From 26379aad1cab647cee3231ae9b72d1bfafb4f482 Mon Sep 17 00:00:00 2001
From: Roland Conybeare <rconybeare@gmail.com>
Date: Fri, 10 Apr 2026 20:31:26 -0400
Subject: [PATCH] xo-object2: DArray: + DArray.pop_back + no ub on oom

---
 xo-object2/include/xo/object2/DArray.hpp |  8 ++++-
 xo-object2/src/object2/DArray.cpp        | 42 +++++++++++++++++++-----
 2 files changed, 40 insertions(+), 10 deletions(-)

diff --git a/xo-object2/include/xo/object2/DArray.hpp b/xo-object2/include/xo/object2/DArray.hpp
index 27b0961d..1d3499aa 100644
--- a/xo-object2/include/xo/object2/DArray.hpp
+++ b/xo-object2/include/xo/object2/DArray.hpp
@@ -111,10 +111,16 @@ namespace xo {
             bool assign_at(size_type index, obj<AGCObject> elt) noexcept;
 
             /** append @p elt at the end of array.
-             *  true on success, false otherwise
+             *  true on success, false otherwise.
+             *  on failure array is unaltered
              **/
             bool push_back(obj<AGCObject> elt) noexcept;
 
+            /** store last element in array into @p elt and decrement array size.
+             *  true on success; false on failure (implies array was empty)
+             **/
+            bool pop_back(obj<AGCObject> * p_elt = nullptr) noexcept;
+
             ///@}
             /** @defgroup darray-general general methods **/
             ///@{
diff --git a/xo-object2/src/object2/DArray.cpp b/xo-object2/src/object2/DArray.cpp
index c6e83ee6..a181f9c3 100644
--- a/xo-object2/src/object2/DArray.cpp
+++ b/xo-object2/src/object2/DArray.cpp
@@ -18,6 +18,7 @@ namespace xo {
     using xo::facet::typeseq;
 
     namespace scm {
+        // TODO: error reporting?
         DArray *
         DArray::empty(obj<AAllocator> mm,
                       size_type cap)
@@ -27,12 +28,14 @@ namespace xo {
             void * mem = mm.alloc(typeseq::id<DArray>(),
                                   sizeof(DArray) + cap * sizeof(obj<AGCObject>));
 
-            result = new (mem) DArray();
+            if (mem) [[likely]] {
+                result = new (mem) DArray();
 
-            assert(result);
+                assert(result);
 
-            result->capacity_ = cap;
-            result->size_ = 0;
+                result->capacity_ = cap;
+                result->size_ = 0;
+            }
 
             return result;
         }
@@ -44,12 +47,14 @@ namespace xo {
         {
             DArray * dest = empty(mm, new_cap);
 
-            /** could just memcpy here **/
-            for (size_type i = 0, n = src->size(); i < n; ++i) {
-                dest->elts_[i] = src->elts_[i];
-            }
+            if (dest) [[likely]] {
+                /** could just memcpy here **/
+                for (size_type i = 0, n = src->size(); i < n; ++i) {
+                    dest->elts_[i] = src->elts_[i];
+                }
 
-            dest->size_ = src->size();
+                dest->size_ = src->size();
+            }
 
             return dest;
         }
@@ -98,6 +103,25 @@ namespace xo {
             }
         }
 
+        bool
+        DArray::pop_back(obj<AGCObject> * p_elt) noexcept
+        {
+            if (size_ > 0) {
+                --size_;
+
+                obj<AGCObject> & last = elts_[size_];
+
+                if (p_elt)
+                    *p_elt = last;
+
+                last.reset(); // hygiene
+
+                return true;
+            }
+
+            return false;
+        }
+
         bool
         DArray::resize(size_type new_z) noexcept
         {