stabilize build system: depends, installer, boost/bdb fixes, cross targets groundwork

2026-02-24 18:38:47 +00:00
parent da8c28aaeb
commit 65cb2619a7
13106 changed files with 2484322 additions and 1804 deletions
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+/*!
+@file
+Forward declares `boost::hana::Metafunction`.
+
+@copyright Louis Dionne 2013-2017
+Distributed under the Boost Software License, Version 1.0.
+(See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt)
+ */
+
+#ifndef BOOST_HANA_FWD_CONCEPT_METAFUNCTION_HPP
+#define BOOST_HANA_FWD_CONCEPT_METAFUNCTION_HPP
+
+#include <boost/hana/config.hpp>
+
+
+BOOST_HANA_NAMESPACE_BEGIN
+    //! @ingroup group-concepts
+    //! @defgroup group-Metafunction Metafunction
+    //! A `Metafunction` is a function that takes `hana::type`s as inputs and
+    //! returns a `hana::type` as output.
+    //!
+    //! A `Metafunction` is an object satisfying the [FunctionObject][1]
+    //! concept, but with additional requirements. First, it must be possible
+    //! to apply a `Metafunction` to arguments whose tag is `type_tag`, and
+    //! the result of such an application must be an object whose tag is also
+    //! `type_tag`. Note that `hana::type` and `hana::basic_type` are the
+    //! only such types.
+    //!
+    //! Secondly, a `Metafunction` must provide a nested `::%apply` template
+    //! which allows performing the same type-level computation as is done by
+    //! the call operator. In Boost.MPL parlance, a `Metafunction` `F` is
+    //! hence a [MetafunctionClass][2] in addition to being a `FunctionObject`.
+    //! Rigorously, the following must be satisfied by any object `f` of type
+    //! `F` which is a `Metafunction`, and for arbitrary types `T...`:
+    //! @code
+    //!     f(hana::type_c<T>...) == hana::type_c<F::apply<T...>::type>
+    //! @endcode
+    //!
+    //! Thirdly, to ease the inter-operation of values and types,
+    //! `Metafunction`s must also allow being called with arguments that
+    //! are not `hana::type`s. In that case, the result is equivalent to
+    //! calling the metafunction on the types of the arguments. Rigorously,
+    //! this means that for arbitrary objects `x...`,
+    //! @code
+    //!     f(x...) == f(hana::type_c<decltype(x)>...)
+    //! @endcode
+    //!
+    //!
+    //! Minimal complete definition
+    //! ---------------------------
+    //! The `Metafunction` concept does not have a minimal complete definition
+    //! in terms of tag-dispatched methods. Instead, the syntactic requirements
+    //! documented above should be satisfied, and the `Metafunction` struct
+    //! should be specialized explicitly in Hana's namespace.
+    //!
+    //!
+    //! Concrete models
+    //! ---------------
+    //! `hana::metafunction`, `hana::metafunction_class`, `hana::template_`
+    //!
+    //!
+    //! Rationale: Why aren't `Metafunction`s `Comparable`?
+    //! ---------------------------------------------------
+    //! When seeing `hana::template_`, a question that naturally arises is
+    //! whether `Metafunction`s should be made `Comparable`. Indeed, it
+    //! would seem to make sense to compare two templates `F` and `G` with
+    //! `template_<F> == template_<G>`. However, in the case where `F` and/or
+    //! `G` are alias templates, it makes sense to talk about two types of
+    //! comparisons. The first one is _shallow_ comparison, and it determines
+    //! that two alias templates are equal if they are the same alias
+    //! template. The second one is _deep_ comparison, and it determines
+    //! that two template aliases are equal if they alias the same type for
+    //! any template argument. For example, given `F` and `G` defined as
+    //! @code
+    //!     template <typename T>
+    //!     using F = void;
+    //!
+    //!     template <typename T>
+    //!     using G = void;
+    //! @endcode
+    //!
+    //! shallow comparison would determine that `F` and `G` are different
+    //! because they are two different template aliases, while deep comparison
+    //! would determine that `F` and `G` are equal because they always
+    //! expand to the same type, `void`. Unfortunately, deep comparison is
+    //! impossible to implement because one would have to check `F` and `G`
+    //! on all possible types. On the other hand, shallow comparison is not
+    //! satisfactory because `Metafunction`s are nothing but functions on
+    //! `type`s, and the equality of two functions is normally defined with
+    //! deep comparison. Hence, we adopt a conservative stance and avoid
+    //! providing comparison for `Metafunction`s.
+    //!
+    //! [1]: http://en.cppreference.com/w/cpp/concept/FunctionObject
+    //! [2]: http://www.boost.org/doc/libs/release/libs/mpl/doc/refmanual/metafunction-class.html
+    template <typename F>
+    struct Metafunction;
+BOOST_HANA_NAMESPACE_END
+
+#endif // !BOOST_HANA_FWD_CONCEPT_METAFUNCTION_HPP