functor: add constant functor

CMakeLists.txt

@@ -222,6 +222,7 @@ list (
     format.hpp
     fourcc.cpp
     fourcc.hpp
+    functor.hpp
     geom/fwd.hpp
     geom/aabb.cpp
     geom/aabb.hpp

functor.hpp

@@ -0,0 +1,54 @@
+/*
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ *
+ * Copyright 2018 Danny Robson <danny@nerdcruft.net>
+ */
+
+#pragma once
+
+#include <utility>
+
+
+namespace util::functor {
+    ///////////////////////////////////////////////////////////////////////////
+    /// returns the value provided at construction time regardless of the
+    /// arguments supplied in the call operator.
+    template <typename ValueT>
+    class constant {
+    public:
+        constant (const ValueT &_value):
+            m_value (_value)
+        { ; }
+
+
+        template <typename ...Args>
+        ValueT&
+        operator() (Args&&...) noexcept
+        { return m_value; }
+
+
+        template <typename ...Args>
+        const ValueT&
+        operator() (Args&&...) const noexcept
+        { return m_value; }
+
+
+    private:
+        ValueT m_value;
+    };
+
+
+    //-------------------------------------------------------------------------
+    template <typename ValueT>
+    constant (ValueT) -> constant<std::decay_t<ValueT>>;
+};