# UNDF: UNDF-2026-000000281
Fixes sfml-0004: WindowImplX11 destructor uses std::find() on allWindows vector.

--- a/src/SFML/Window/Unix/WindowImplX11.cpp
+++ b/src/SFML/Window/Unix/WindowImplX11.cpp
@@ -63,7 +63,8 @@
 namespace
 {
-    std::vector<sf::priv::WindowImplX11*> allWindows;
+    std::set<sf::priv::WindowImplX11*>    allWindows;   // FIX sfml-0004: O(1) insert/erase — CWE-407
     sf::Mutex                              allWindowsMutex;
+    // Note: std::vector removed; iteration in allWindows loop at line 1153 uses range-for
 }

@@ -777,7 +777,7 @@ WindowImplX11::~WindowImplX11()
     Lock lock(allWindowsMutex);
-    allWindows.erase(std::find(allWindows.begin(), allWindows.end(), this));
-    // was: O(n) find then O(n) erase — CWE-407; closing N windows = O(n²)
+    allWindows.erase(this);   // O(log n)

@@ -1598,7 +1598,7 @@ void WindowImplX11::initialize()
     Lock lock(allWindowsMutex);
-    allWindows.push_back(this);
+    allWindows.insert(this);   // O(log n)

# The iteration at line 1153 (for itr : allWindows) works unchanged with std::set.
# No ordering guarantees were relied upon with the vector (LIFO ordering not used).