FM / fwd

/usr/include/boost/hana/fwd

Name	Size	Mode	Actions
concept/	-	0755	rm
core/	-	0755	rm
accessors.hpp	1519	0644	edit dl rm
adapt_adt.hpp	1596	0644	edit dl rm
adapt_struct.hpp	1586	0644	edit dl rm
adjust.hpp	1787	0644	edit dl rm
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all.hpp	1185	0644	edit dl rm
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any.hpp	1178	0644	edit dl rm
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basic_tuple.hpp	2292	0644	edit dl rm
bool.hpp	384	0644	edit dl rm
cartesian_product.hpp	1837	0644	edit dl rm
chain.hpp	1964	0644	edit dl rm
comparing.hpp	1924	0644	edit dl rm
concat.hpp	1838	0644	edit dl rm
contains.hpp	2378	0644	edit dl rm
core.hpp	619	0644	edit dl rm
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define_struct.hpp	1582	0644	edit dl rm
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integral_constant.hpp	4791	0644	edit dl rm
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intersperse.hpp	1780	0644	edit dl rm
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lift.hpp	1686	0644	edit dl rm
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Edit: /usr/include/boost/hana/fwd/type.hpp (19848B)

/*!
@file
Forward declares `boost::hana::type` and related utilities.

@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_TYPE_HPP
#define BOOST_HANA_FWD_TYPE_HPP

#include <boost/hana/config.hpp>
#include <boost/hana/fwd/core/make.hpp>

BOOST_HANA_NAMESPACE_BEGIN
    //! Base class of `hana::type`; used for pattern-matching.
    //! @relates hana::type
    //!
    //! Example
    //! -------
    //! @include example/type/basic_type.cpp
    template <typename T>
    struct basic_type;

//! @ingroup group-datatypes
    //! C++ type in value-level representation.
    //!
    //! A `type` is a special kind of object representing a C++ type like
    //! `int`, `void`, `std::vector<float>` or anything else you can imagine.
    //!
    //! This page explains how `type`s work at a low level. To gain
    //! intuition about type-level metaprogramming in Hana, you should
    //! read the [tutorial section](@ref tutorial-type) on type-level
    //! computations.
    //!
    //!
    //! @note
    //! For subtle reasons, the actual representation of `hana::type` is
    //! implementation-defined. In particular, `hana::type` may be a dependent
    //! type, so one should not attempt to do pattern matching on it. However,
    //! one can assume that `hana::type` _inherits_ from `hana::basic_type`,
    //! which can be useful when declaring overloaded functions:
    //! @code
    //!     template <typename T>
    //!     void f(hana::basic_type<T>) {
    //!         // do something with T
    //!     }
    //! @endcode
    //! The full story is that [ADL][] causes template arguments to be
    //! instantiated. Hence, if `hana::type` were defined naively, expressions
    //! like `hana::type<T>{} == hana::type<U>{}` would cause both `T` and `U`
    //! to be instantiated. This is usually not a problem, except when `T` or
    //! `U` should not be instantiated. To avoid these instantiations,
    //! `hana::type` is implemented using some cleverness, and that is
    //! why the representation is implementation-defined. When that
    //! behavior is not required, `hana::basic_type` can be used instead.
    //!
    //!
    //! @anchor type_lvalues_and_rvalues
    //! Lvalues and rvalues
    //! -------------------
    //! When storing `type`s in heterogeneous containers, some algorithms
    //! will return references to those objects. Since we are primarily
    //! interested in accessing their nested `::%type`, receiving a reference
    //! is undesirable; we would end up trying to fetch the nested `::%type`
    //! inside a reference type, which is a compilation error:
    //! @code
    //!   auto ts = make_tuple(type_c<int>, type_c<char>);
    //!   using T = decltype(ts[0_c])::type; // error: 'ts[0_c]' is a reference!
    //! @endcode
    //!
    //! For this reason, `type`s provide an overload of the unary `+`
    //! operator that can be used to turn a lvalue into a rvalue. So when
    //! using a result which might be a reference to a `type` object, one
    //! can use `+` to make sure a rvalue is obtained before fetching its
    //! nested `::%type`:
    //! @code
    //!   auto ts = make_tuple(type_c<int>, type_c<char>);
    //!   using T = decltype(+ts[0_c])::type; // ok: '+ts[0_c]' is an rvalue
    //! @endcode
    //!
    //!
    //! Modeled concepts
    //! ----------------
    //! 1. `Comparable`\n
    //! Two types are equal if and only if they represent the same C++ type.
    //! Hence, equality is equivalent to the `std::is_same` type trait.
    //! @include example/type/comparable.cpp
    //!
    //! 2. `Hashable`\n
    //! The hash of a type is just that type itself. In other words, `hash`
    //! is the identity function on `hana::type`s.
    //! @include example/type/hashable.cpp
    //!
    //! [ADL]: http://en.cppreference.com/w/cpp/language/adl
#ifdef BOOST_HANA_DOXYGEN_INVOKED
    template <typename T>
    struct type {
        //! Returns rvalue of self.
        //! See @ref type_lvalues_and_rvalues "description".
        constexpr auto operator+() const;

