[libcxx-commits] [libcxx] [libc++][vector] Make constructor vector(count, value, allocator) exception-safe (PR #82033)

[via libcxx-commits]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-libcxx

Author: Mateusz Zych (mtezych)

<details>
<summary>Changes</summary>

The constructor `std::vector(count, value, allocator)` was missing an exception guard,
responsible for destroying successfully created elements, in case an exception was thrown:
- https://godbolt.org/z/MeK37n8fb

Since all constructors filling `std::vector<>` with elements of the same value:
 - `vector(count)`
 - `vector(count, allocator)`
 - `vector(count, value)`
 - `vector(count, value, allocator)`

have nearly identical implementation, their definitions should be physically close to each other.

In order to achieve that goal, the constructor `std::vector(count, value, allocator)` is now defined
outside of the `std::vector<>` class template and has sightly different `enable_if<>` expression,
matching to those used in the constructors taking two iterators:
 - `vector(iterator first, iterator last)`
 - `vector(iterator first, iterator last, allocator)`

---
Full diff: https://github.com/llvm/llvm-project/pull/82033.diff


1 Files Affected:

- (modified) libcxx/include/vector (+15-8) 


``````````diff
diff --git a/libcxx/include/vector b/libcxx/include/vector
index ce7df7a9f04207..62d36d9318bd69 100644
--- a/libcxx/include/vector
+++ b/libcxx/include/vector
@@ -429,15 +429,9 @@ public:
 #endif
   _LIBCPP_CONSTEXPR_SINCE_CXX20 _LIBCPP_HIDE_FROM_ABI vector(size_type __n, const value_type& __x);
 
-  template <class = __enable_if_t<__is_allocator<_Allocator>::value> >
+  template <__enable_if_t<__is_allocator<_Allocator>::value, int> = 0>
   _LIBCPP_CONSTEXPR_SINCE_CXX20 _LIBCPP_HIDE_FROM_ABI
-  vector(size_type __n, const value_type& __x, const allocator_type& __a)
-      : __end_cap_(nullptr, __a) {
-    if (__n > 0) {
-      __vallocate(__n);
-      __construct_at_end(__n, __x);
-    }
-  }
+  vector(size_type __n, const value_type& __x, const allocator_type& __a);
 
   template <class _InputIterator,
             __enable_if_t<__has_exactly_input_iterator_category<_InputIterator>::value &&
@@ -1165,6 +1159,19 @@ _LIBCPP_CONSTEXPR_SINCE_CXX20 vector<_Tp, _Allocator>::vector(size_type __n, con
   __guard.__complete();
 }
 
+template <class _Tp, class _Allocator>
+template <__enable_if_t<__is_allocator<_Allocator>::value, int>>
+_LIBCPP_CONSTEXPR_SINCE_CXX20
+vector<_Tp, _Allocator>::vector(size_type __n, const value_type& __x, const allocator_type& __a)
+    : __end_cap_(nullptr, __a) {
+  auto __guard = std::__make_exception_guard(__destroy_vector(*this));
+  if (__n > 0) {
+    __vallocate(__n);
+    __construct_at_end(__n, __x);
+  }
+  __guard.__complete();
+}
+
 template <class _Tp, class _Allocator>
 template <class _InputIterator,
           __enable_if_t<__has_exactly_input_iterator_category<_InputIterator>::value &&

``````````

</details>


https://github.com/llvm/llvm-project/pull/82033