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