reserve()

The String reserve() function allows you to allocate a buffer in memory for manipulating Strings. myString.reserve(size)
myString: a variable of type String. size: the number of bytes in memory to save for String manipulation. Allowed data types: unsigned int. Nothing
		
String myString;

void setup() {
  // initialize serial and wait for port to open:
  Serial.begin(115200);

  myString.reserve(26);
  myString = "i=";
  myString += "1234";
  myString += ", is that ok?";

  // print the String:
  Serial.println(myString);
}

void loop() {
  // nothing to do here
}