Does the StringObject.reserve() function prevent memory fragmentation/leaks?

Question

I'm currently writing and testing a professional Arduino application, I'm aware of the dangers of using the String class, so I began to implement C-style strings in some places, but it's just way harder using them to receive data from a Software Serial port that's variable in size and needs some manipulation.

I've upgraded from an Arduino Uno to a Leonardo so my current minimum SRAM usage is 1325 bytes (51%)

Here's the relevant part of my code:

String serialContents;
char emailChar[SIZEOF_ARRAY][SIZEOF_CHAR];

void setup(){ 
  serialContents.reserve(70);
  SerialOne.begin(9600);
}

void loop{

  if (SerialOne.available() > 0) //something is there to be read
  {
    serialContents = SerialOne.readStringUntil('\n'); //make a string of the characters coming on serial
  }

  if (serialContents.length() >= 60)
  {
   serialContents = ""; //ignore serial data
  }
 else{
  serialContents = cutString(serialContents); //some String manipulation, no adding only cutting
  serialContents.replace("@", "%40");   
  serialContents.toCharArray(emailChar[count], 31); 
  serialContents = ""; //String reset
}

So my question is, does something like this make my code unstable? Does the reserve() function help at all?

Edit: Here's the cutString() function

String cutString(String request) {

  int points = request.indexOf(':');

  if (request.substring(0, points) == "CHECKOUT")
  {
    modoPedido[count] = 'o'; //char array
  }
  else if (request.substring(0, points) == "CHECKIN")
  {
    modoPedido[count] = 'i';
  }

  request = request.substring(points + 1);

  return request;
}

Answer 1

Reserving space will help reduce memory fragmentation, but much of what else you are doing is swamping that small improvement you have made.

The biggest area of concern is your cutString function, which you have omitted from your code above. Passing a String as a parameter, modifying it, and then returning it again, ends up in new String objects being created and destroyed multiple times, which will make a mess of your heap.

Instead you should pass a String object by reference, and modify that object in-place. No need to return anything.

void cutString(String &theString) {
    // Modify theString here.
}

Of course, what you do in cutString could cause more problems - you should include that function in your code above so we can check it out.