cow_darray: Fail safe API: Only [c]begin() iterator retrieval, leave other operations on the cow_[ro|rw]_iterator avoding data races

These changes have been adopted to cow_vector as well, still deprecated now.

To allow data-race free operations using iterators from a potentially mutated CoW,
only one cow_darray::begin() const_iterator or iterator should be retrieved from this CoW
and all further operations shall use its
jau::cow_ro_iterator::size(), jau::cow_ro_iterator::begin() and jau::cow_ro_iterator::end()
- or its respective variant from jau::cow_rw_iterator.

Removed from cow_darray:
- 'const_iterator begin() const' // ambiguous mutable/immutable overload
- 'const_iterator cend()'        // fetch from cow_ro_iterator::end()
- 'iterator end()'               // fetch from cow_rw_iterator::end()

cow_[ro|rw]_iterator:
- private constructor w/ fried to cow_darray, .. only allow copy-ctor etc, not allowing to set false pointer
- Added size(), begin() and end(): Allowing data-race free operations
- full typedefs
- Holds the 'begin iterator' internally to implement begin() and end()

+++

Added new convenience template Type Traits and functions:

- 'template< class T > is_cow_type<T>::value' compile-time Type Trait,
  determining whether the given template class is a CoW type, e.g. jau::cow_darray,
  jau::cow_vector or any of their iterator.

For both, cow_type or any other using above 'is_cow_type<T>' trait:
- find_const(T& data)

- for_each_const(T& data)
- for_each_fidelity(T& data)

++++

Performance: No regression, even a little more speed-up.

1 files changed, 15 insertions, 27 deletions

diff --git a/include/jau/cow_vector.hpp b/include/jau/cow_vector.hpp
index 5223f82..365d776 100644
--- a/include/jau/cow_vector.hpp
+++ b/include/jau/cow_vector.hpp
@@ -117,23 +117,19 @@ namespace jau {
             typedef std::vector<value_type, allocator_type>     storage_t;
             typedef std::shared_ptr<storage_t>                  storage_ref_t;
 
+            typedef cow_vector<value_type, allocator_type>      cow_container_t;
+
             /**
-             * Immutable, read-only const_iterator, lock-free,
-             * holding the current shared store reference until destruction.
-             * <p>
-             * Using jau::cow_vector::get_snapshot() at construction.
-             * </p>
+             * @see jau::cow_darray::const_iterator
+             * @see jau::cow_ro_iterator
              */
-            typedef cow_ro_iterator<storage_t, storage_ref_t, cow_vector> const_iterator;
+            typedef cow_ro_iterator<storage_t, storage_ref_t, cow_container_t> const_iterator;
 
             /**
-             * Mutable, read-write iterator, holding the write-lock and a store copy until destruction.
-             * <p>
-             * Using jau::cow_vector::get_write_mutex(), jau::cow_vector::copy_store() at construction<br>
-             * and jau::cow_vector::set_store() at destruction.
-             * </p>
+             * @see jau::cow_darray::iterator
+             * @see jau::cow_rw_iterator
              */
-            typedef cow_rw_iterator<storage_t, storage_ref_t, cow_vector> iterator;
+            typedef cow_rw_iterator<storage_t, storage_ref_t, cow_container_t> iterator;
 
         private:
             static constexpr size_type DIFF_MAX = std::numeric_limits<difference_type>::max();
@@ -278,32 +274,24 @@ namespace jau {
 
             // const_iterator, non mutable, read-only
 
-            constexpr const_iterator begin() const noexcept {
-                return const_iterator(get_snapshot(), store_ref->cbegin());
-            }
+            // Removed for clarity: "constexpr const_iterator begin() const noexcept"
 
+            /**
+             * See description in jau::cow_darray::cbegin()
+             */
             constexpr const_iterator cbegin() const noexcept {
                 return const_iterator(get_snapshot(), store_ref->cbegin());
             }
 
-            constexpr const_iterator end() const noexcept {
-                return const_iterator(get_snapshot(), store_ref->cend());
-            }
-
-            constexpr const_iterator cend() const noexcept {
-                return const_iterator(get_snapshot(), store_ref->cend());
-            }
-
             // iterator, mutable, read-write
 
+            /**
+             * See description in jau::cow_darray::begin()
+             */
             constexpr iterator begin() noexcept {
                 return iterator(*this, [](storage_ref_t& new_store) -> typename storage_t::iterator { return new_store->begin(); } );
             }
 
-            constexpr iterator end() noexcept {
-                return iterator(*this, [](storage_ref_t& new_store) -> typename storage_t::iterator { return new_store->end(); } );
-            }
-
             // read access
 
             allocator_type get_allocator() const noexcept {