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+///////////////////////////////////////////////////////////////////////////////
+//
+// Copyright (c) 2015 Microsoft Corporation. All rights reserved.
+//
+// This code is licensed under the MIT License (MIT).
+//
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+// THE SOFTWARE.
+//
+///////////////////////////////////////////////////////////////////////////////
+
+#pragma once
+
+#ifndef GSL_UTIL_H
+#define GSL_UTIL_H
+
+#include "gsl_assert" // Ensures/Expects
+#include <array>
+#include <exception>
+#include <type_traits>
+#include <utility>
+
+#ifdef _MSC_VER
+
+// No MSVC does constexpr fully yet
+#pragma push_macro("constexpr")
+#define constexpr /*constexpr*/
+
+#pragma warning(push)
+#pragma warning(disable : 4127) // conditional expression is constant
+
+// MSVC 2013 workarounds
+#if _MSC_VER <= 1800
+// noexcept is not understood
+#pragma push_macro("noexcept")
+#define noexcept /*noexcept*/
+
+// turn off some misguided warnings
+#pragma warning(push)
+#pragma warning(disable : 4351) // warns about newly introduced aggregate initializer behavior
+
+#endif // _MSC_VER <= 1800
+
+#endif // _MSC_VER
+
+namespace gsl
+{
+//
+// GSL.util: utilities
+//
+
+// final_act allows you to ensure something gets run at the end of a scope
+template <class F>
+class final_act
+{
+public:
+    explicit final_act(F f) noexcept : f_(std::move(f)), invoke_(true) {}
+
+    final_act(final_act&& other) noexcept : f_(std::move(other.f_)), invoke_(other.invoke_)
+    {
+        other.invoke_ = false;
+    }
+
+    final_act(const final_act&) = delete;
+    final_act& operator=(const final_act&) = delete;
+
+    ~final_act() noexcept
+    {
+        if (invoke_) f_();
+    }
+
+private:
+    F f_;
+    bool invoke_;
+};
+
+// finally() - convenience function to generate a final_act
+template <class F>
+inline final_act<F> finally(const F& f) noexcept
+{
+    return final_act<F>(f);
+}
+
+template <class F>
+inline final_act<F> finally(F&& f) noexcept
+{
+    return final_act<F>(std::forward<F>(f));
+}
+
+// narrow_cast(): a searchable way to do narrowing casts of values
+#if _MSC_VER <= 1800
+template <class T, class U>
+inline constexpr T narrow_cast(U u) noexcept
+{
+    return static_cast<T>(u);
+}
+#else
+template <class T, class U>
+inline constexpr T narrow_cast(U&& u) noexcept
+{
+    return static_cast<T>(std::forward<U>(u));
+}
+#endif
+
+struct narrowing_error : public std::exception
+{
+};
+
+namespace details
+{
+    template <class T, class U>
+    struct is_same_signedness
+        : public std::integral_constant<bool, std::is_signed<T>::value == std::is_signed<U>::value>
+    {
+    };
+}
+
+// narrow() : a checked version of narrow_cast() that throws if the cast changed the value
+template <class T, class U>
+inline T narrow(U u)
+{
+    T t = narrow_cast<T>(u);
+    if (static_cast<U>(t) != u) throw narrowing_error();
+    if (!details::is_same_signedness<T, U>::value && ((t < T{}) != (u < U{})))
+        throw narrowing_error();
+    return t;
+}
+
+//
+// at() - Bounds-checked way of accessing static arrays, std::array, std::vector
+//
+template <class T, size_t N>
+constexpr T& at(T (&arr)[N], size_t index)
+{
+    Expects(index < N);
+    return arr[index];
+}
+
+template <class T, size_t N>
+constexpr T& at(std::array<T, N>& arr, size_t index)
+{
+    Expects(index < N);
+    return arr[index];
+}
+
+template <class Cont>
+constexpr typename Cont::value_type& at(Cont& cont, size_t index)
+{
+    Expects(index < cont.size());
+    return cont[index];
+}
+
+template <class T>
+constexpr const T& at(std::initializer_list<T> cont, size_t index)
+{
+    Expects(index < cont.size());
+    return *(cont.begin() + index);
+}
+
+} // namespace gsl
+
+#ifdef _MSC_VER
+
+#pragma warning(pop)
+
+#undef constexpr
+#pragma pop_macro("constexpr")
+
+#if _MSC_VER <= 1800
+
+#undef noexcept
+#pragma pop_macro("noexcept")
+
+#pragma warning(pop)
+
+#endif // _MSC_VER <= 1800
+
+#endif // _MSC_VER
+
+#endif // GSL_UTIL_H