fixed_string: add a compile type string suitable for template params

CMakeLists.txt

@@ -329,6 +329,7 @@ list (
     extent.hpp
     fixed.cpp
     fixed.hpp
+    fixed_string.hpp
     float.cpp
     float.hpp
     ${CMAKE_CURRENT_BINARY_DIR}/format.cpp

fixed_string.hpp

@@ -0,0 +1,38 @@
+/*
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/.
+ *
+ * Copyright 2019, Danny Robson <danny@nerdcruft.net>
+ */
+
+#pragma once
+
+#include <algorithm>
+
+
+///////////////////////////////////////////////////////////////////////////////
+namespace cruft {
+    /// A simple string-like class that allows constexpr representation of
+    /// strings.
+    ///
+    /// This allows strings to be represented as instances of fixed_string in
+    /// template parameters (rather than using vendor specific array
+    /// extensions).
+    template <std::size_t N>
+    struct fixed_string {
+        char value[N + 1] {};
+
+        constexpr
+        fixed_string (char const _value[N+1]) noexcept
+        {
+            std::copy_n (std::begin (_value), N, std::begin (value));
+        }
+
+        constexpr operator char const* () const { return value; }
+    };
+
+
+    template <std::size_t N>
+    fixed_string (char const (&)[N]) -> fixed_string<N - 1>;
+}