refs #583 as_const

src/blackmisc/range.h

@@ -38,6 +38,18 @@ namespace BlackMisc
 
     template <class> class CRange;
 
+    /*!
+     * Own implementation of C++17 std::as_const. Adds const to any lvalue.
+     * Useful with non-mutating range-for loops to avoid unnecessary detachment of Qt implicitly shared containers.
+     * Does not allow rvalues, as that could easily lead to undefined behaviour.
+     */
+    //! @{
+    template <class T>
+    Q_CONSTEXPR typename std::add_const<T>::type &as_const(T &v) Q_DECL_NOEXCEPT { return v; }
+    template <class T>
+    void as_const(const T &&) = delete;
+    //! @}
+
     /*!
      * Any container class with begin and end iterators can inherit from this CRTP class
      * to gain some useful algorithms as member functions.