From 4714b77f45b8de4c66c224d67e4230cc56fa550c Mon Sep 17 00:00:00 2001
From: "J. Nick Koston" <nick@koston.org>
Date: Wed, 16 Sep 2026 03:37:28 -0500
Subject: [PATCH] [core] Construct App without value initialization (#19109)

---
 esphome/core/application.h           |  2 +-
 esphome/core/config.py               |  5 +++--
 tests/unit_tests/core/test_config.py | 21 +++++++++++++++++++++
 3 files changed, 25 insertions(+), 3 deletions(-)

diff --git a/esphome/core/application.h b/esphome/core/application.h
index a12cdc4ac88..f1cf6fcca02 100644
--- a/esphome/core/application.h
+++ b/esphome/core/application.h
@@ -528,7 +528,7 @@ class Application {
 
   // 1-byte members (grouped together to minimize padding)
   uint8_t app_state_{0};
-  bool name_add_mac_suffix_;
+  bool name_add_mac_suffix_{false};
   bool in_loop_{false};
   volatile bool has_pending_enable_loop_requests_{false};
 
diff --git a/esphome/core/config.py b/esphome/core/config.py
index 67a7b5210ee..8a4eb0fc379 100644
--- a/esphome/core/config.py
+++ b/esphome/core/config.py
@@ -717,9 +717,10 @@ async def to_code(config: ConfigType) -> None:
     cg.add_global(cg.RawExpression("using std::min"))
     cg.add_global(cg.RawExpression("using std::max"))
 
-    # Construct App via placement new — see application.cpp for storage details
+    # Construct App via placement new — see application.cpp for storage details.
+    # No parens: `Application()` would zero-fill storage that is already zero.
     cg.add_global(cg.RawStatement("#include <new>"))
-    cg.add(cg.RawExpression("new (&App) Application()"))
+    cg.add(cg.RawExpression("new (&App) Application"))
     name = config[CONF_NAME]
     friendly_name = config[CONF_FRIENDLY_NAME]
     name_add_mac_suffix = config[CONF_NAME_ADD_MAC_SUFFIX]
diff --git a/tests/unit_tests/core/test_config.py b/tests/unit_tests/core/test_config.py
index 8ab3ad5d153..07cff003cdd 100644
--- a/tests/unit_tests/core/test_config.py
+++ b/tests/unit_tests/core/test_config.py
@@ -175,6 +175,27 @@ async def test_core_area_recorded_at_config_load(
     assert CORE.area == expected_area
 
 
+@pytest.mark.asyncio
+async def test_app_is_default_initialized(
+    yaml_file: Callable[[str], Path],
+) -> None:
+    """App is constructed with `new (&App) Application`, no parentheses.
+
+    `Application()` would value-initialize and memset the whole object into
+    storage that is already zero."""
+    result = load_config_from_fixture(yaml_file, "valid_area_device.yaml", FIXTURES_DIR)
+    assert result is not None
+
+    with patch("esphome.core.config.cg") as mock_cg:
+        mock_cg.RawStatement.side_effect = lambda *args, **kwargs: MagicMock()
+        mock_cg.RawExpression.side_effect = lambda *args, **kwargs: MagicMock()
+        await config.to_code(result[CONF_ESPHOME])
+
+    raw_expressions = [c.args[0] for c in mock_cg.RawExpression.call_args_list]
+    assert "new (&App) Application" in raw_expressions
+    assert "new (&App) Application()" not in raw_expressions
+
+
 def test_config_load_without_area_clears_stale_core_area(
     yaml_file: Callable[[str], Path],
 ) -> None: