1/*============================================================================= 2 Copyright (c) 2014 Paul Fultz II 3 function.h 4 Distributed under the Boost Software License, Version 1.0. (See accompanying 5 file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt) 6==============================================================================*/ 7 8#ifndef BOOST_HOF_GUARD_FUNCTION_FUNCTION_H 9#define BOOST_HOF_GUARD_FUNCTION_FUNCTION_H 10 11/// BOOST_HOF_STATIC_FUNCTION 12/// =================== 13/// 14/// Description 15/// ----------- 16/// 17 18/// The `BOOST_HOF_STATIC_FUNCTION` macro allows initializing a function object from a 19/// `constexpr` expression. It uses the best practices as outlined in 20/// [N4381](http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2015/n4381.html). 21/// This includes using `const` to avoid global state, compile-time 22/// initialization of the function object to avoid the [static initialization 23/// order fiasco](https://isocpp.org/wiki/faq/ctors#static-init-order), and an 24/// external address of the function object that is the same across translation 25/// units to avoid possible One-Definition-Rule(ODR) violations. 26/// 27/// In C++17, this achieved using the `inline` keyword. However, on older 28/// compilers it is initialized using a reference to a static member variable. 29/// The static member variable is default constructed, as such the user variable 30/// is always default constructed regardless of the expression. 31/// 32/// By default, all functions defined with `BOOST_HOF_STATIC_FUNCTION` use the 33/// [`boost::hof::reveal`](/include/boost/hof/reveal) adaptor to improve error messages. 34/// 35/// Example 36/// ------- 37/// 38/// #include <boost/hof.hpp> 39/// #include <cassert> 40/// 41/// struct sum_f 42/// { 43/// template<class T, class U> 44/// T operator()(T x, U y) const 45/// { 46/// return x+y; 47/// } 48/// }; 49/// 50/// BOOST_HOF_STATIC_FUNCTION(sum) = sum_f(); 51/// BOOST_HOF_STATIC_FUNCTION(partial_sum) = boost::hof::partial(sum_f()); 52/// 53/// int main() { 54/// assert(sum(1, 2) == partial_sum(1)(2)); 55/// } 56/// 57 58#include <boost/hof/reveal.hpp> 59#if !BOOST_HOF_HAS_INLINE_VARIABLES 60#include <boost/hof/detail/static_const_var.hpp> 61#include <boost/hof/detail/constexpr_deduce.hpp> 62#endif 63 64namespace boost { namespace hof { 65 66namespace detail { 67 68struct reveal_static_const_factory 69{ 70 constexpr reveal_static_const_factory() 71 {} 72 template<class F> 73 constexpr reveal_adaptor<F> operator=(const F& f) const 74 { 75#if BOOST_HOF_HAS_INLINE_VARIABLES 76#else 77 static_assert(BOOST_HOF_IS_DEFAULT_CONSTRUCTIBLE(F), "Static functions must be default constructible"); 78#endif 79 return reveal_adaptor<F>(f); 80 } 81}; 82}}} // namespace boost::hof 83 84#if BOOST_HOF_HAS_INLINE_VARIABLES 85#define BOOST_HOF_STATIC_FUNCTION(name) inline const constexpr auto name = boost::hof::detail::reveal_static_const_factory() 86#else 87#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() 88#endif 89 90#endif 91