[sntp] Replace std::vector<std::string> with std::array<const char*> to save heap memory (#11525)

esphome/components/sntp/sntp_component.cpp

@@ -27,7 +27,7 @@ void SNTPComponent::setup() {
   esp_sntp_setoperatingmode(ESP_SNTP_OPMODE_POLL);
   size_t i = 0;
   for (auto &server : this->servers_) {
-    esp_sntp_setservername(i++, server.c_str());
+    esp_sntp_setservername(i++, server);
   }
   esp_sntp_set_sync_interval(this->get_update_interval());
   esp_sntp_set_time_sync_notification_cb([](struct timeval *tv) {
@@ -42,7 +42,7 @@ void SNTPComponent::setup() {
 
   size_t i = 0;
   for (auto &server : this->servers_) {
-    sntp_setservername(i++, server.c_str());
+    sntp_setservername(i++, server);
   }
 
 #if defined(USE_ESP8266)
@@ -59,7 +59,7 @@ void SNTPComponent::dump_config() {
   ESP_LOGCONFIG(TAG, "SNTP Time:");
   size_t i = 0;
   for (auto &server : this->servers_) {
-    ESP_LOGCONFIG(TAG, "  Server %zu: '%s'", i++, server.c_str());
+    ESP_LOGCONFIG(TAG, "  Server %zu: '%s'", i++, server);
   }
 }
 void SNTPComponent::update() {

esphome/components/sntp/sntp_component.h

@@ -2,10 +2,14 @@
 
 #include "esphome/core/component.h"
 #include "esphome/components/time/real_time_clock.h"
+#include <array>
 
 namespace esphome {
 namespace sntp {
 
+// Server count is calculated at compile time by Python codegen
+// SNTP_SERVER_COUNT will always be defined
+
 /// The SNTP component allows you to configure local timekeeping via Simple Network Time Protocol.
 ///
 /// \note
@@ -14,10 +18,7 @@ namespace sntp {
 /// \see https://www.gnu.org/software/libc/manual/html_node/TZ-Variable.html
 class SNTPComponent : public time::RealTimeClock {
  public:
-  SNTPComponent(const std::vector<std::string> &servers) : servers_(servers) {}
+  SNTPComponent(const std::array<const char *, SNTP_SERVER_COUNT> &servers) : servers_(servers) {}
-
-  // Note: set_servers() has been removed and replaced by a constructor - calling set_servers after setup would
-  // have had no effect anyway, and making the strings immutable avoids the need to strdup their contents.
 
   void setup() override;
   void dump_config() override;
@@ -29,7 +30,10 @@ class SNTPComponent : public time::RealTimeClock {
   void time_synced();
 
  protected:
-  std::vector<std::string> servers_;
+  // Store const char pointers to string literals
+  // ESP8266: strings in rodata (RAM), but avoids std::string overhead (~24 bytes each)
+  // Other platforms: strings in flash
+  std::array<const char *, SNTP_SERVER_COUNT> servers_;
   bool has_time_{false};
 
 #if defined(USE_ESP32)

esphome/components/sntp/time.py

@@ -43,6 +43,11 @@ CONFIG_SCHEMA = cv.All(
 
 async def to_code(config):
     servers = config[CONF_SERVERS]
+
+    # Define server count at compile time
+    cg.add_define("SNTP_SERVER_COUNT", len(servers))
+
+    # Pass string literals to constructor - stored in flash/rodata by compiler
     var = cg.new_Pvariable(config[CONF_ID], servers)
 
     await cg.register_component(var, config)

esphome/core/defines.h

@@ -87,6 +87,7 @@
 #define USE_MDNS_STORE_SERVICES
 #define MDNS_SERVICE_COUNT 3
 #define MDNS_DYNAMIC_TXT_COUNT 3
+#define SNTP_SERVER_COUNT 3
 #define USE_MEDIA_PLAYER
 #define USE_NEXTION_TFT_UPLOAD
 #define USE_NUMBER