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+/*=============================================================================
+ Copyright (c) 2014 Paul Fultz II
+ mutable.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_MUTABLE_H
+#define BOOST_HOF_GUARD_FUNCTION_MUTABLE_H
+
+/// mutable
+/// =======
+///
+/// Description
+/// -----------
+///
+/// The `mutable` function adaptor allows using a non-const function object
+/// inside of a const-function object. In Fit, all the function adaptors use
+/// `const` call overloads, so if there is a function that has a non-const
+/// call operator, it couldn't be used directly. So, `mutable_` allows the
+/// function to be used inside of the call operator.
+///
+/// NOTE: This function should be used with caution since many functions are
+/// copied, so relying on some internal shared state can be error-prone.
+///
+/// Synopsis
+/// --------
+///
+/// template<class F>
+/// mutable_adaptor<F> mutable_(F f)
+///
+/// Requirements
+/// ------------
+///
+/// F must be:
+///
+/// * [MutableFunctionObject](MutableFunctionObject)
+/// * MoveConstructible
+///
+
+#include <boost/hof/detail/result_of.hpp>
+#include <boost/hof/detail/delegate.hpp>
+#include <boost/hof/detail/move.hpp>
+#include <boost/hof/detail/make.hpp>
+#include <boost/hof/detail/static_const_var.hpp>
+
+namespace boost { namespace hof {
+
+template<class F>
+struct mutable_adaptor
+{
+ mutable F f;
+
+ BOOST_HOF_DELEGATE_CONSTRUCTOR(mutable_adaptor, F, f);
+
+ BOOST_HOF_RETURNS_CLASS(mutable_adaptor);
+
+ template<class... Ts>
+ BOOST_HOF_SFINAE_RESULT(F, id_<Ts>...)
+ operator()(Ts&&... xs) const BOOST_HOF_SFINAE_RETURNS(BOOST_HOF_CONST_THIS->f(BOOST_HOF_FORWARD(Ts)(xs)...));
+};
+
+BOOST_HOF_DECLARE_STATIC_VAR(mutable_, detail::make<mutable_adaptor>);
+
+}} // namespace boost::hof
+
+
+#endif