blob: ddc524672d0bd024862e72a0931a48cf2f1455df (
plain)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
|
/*!
@file
Forward declares `boost::hana::is_empty`.
@copyright Louis Dionne 2013-2016
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_IS_EMPTY_HPP
#define BOOST_HANA_FWD_IS_EMPTY_HPP
#include <boost/hana/config.hpp>
#include <boost/hana/core/when.hpp>
BOOST_HANA_NAMESPACE_BEGIN
//! Returns whether the iterable is empty.
//! @ingroup group-Iterable
//!
//! Given an `Iterable` `xs`, `is_empty` returns whether `xs` contains
//! no more elements. In other words, it returns whether trying to
//! extract the tail of `xs` would be an error. In the current version
//! of the library, `is_empty` must return an `IntegralConstant` holding
//! a value convertible to `bool`. This is because only compile-time
//! `Iterable`s are supported right now.
//!
//!
//! Example
//! -------
//! @include example/is_empty.cpp
#ifdef BOOST_HANA_DOXYGEN_INVOKED
constexpr auto is_empty = [](auto const& xs) {
return tag-dispatched;
};
#else
template <typename It, typename = void>
struct is_empty_impl : is_empty_impl<It, when<true>> { };
struct is_empty_t {
template <typename Xs>
constexpr auto operator()(Xs const& xs) const;
};
constexpr is_empty_t is_empty{};
#endif
BOOST_HANA_NAMESPACE_END
#endif // !BOOST_HANA_FWD_IS_EMPTY_HPP
|