[!700] dense vectors: add some more methods expected for containers

Merge branch 'feature/make-vectors-more-like-containers' into 'master'

ref:core/dune-common This merge request makes dense vectors look a little bit
more like standard containers by adding front(), back() and empty() member
functions.

For dense vectors where the implementation uses a contiguous array, a data()
member function is also provided.

See merge request [!700]

  [!700]: gitlab.dune-project.org/core/dune-common/merge_requests/700

dune/common/densevector.hh

@@ -300,6 +300,36 @@ namespace Dune {
       return asImp()[i];
     }
 
+    //! return reference to first element
+    value_type& front()
+    {
+      return asImp()[0];
+    }
+
+    //! return reference to first element
+    const value_type& front() const
+    {
+      return asImp()[0];
+    }
+
+    //! return reference to last element
+    value_type& back()
+    {
+      return asImp()[size()-1];
+    }
+
+    //! return reference to last element
+    const value_type& back() const
+    {
+      return asImp()[size()-1];
+    }
+
+    //! checks whether the container is empty
+    bool empty() const
+    {
+      return size() == 0;
+    }
+
     //! size method
     size_type size() const
     {

dune/common/dynvector.hh

@@ -156,6 +156,18 @@ namespace Dune {
       return _data[i];
     }
 
+    //! return pointer to underlying array
+    K* data() noexcept
+    {
+      return _data.data();
+    }
+
+    //! return pointer to underlying array
+    const K* data() const noexcept
+    {
+      return _data.data();
+    }
+
     const container_type &container () const { return _data; }
     container_type &container () { return _data; }
   };

dune/common/fvector.hh

@@ -196,6 +196,18 @@ namespace Dune {
       DUNE_ASSERT_BOUNDS(i < SIZE);
       return _data[i];
     }
+
+    //! return pointer to underlying array
+    K* data() noexcept
+    {
+      return _data.data();
+    }
+
+    //! return pointer to underlying array
+    const K* data() const noexcept
+    {
+      return _data.data();
+    }
   };
 
   /** \brief Read a FieldVector from an input stream
@@ -333,6 +345,18 @@ namespace Dune {
       return _data;
     }
 
+    //! return pointer to underlying array
+    K* data() noexcept
+    {
+      return &_data;
+    }
+
+    //! return pointer to underlying array
+    const K* data() const noexcept
+    {
+      return &_data;
+    }
+
     //===== conversion operator
 
     /** \brief Conversion operator */

dune/common/test/fvectortest.cc

@@ -7,6 +7,7 @@
 #include <complex>
 #include <iostream>
 #include <limits>
+#include <memory>
 #include <typeinfo>
 #include <type_traits>
 
@@ -123,6 +124,13 @@ struct FieldVectorMainTestCommons
     // test container methods
     typename FieldVector<ft,d>::size_type size = FieldVector<ft,d>::dimension;
     FVECTORTEST_ASSERT(size == w.size());
+
+    if (w.size() > 0) {
+      FVECTORTEST_ASSERT(!w.empty());
+      FVECTORTEST_ASSERT(std::addressof(w[0]) == std::addressof(w.front()));
+      FVECTORTEST_ASSERT(std::addressof(w[0]) == w.data());
+      FVECTORTEST_ASSERT(std::addressof(w[d-1]) == std::addressof(w.back()));
+    }
   }
 };