Added thirdparty: boost library

thirdparty/include/boost/histogram/detail/try_cast.hpp

@@ -0,0 +1,61 @@
+// Copyright 2019 Hans Dembinski
+//
+// 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_HISTOGRAM_DETAIL_TRY_CAST_HPP
+#define BOOST_HISTOGRAM_DETAIL_TRY_CAST_HPP
+
+#include <boost/config.hpp> // BOOST_NORETURN
+#include <boost/throw_exception.hpp>
+#include <type_traits>
+
+namespace boost {
+namespace histogram {
+namespace detail {
+
+template <class T, class U>
+constexpr T* ptr_cast(U*) noexcept {
+  return nullptr;
+}
+
+template <class T>
+constexpr T* ptr_cast(T* p) noexcept {
+  return p;
+}
+
+template <class T>
+constexpr const T* ptr_cast(const T* p) noexcept {
+  return p;
+}
+
+template <class T, class E, class U>
+BOOST_NORETURN T try_cast_impl(std::false_type, std::false_type, U&&) {
+  BOOST_THROW_EXCEPTION(E("type cast error"));
+}
+
+// converting cast
+template <class T, class E, class U>
+T try_cast_impl(std::false_type, std::true_type, U&& u) noexcept {
+  return static_cast<T>(u); // cast to avoid warnings
+}
+
+// pass-through cast
+template <class T, class E>
+T&& try_cast_impl(std::true_type, std::true_type, T&& t) noexcept {
+  return std::forward<T>(t);
+}
+
+// cast fails at runtime with exception E instead of compile-time, T must be a value
+template <class T, class E, class U>
+T try_cast(U&& u) noexcept(std::is_convertible<U, T>::value) {
+  return try_cast_impl<T, E>(std::is_same<U, T>{}, std::is_convertible<U, T>{},
+                             std::forward<U>(u));
+}
+
+} // namespace detail
+} // namespace histogram
+} // namespace boost
+
+#endif