//! Equivalent to `hana::equal`
        template <typename X, typename Y>
        friend constexpr auto operator==(X&& x, Y&& y);

//! Equivalent to `hana::not_equal`
        template <typename X, typename Y>
        friend constexpr auto operator!=(X&& x, Y&& y);
    };
#else
    template <typename T>
    struct type_impl;

template <typename T>
    using type = typename type_impl<T>::_;
#endif

//! Tag representing `hana::type`.
    //! @relates hana::type
    struct type_tag { };

//! Creates an object representing the C++ type `T`.
    //! @relates hana::type
    template <typename T>
    constexpr type<T> type_c{};

//! `decltype` keyword, lifted to Hana.
    //! @relates hana::type
    //!
    //! @deprecated
    //! The semantics of `decltype_` can be confusing, and `hana::typeid_`
    //! should be preferred instead. `decltype_` may be removed in the next
    //! major version of the library.
    //!
    //! `decltype_` is somewhat equivalent to `decltype` in that it returns
    //! the type of an object, except it returns it as a `hana::type` which
    //! is a first-class citizen of Hana instead of a raw C++ type.
    //! Specifically, given an object `x`, `decltype_` satisfies
    //! @code
    //!   decltype_(x) == type_c<decltype(x) with references stripped>
    //! @endcode
    //!
    //! As you can see, `decltype_` will strip any reference from the
    //! object's actual type. The reason for doing so is explained below.
    //! However, any `cv`-qualifiers will be retained. Also, when given a
    //! `hana::type`, `decltype_` is just the identity function. Hence,
    //! for any C++ type `T`,
    //! @code
    //!   decltype_(type_c<T>) == type_c<T>
    //! @endcode
    //!
    //! In conjunction with the way `metafunction` & al. are specified, this
    //! behavior makes it easier to interact with both types and values at
    //! the same time. However, it does make it impossible to create a `type`
    //! containing another `type` with `decltype_`. In other words, it is
    //! not possible to create a `type_c<decltype(type_c<T>)>` with this
    //! utility, because `decltype_(type_c<T>)` would be just `type_c<T>`
    //! instead of `type_c<decltype(type_c<T>)>`. This use case is assumed
    //! to be rare and a hand-coded function can be used if this is needed.
    //!
    //!
    //! ### Rationale for stripping the references
    //! The rules for template argument deduction are such that a perfect
    //! solution that always matches `decltype` is impossible. Hence, we
    //! have to settle on a solution that's good and and consistent enough
    //! for our needs. One case where matching `decltype`'s behavior is
    //! impossible is when the argument is a plain, unparenthesized variable
    //! or function parameter. In that case, `decltype_`'s argument will be
    //! deduced as a reference to that variable, but `decltype` would have
    //! given us the actual type of that variable, without references. Also,
    //! given the current definition of `metafunction` & al., it would be
    //! mostly useless if `decltype_` could return a reference, because it
    //! is unlikely that `F` expects a reference in its simplest use case:
    //! @code
    //!   int i = 0;
    //!   auto result = metafunction<F>(i);
    //! @endcode
    //!
    //! Hence, always discarding references seems to be the least painful
    //! solution.
    //!
    //!
    //! Example
    //! -------
    //! @include example/type/decltype.cpp
#ifdef BOOST_HANA_DOXYGEN_INVOKED
    constexpr auto decltype_ = see documentation;
#else
    struct decltype_t {
        template <typename T>
        constexpr auto operator()(T&&) const;
    };

