From 8faf1f951011f974b62f9226a725e2af59dd9a15 Mon Sep 17 00:00:00 2001
From: naoto <naotofujiwara890@gmail.com>
Date: Thu, 27 Aug 2026 22:18:03 +0900
Subject: [PATCH] Avoid copying MetricFamily in Family::Collect

Collect() built the family in a local variable and returned it as
`return {family};`. A braced-init-list copies its elements, so every
collection deep-copied the whole metric vector, including each
ClientMetric and all of its labels, and temporarily required twice the
memory of the collected family.

Move the family into the returned vector instead. Registry::Collect()
already moves the families out, so this removes the last copy on the
collection path.

Relates to #741.
---
 core/src/family.cc | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/core/src/family.cc b/core/src/family.cc
index 6a994516..d8ac2921 100644
--- a/core/src/family.cc
+++ b/core/src/family.cc
@@ -86,8 +86,9 @@ template <typename T>
 std::vector<MetricFamily> Family<T>::Collect() const {
   std::lock_guard<std::mutex> lock{mutex_};
 
+  auto families = std::vector<MetricFamily>{};
   if (metrics_.empty()) {
-    return {};
+    return families;
   }
 
   auto family = MetricFamily{};
@@ -98,7 +99,8 @@ std::vector<MetricFamily> Family<T>::Collect() const {
   for (const auto& [metric_labels, metric] : metrics_) {
     family.metric.push_back(CollectMetric(metric_labels, metric.get()));
   }
-  return {family};
+  families.push_back(std::move(family));
+  return families;
 }
 
 template <typename T>