Imported Upstream version 1.67.0upstream/1.67.0

1 files changed, 90 insertions, 0 deletions

diff --git a/boost/hof/function.hpp b/boost/hof/function.hpp
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+/*=============================================================================
+    Copyright (c) 2014 Paul Fultz II
+    function.h
+    Distributed under the Boost Software License, Version 1.0. (See accompanying
+    file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
+==============================================================================*/
+
+#ifndef BOOST_HOF_GUARD_FUNCTION_FUNCTION_H
+#define BOOST_HOF_GUARD_FUNCTION_FUNCTION_H
+
+/// BOOST_HOF_STATIC_FUNCTION
+/// ===================
+/// 
+/// Description
+/// -----------
+/// 
+
+/// The `BOOST_HOF_STATIC_FUNCTION` macro allows initializing a function object from a
+/// `constexpr` expression. It uses the best practices as outlined in
+/// [N4381](http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2015/n4381.html).
+/// This includes using `const` to avoid global state, compile-time
+/// initialization of the function object to avoid the [static initialization
+/// order fiasco](https://isocpp.org/wiki/faq/ctors#static-init-order), and an
+/// external address of the function object that is the same across translation
+/// units to avoid possible One-Definition-Rule(ODR) violations.
+/// 
+/// In C++17, this achieved using the `inline` keyword. However, on older
+/// compilers it is initialized using a reference to a static member variable.
+/// The static member variable is default constructed, as such the user variable
+/// is always default constructed regardless of the expression.
+/// 
+/// By default, all functions defined with `BOOST_HOF_STATIC_FUNCTION` use the
+/// [`boost::hof::reveal`](/include/boost/hof/reveal) adaptor to improve error messages.
+/// 
+/// Example
+/// -------
+/// 
+///     #include <boost/hof.hpp>
+///     #include <cassert>
+/// 
+///     struct sum_f
+///     {
+///         template<class T, class U>
+///         T operator()(T x, U y) const
+///         {
+///             return x+y;
+///         }
+///     };
+/// 
+///     BOOST_HOF_STATIC_FUNCTION(sum) = sum_f();
+///     BOOST_HOF_STATIC_FUNCTION(partial_sum) = boost::hof::partial(sum_f());
+/// 
+///     int main() {
+///         assert(sum(1, 2) == partial_sum(1)(2));
+///     }
+/// 
+
+#include <boost/hof/reveal.hpp>
+#if !BOOST_HOF_HAS_INLINE_VARIABLES
+#include <boost/hof/detail/static_const_var.hpp>
+#include <boost/hof/detail/constexpr_deduce.hpp>
+#endif
+
+namespace boost { namespace hof {
+
+namespace detail {
+
+struct reveal_static_const_factory
+{
+    constexpr reveal_static_const_factory()
+    {}
+    template<class F>
+    constexpr reveal_adaptor<F> operator=(const F& f) const
+    {
+#if BOOST_HOF_HAS_INLINE_VARIABLES
+#else
+        static_assert(BOOST_HOF_IS_DEFAULT_CONSTRUCTIBLE(F), "Static functions must be default constructible");
+#endif
+        return reveal_adaptor<F>(f);
+    }
+};
+}}} // namespace boost::hof
+
+#if BOOST_HOF_HAS_INLINE_VARIABLES
+#define BOOST_HOF_STATIC_FUNCTION(name) inline const constexpr auto name = boost::hof::detail::reveal_static_const_factory()
+#else
+#define BOOST_HOF_STATIC_FUNCTION(name) BOOST_HOF_STATIC_CONST_VAR(name) = BOOST_HOF_DETAIL_MSVC_CONSTEXPR_DEDUCE boost::hof::detail::reveal_static_const_factory()
+#endif
+
+#endif