constexpr decltype_t decltype_{};
#endif

//! Returns a `hana::type` representing the type of a given object.
    //! @relates hana::type
    //!
    //! `hana::typeid_` is somewhat similar to `typeid` in that it returns
    //! something that represents the type of an object. However, what
    //! `typeid` returns represent the _runtime_ type of the object, while
    //! `hana::typeid_` returns the _static_ type of the object. Specifically,
    //! given an object `x`, `typeid_` satisfies
    //! @code
    //!   typeid_(x) == type_c<decltype(x) with ref and cv-qualifiers stripped>
    //! @endcode
    //!
    //! As you can see, `typeid_` strips any reference and cv-qualifier from
    //! the object's actual type. The reason for doing so is that it faithfully
    //! models how the language's `typeid` behaves with respect to reference
    //! and cv-qualifiers, and it also turns out to be the desirable behavior
    //! most of the time. Also, when given a `hana::type`, `typeid_` is just
    //! the identity function. Hence, for any C++ type `T`,
    //! @code
    //!   typeid_(type_c<T>) == type_c<T>
    //! @endcode
    //!
    //! In conjunction with the way `metafunction` & al. are specified, this
    //! behavior makes it easier to interact with both types and values at
    //! the same time. However, it does make it impossible to create a `type`
    //! containing another `type` using `typeid_`. This use case is assumed
    //! to be rare and a hand-coded function can be used if this is needed.
    //!
    //!
    //! Example
    //! -------
    //! @include example/type/typeid.cpp
#ifdef BOOST_HANA_DOXYGEN_INVOKED
    constexpr auto typeid_ = see documentation;
#else
    struct typeid_t {
        template <typename T>
        constexpr auto operator()(T&&) const;
    };

constexpr typeid_t typeid_{};
#endif

#ifdef BOOST_HANA_DOXYGEN_INVOKED
    //! Equivalent to `decltype_`, provided for convenience.
    //! @relates hana::type
    //!
    //!
    //! Example
    //! -------
    //! @include example/type/make.cpp
    template <>
    constexpr auto make<type_tag> = hana::decltype_;
#endif

//! Equivalent to `make<type_tag>`, provided for convenience.
    //! @relates hana::type
    //!
    //!
    //! Example
    //! -------
    //! @include example/type/make.cpp
    constexpr auto make_type = hana::make<type_tag>;

//! `sizeof` keyword, lifted to Hana.
    //! @relates hana::type
    //!
    //! `sizeof_` is somewhat equivalent to `sizeof` in that it returns the
    //! size of an expression or type, but it takes an arbitrary expression
    //! or a `hana::type` and returns its size as an `integral_constant`.
    //! Specifically, given an expression `expr`, `sizeof_` satisfies
    //! @code
    //!   sizeof_(expr) == size_t<sizeof(decltype(expr) with references stripped)>
    //! @endcode
    //!
    //! However, given a `type`, `sizeof_` will simply fetch the size
    //! of the C++ type represented by that object. In other words,
    //! @code
    //!   sizeof_(type_c<T>) == size_t<sizeof(T)>
    //! @endcode
    //!
    //! The behavior of `sizeof_` is consistent with that of `decltype_`.
    //! In particular, see `decltype_`'s documentation to understand why
    //! references are always stripped by `sizeof_`.
    //!
    //!
    //! Example
    //! -------
    //! @include example/type/sizeof.cpp
#ifdef BOOST_HANA_DOXYGEN_INVOKED
    constexpr auto sizeof_ = [](auto&& x) {
        using T = typename decltype(hana::decltype_(x))::type;
        return hana::size_c<sizeof(T)>;
    };
#else
    struct sizeof_t {
        template <typename T>
        constexpr auto operator()(T&&) const;
    };

