[libcxx-commits] [libcxx] [libcxx] adds size-based `__split_buffer` representation to unstable ABI (PR #139632)

[Nikolas Klauser via libcxx-commits]

================
@@ -78,23 +80,232 @@ public:
                       __split_buffer,
                       void>;
 
-  pointer __first_;
-  pointer __begin_;
-  pointer __end_;
-  _LIBCPP_COMPRESSED_PAIR(pointer, __cap_, allocator_type, __alloc_);
+  struct __data {
+    pointer __first_ = nullptr;
+    pointer __begin_ = nullptr;
+#ifdef _LIBCPP_ABI_SIZE_BASED_CONTAINERS
+    size_type __size_ = 0;
+    size_type __cap_ = 0;
+    allocator_type __alloc_;
+#else
+    pointer __end_ = nullptr;
+    _LIBCPP_COMPRESSED_PAIR(pointer, __cap_ = nullptr, allocator_type, __alloc_);
+#endif
+
+    _LIBCPP_CONSTEXPR_SINCE_CXX20 _LIBCPP_HIDE_FROM_ABI __data() = default;
+
+    _LIBCPP_CONSTEXPR_SINCE_CXX20 _LIBCPP_HIDE_FROM_ABI explicit __data(const allocator_type& __alloc)
+    : __alloc_(__alloc)
+    {}
+
+    _LIBCPP_CONSTEXPR_SINCE_CXX20 _LIBCPP_HIDE_FROM_ABI pointer begin() _NOEXCEPT {
+      return __begin_;
+    }
+    _LIBCPP_CONSTEXPR_SINCE_CXX20 _LIBCPP_HIDE_FROM_ABI const_pointer begin() const _NOEXCEPT {
+      return __begin_;
+    }
+
+    _LIBCPP_CONSTEXPR_SINCE_CXX20 _LIBCPP_HIDE_FROM_ABI pointer end() _NOEXCEPT {
+#ifdef _LIBCPP_ABI_SIZE_BASED_CONTAINERS
+      return __begin_ + __size_;
+#else
+      return __end_;
+#endif
+    }
+
+    _LIBCPP_CONSTEXPR_SINCE_CXX20 _LIBCPP_HIDE_FROM_ABI pointer end() const _NOEXCEPT {
+#ifdef _LIBCPP_ABI_SIZE_BASED_CONTAINERS
+      return __begin_ + __size_;
+#else
+      return __end_;
+#endif
+    }
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philnik777 wrote:

Can we remove these and instead implement `__split_buffer`s `end()` as `begin() + size()`?

https://github.com/llvm/llvm-project/pull/139632