refs #414 Efficient findFirstBy methods in CRangeBase.

src/blackmisc/range.h

@@ -46,6 +46,9 @@ namespace BlackMisc
     template <class Derived, class CIt>
     class CRangeBase
     {
+        using value_type = typename std::iterator_traits<CIt>::value_type;
+        using const_reference = typename std::iterator_traits<CIt>::reference;
+
     public:
         /*!
          * Return a new container generated by applying some transformation function to all elements of this one.
@@ -71,6 +74,30 @@ namespace BlackMisc
         inline auto findBy(K0 k0, V0 v0, KeysValues... keysValues) const
             -> CRange<Iterators::ConditionalIterator<CIt, decltype(BlackMisc::Predicates::MemberEqual(k0, v0, keysValues...))>>;
 
+        /*!
+         * Return a reference to the first element for which a given predicate returns true. Undefined if there is none.
+         */
+        template <class Predicate>
+        const_reference findFirstBy(Predicate p) const { return findBy(p).front(); }
+
+        /*!
+         * Return a reference to the first element matching some particular key/value pair(s). Undefined if there is none.
+         */
+        template <class K, class V>
+        const_reference findFirstBy(K key, V value) const { return findBy(key, value).front(); }
+
+        /*!
+         * Return a copy of the first element for which a given predicate returns true, or a default value if there is none.
+         */
+        template <class Predicate>
+        value_type findFirstByOrDefault(Predicate p, const value_type &def = value_type{}) const { return findBy(p).frontOrDefault(def); }
+
+        /*!
+         * Return a copy of the first element matching some particular key/value pair(s), or a default value if there is none.
+         */
+        template <class K, class V>
+        value_type findFirstByOrDefault(K key, V value, const value_type &def = value_type{}) const { return findBy(key, value).frontOrDefault(def); }
+
         /*!
          * \brief Return true if there is an element for which a given predicate returns true.
          */
@@ -161,7 +188,7 @@ namespace BlackMisc
         //! @}
 
         //! Returns the element at the beginning of the range. Undefined if the range is empty.
-        const_reference front() const { return *begin(); }
+        const_reference front() const { Q_ASSERT(!empty()); return *begin(); }
 
         //! Returns the element at the beginning of the range, or a default value if the range is empty.
         const_reference frontOrDefault() const