Make row iterator a proper random_access_iterator.

Fixes: #846

This involves defining `operator[]` on the iterator, which indexes
at an offset: `i[n] == *(i + n)`.

Unfortunately I can't do the same for the _result_ iterator.  Way back
when, unaware of this requirement, I defined array indexing on a result
iterator as `i[n] == (*i)[n]`.  It made some code a lot more convenient
and seemed very logical.

include/pqxx/internal/result_iterator.hxx

@@ -55,9 +55,9 @@ public:
    * @name Dereferencing operators
    *
    * An iterator "points to" its own row, which is also itself.  This makes it
-   * easy to address a @ref result as a two-dimensional container, without
-   * going through the intermediate step of dereferencing the iterator.  It
-   * makes the interface similar to C pointer/array semantics.
+   * easy to address a @ref pqxx::result as a two-dimensional container,
+   * without going through the intermediate step of dereferencing the iterator.
+   * It makes the interface similar to C pointer/array semantics.
    *
    * IIRC Alex Stepanov, the inventor of the STL, once remarked that having
    * this as standard behaviour for pointers would be useful in some

include/pqxx/row.hxx

@@ -397,6 +397,9 @@ public:
   operator-(difference_type) const noexcept;
   [[nodiscard]] inline difference_type
   operator-(const_row_iterator const &) const noexcept;
+
+  [[nodiscard]] inline field operator[](difference_type offset) const noexcept
+  { return *(*this + offset); }
   //@}
 };
 
@@ -487,6 +490,8 @@ public:
   {
     return rhs.const_row_iterator::operator-(*this);
   }
+  [[nodiscard]] inline field operator[](difference_type offset) const noexcept
+  { return *(*this + offset); }
   //@}
 
   /**

test/unit/test_row.cxx

@@ -78,7 +78,19 @@ void test_row_as()
 }
 
 
+// In a random access iterator i, i[n] == *(i + n).
+void test_row_iterator_array_index_offsets_iterator()
+{
+  pqxx::connection conn;
+  pqxx::work tx{conn};
+  auto const row{tx.exec1("SELECT 5, 4, 3, 2")};
+  PQXX_CHECK_EQUAL(row.begin()[1].as<std::string>(), "4", "Row iterator indexing went wrong.");
+  PQXX_CHECK_EQUAL(row.rbegin()[1].as<std::string>(), "3", "Reverse row iterator indexing went wrong.");
+}
+
+
 PQXX_REGISTER_TEST(test_row);
 PQXX_REGISTER_TEST(test_row_iterator);
 PQXX_REGISTER_TEST(test_row_as);
+PQXX_REGISTER_TEST(test_row_iterator_array_index_offsets_iterator);
 } // namespace