constexpr sizeof_t sizeof_{};
#endif

//! `alignof` keyword, lifted to Hana.
    //! @relates hana::type
    //!
    //! `alignof_` is somewhat equivalent to `alignof` in that it returns the
    //! alignment required by any instance of a type, but it takes a `type`
    //! and returns its alignment as an `integral_constant`. Like `sizeof`
    //! which works for expressions and type-ids, `alignof_` can also be
    //! called on an arbitrary expression. Specifically, given an expression
    //! `expr` and a C++ type `T`, `alignof_` satisfies
    //! @code
    //!   alignof_(expr) == size_t<alignof(decltype(expr) with references stripped)>
    //!   alignof_(type_c<T>) == size_t<alignof(T)>
    //! @endcode
    //!
    //! The behavior of `alignof_` is consistent with that of `decltype_`.
    //! In particular, see `decltype_`'s documentation to understand why
    //! references are always stripped by `alignof_`.
    //!
    //!
    //! Example
    //! -------
    //! @include example/type/alignof.cpp
#ifdef BOOST_HANA_DOXYGEN_INVOKED
    constexpr auto alignof_ = [](auto&& x) {
        using T = typename decltype(hana::decltype_(x))::type;
        return hana::size_c<alignof(T)>;
    };
#else
    struct alignof_t {
        template <typename T>
        constexpr auto operator()(T&&) const;
    };

constexpr alignof_t alignof_{};
#endif

//! Checks whether a SFINAE-friendly expression is valid.
    //! @relates hana::type
    //!
    //! Given a SFINAE-friendly function, `is_valid` returns whether the
    //! function call is valid with the given arguments. Specifically, given
    //! a function `f` and arguments `args...`,
    //! @code
    //!   is_valid(f, args...) == whether f(args...) is valid
    //! @endcode
    //!
    //! The result is returned as a compile-time `Logical`. Furthermore,
    //! `is_valid` can be used in curried form as follows:
    //! @code
    //!   is_valid(f)(args...)
    //! @endcode
    //!
    //! This syntax makes it easy to create functions that check the validity
    //! of a generic expression on any given argument(s).
    //!
    //! @warning
    //! To check whether calling a nullary function `f` is valid, one should
    //! use the `is_valid(f)()` syntax. Indeed, `is_valid(f /* no args */)`
    //! will be interpreted as the currying of `is_valid` to `f` rather than
    //! the application of `is_valid` to `f` and no arguments.
    //!
    //!
    //! Example
    //! -------
    //! @include example/type/is_valid.cpp
#ifdef BOOST_HANA_DOXYGEN_INVOKED
    constexpr auto is_valid = [](auto&& f) {
        return [](auto&& ...args) {
            return whether f(args...) is a valid expression;
        };
    };
#else
    struct is_valid_t {
        template <typename F>
        constexpr auto operator()(F&&) const;

template <typename F, typename ...Args>
        constexpr auto operator()(F&&, Args&&...) const;
    };

constexpr is_valid_t is_valid{};
#endif

//! Lift a template to a Metafunction.
    //! @ingroup group-Metafunction
    //!
    //! Given a template class or template alias `f`, `template_<f>` is a
    //! `Metafunction` satisfying
    //! @code
    //!     template_<f>(type_c<x>...) == type_c<f<x...>>
    //!     decltype(template_<f>)::apply<x...>::type == f<x...>
    //! @endcode
    //!
    //! @note
    //! In a SFINAE context, the expression `template_<f>(type_c<x>...)` is
    //! valid whenever the expression `f<x...>` is valid.
    //!
    //!
    //! Example
    //! -------
    //! @include example/type/template.cpp
#ifdef BOOST_HANA_DOXYGEN_INVOKED
    template <template <typename ...> class F>
    constexpr auto template_ = [](basic_type<T>...) {
        return hana::type_c<F<T...>>;
    };
#else
    template <template <typename ...> class F>
    struct template_t;

template <template <typename ...> class F>
    constexpr template_t<F> template_{};
#endif

