Add str_sprintf function that returns std::string (#2408)

esphome/core/helpers.cpp

@@ -351,6 +351,21 @@ bool str_startswith(const std::string &full, const std::string &start) { return
 bool str_endswith(const std::string &full, const std::string &ending) {
   return full.rfind(ending) == (full.size() - ending.size());
 }
+std::string str_sprintf(const char *fmt, ...) {
+  std::string str;
+  va_list args;
+
+  va_start(args, fmt);
+  size_t length = vsnprintf(nullptr, 0, fmt, args);
+  va_end(args);
+
+  str.resize(length);
+  va_start(args, fmt);
+  vsnprintf(&str[0], length + 1, fmt, args);
+  va_end(args);
+
+  return str;
+}
 
 uint16_t encode_uint16(uint8_t msb, uint8_t lsb) { return (uint16_t(msb) << 8) | uint16_t(lsb); }
 std::array<uint8_t, 2> decode_uint16(uint16_t value) {

esphome/core/helpers.h

@@ -59,6 +59,9 @@ bool str_equals_case_insensitive(const std::string &a, const std::string &b);
 bool str_startswith(const std::string &full, const std::string &start);
 bool str_endswith(const std::string &full, const std::string &ending);
 
+/// sprintf-like function returning std::string instead of writing to char array.
+std::string __attribute__((format(printf, 1, 2))) str_sprintf(const char *fmt, ...);
+
 class HighFrequencyLoopRequester {
  public:
   void start();