later version.
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This program is distributed in the hope that it will be useful,
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but WITHOUT ANY WARRANTY; without even the implied warranty of
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MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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GNU General Public License for more details.
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You should have received a copy of the GNU General Public License
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along with this program; if not, write to the Free Software
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Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.
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*/
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/** @file builder-qualifier-support.hpp
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** Mix-in to support builder functions to accept optional qualifier terms.
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** For the classical _builder pattern_ we define a dedicated *builder* class,
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** which in turn offers explicitly named setter or configuration functions.
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** However, sometimes it is desirable just to offer some builder-like extension
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** on regular constructor syntax, where optional additional parameters may be passed
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** to adapt the basic setup. Unfortunately, a list of optional further argument tends
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** to become hard to read, especially when some dummy arguments must be passed in order
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** to supply a setting further down the list.
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**
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** This mix-in template instead allows to define _qualifier terms,_ and to pass an arbitrary
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** sequence of such terms to a constructor or builder function. These qualifiers look like
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** "algebraic data types" at invocation site, i.e. they are named terms, optionally even accepting
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** further arguments. Yet for the implementation, these qualifiers return a wrapped functor. When
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** defined as _friend function,_ the generated qualifier functors may even manipulate private internals
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** of the "Builder" or "Strategy" class.
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** @warning since these qualifiers manipulate by side-effect, it is up to the user to ensure
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** consistent settings and behaviour (typically within a terminal `build()` function).
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** @note the `qualify()` function accepts the qualifier functors by-reference; thus it is up
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** to the user to either pass them by-value, by reverence or even by perfect forwarding.
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**
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** @see BuilderQualifierSupport_test
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** @see ElementBoxWidget as usage example
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*/
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#ifndef LIB_BUILDER_QUALIFIER_SUPPORT_H
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#define LIB_BUILDER_QUALIFIER_SUPPORT_H
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#include <functional>
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namespace lib {
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/**
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* Mix-in to accept and apply an arbitrary sequence of qualifier functors.
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* Typically these are used for Builder or Strategy definitions and applied
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* to some target object, which thereby is manipulated by the qualifiers by
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* side-effect.
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* @tparam TAR target type to apply the qualifier functors
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* @remark the intended usage is to mix-in this template privately and then
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* to define _qualifier friend functions,_ each binding a suitable lambda
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* to manipulate the internal settings. This way, at usage site, an arbitrary
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* sequence of clearly labelled "algebraic terms" can be accepted, to tweak
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* and adapt the basic features established on the Builder or Strategy.
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*/
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template<class TAR>
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class BuilderQualifierSupport
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{
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protected:
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using Manipulator = std::function<void(TAR&)>;
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struct Qualifier
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: Manipulator
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{
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using Manipulator::Manipulator;
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};
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/** Main entrance point: apply the given qualifiers in sequence to the `target`. */
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template<class... QUALS>
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friend void qualify(TAR& target, Qualifier& qualifier, QUALS& ...qs)
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{
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qualifier(target);
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qualify(target, qs...);
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}
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friend void qualify(TAR&){ }
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public:
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// default construct and copyable
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};
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}// namespace lib
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#endif /*LIB_BUILDER_QUALIFIER_SUPPORT_H*/
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