//! Lift a MPL-style metafunction to a Metafunction.
    //! @ingroup group-Metafunction
    //!
    //! Given a MPL-style metafunction, `metafunction<f>` is a `Metafunction`
    //! satisfying
    //! @code
    //!     metafunction<f>(type_c<x>...) == type_c<f<x...>::type>
    //!     decltype(metafunction<f>)::apply<x...>::type == f<x...>::type
    //! @endcode
    //!
    //! @note
    //! In a SFINAE context, the expression `metafunction<f>(type_c<x>...)` is
    //! valid whenever the expression `f<x...>::%type` is valid.
    //!
    //!
    //! Example
    //! -------
    //! @include example/type/metafunction.cpp
#ifdef BOOST_HANA_DOXYGEN_INVOKED
    template <template <typename ...> class F>
    constexpr auto metafunction = [](basic_type<T>...) {
        return hana::type_c<typename F<T...>::type>;
    };
#else
    template <template <typename ...> class f>
    struct metafunction_t;

template <template <typename ...> class f>
    constexpr metafunction_t<f> metafunction{};
#endif

//! Lift a MPL-style metafunction class to a Metafunction.
    //! @ingroup group-Metafunction
    //!
    //! Given a MPL-style metafunction class, `metafunction_class<f>` is a
    //! `Metafunction` satisfying
    //! @code
    //!     metafunction_class<f>(type_c<x>...) == type_c<f::apply<x...>::type>
    //!     decltype(metafunction_class<f>)::apply<x...>::type == f::apply<x...>::type
    //! @endcode
    //!
    //! @note
    //! In a SFINAE context, the expression `metafunction_class<f>(type_c<x>...)`
    //! is valid whenever the expression `f::apply<x...>::%type` is valid.
    //!
    //!
    //! Example
    //! -------
    //! @include example/type/metafunction_class.cpp
#ifdef BOOST_HANA_DOXYGEN_INVOKED
    template <typename F>
    constexpr auto metafunction_class = [](basic_type<T>...) {
        return hana::type_c<typename F::template apply<T...>::type>;
    };
#else
    template <typename F>
    struct metafunction_class_t;

template <typename F>
    constexpr metafunction_class_t<F> metafunction_class{};
#endif

//! Turn a `Metafunction` into a function taking `type`s and returning a
    //! default-constructed object.
    //! @ingroup group-Metafunction
    //!
    //! Given a `Metafunction` `f`, `integral` returns a new `Metafunction`
    //! that default-constructs an object of the type returned by `f`. More
    //! specifically, the following holds:
    //! @code
    //!     integral(f)(t...) == decltype(f(t...))::type{}
    //! @endcode
    //!
    //! The principal use case for `integral` is to transform `Metafunction`s
    //! returning a type that inherits from a meaningful base like
    //! `std::integral_constant` into functions returning e.g. a
    //! `hana::integral_constant`.
    //!
    //! @note
    //! - This is not a `Metafunction` because it does not return a `type`.
    //!   As such, it would not make sense to make `decltype(integral(f))`
    //!   a MPL metafunction class like the usual `Metafunction`s are.
    //!
    //! - When using `integral` with metafunctions returning
    //!   `std::integral_constant`s, don't forget to include the
    //!   boost/hana/ext/std/integral_constant.hpp header to ensure
    //!   Hana can interoperate with the result.
    //!
    //! - In a SFINAE context, the expression `integral(f)(t...)` is valid
    //!   whenever the expression `decltype(f(t...))::%type` is valid.
    //!
    //!
    //! Example
    //! -------
    //! @include example/type/integral.cpp
#ifdef BOOST_HANA_DOXYGEN_INVOKED
    constexpr auto integral = [](auto f) {
        return [](basic_type<T>...) {
            return decltype(f)::apply<T...>::type{};
        };
    };
#else
    template <typename F>
    struct integral_t;

struct make_integral_t {
        template <typename F>
        constexpr integral_t<F> operator()(F const&) const
        { return {}; }
    };

constexpr make_integral_t integral{};
#endif

//! Alias to `integral(metafunction<F>)`, provided for convenience.
    //! @ingroup group-Metafunction
    //!
    //!
    //! Example
    //! -------
    //! @include example/type/trait.cpp
    template <template <typename ...> class F>
    constexpr auto trait = hana::integral(hana::metafunction<F>);
BOOST_HANA_NAMESPACE_END

#endif // !BOOST_HANA_FWD_TYPE_HPP

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