C Convert cases of strings (tolower/toupper)

Two functions, used to convert c++ strings from upper case to lowercase and vice versa. Each takes a string argument, and returns the converted string. Please note that parameters should be validated before passing them to the functions.

string StringToUpper(string strToConvert)
{
   //change each element of the string to upper case
   for(unsigned int i=0;i<strToConvert.length();i++)
   {
      strToConvert[i] = toupper(strToConvert[i]);
   }
   return strToConvert;//return the converted string
}

string StringToLower(string strToConvert)
{
   //change each element of the string to lower case
   for(unsigned int i=0;i<strToConvert.length();i++)
   {
      strToConvert[i] = tolower(strToConvert[i]);
   }
   return strToConvert;//return the converted string
}