I'm using a customised C++ implementation of ArrayList based on the method found in Processing. The original code is by Obed Isai Rios, but I've added a version to Github with more explicit file structure. The arraylist is appended to by the add_string_item method, which works ok in the format arraylist->add_string_item("text string") but fails when the same input is a String variable. The following script illustrates:

#include "ArrayList.h"
void setup(){
  ArrayList *list = new ArrayList("text1");
  list->add_string_item("text2");
  String str = "text3";
  list->add_string_item(str);
}
void loop(){}

This fails on the last line with error message:

no matching function for call to 'ArrayList::add_string_item(String&)'

sketch_aug24b.ino: In function 'void setup()':
sketch_aug24b:7: error: no matching function for call to 'ArrayList::add_string_item(String&)'
/Users/robinedwards/Documents/Arduino/libraries/ArrayList/ArrayList.h:17: note: candidates are: void ArrayList::add_string_item(char*)

I'm not familiar with C++ so can't figure out why this method accepts a string "str" but not when assigned to a String object. I'm pretty sure it's because I need to coerce to char* but not sure how to do this. I've tried toCharArray without success, although this seems the right direction. Very grateful for any assistance.

ArrayList.cpp code:

#include "Arduino.h"
#include "ArrayList.h"
 
ArrayList::ArrayList(char* init){
  stringlist = (char**)malloc(10*sizeof(char*));
  stringlist[0] = init;
  this->size = 1; 
}
 
ArrayList::~ArrayList(void)
{
}
 
void ArrayList::add_string_item(char* item){
  char **neulist = (char**)malloc((size+1)*sizeof(char*));
  for(int i=0; i<size; i++){
    neulist[i] = stringlist[i];
   }
   //
   neulist[size] = item;
   stringlist = neulist;
   size = size + 1;
}
 
void ArrayList::set_string_item(char* item, int index){
  stringlist[index] = item;
}
 
void ArrayList::remove_selected_item(int index){
  char **neulist = (char**)malloc((size-1)*sizeof(char*));
  //From Begining
  for(int i=0; i<index; i++){
    neulist[i] = stringlist[i]; 
  }
  //From next Index  
  for(int i=index; i<=size-1; i++){
    neulist[i] = stringlist[i+1];
  }
  //free(matrix);
  stringlist = neulist;
  size = size - 1;
}
 
void ArrayList::empty_list(){
   size = 1;
   char **neulist = (char**)malloc((size)*sizeof(char*));   
   stringlist = neulist;
   stringlist[0] = "EMPTY";
}
 
void ArrayList::display_string_list(){
   Serial.begin(9600);
  for(int i=0; i<size; i++){
    Serial.println(stringlist[i]);
   }
}
 
char** ArrayList::get_stringlist(){
  return this->stringlist;
}
 
char* ArrayList::get_item(int index){
  return this->stringlist[index];
}
 
void ArrayList::set_stringlist(char** stringlist){
  this->stringlist = stringlist;
}
 
int ArrayList::get_size(){
  return this->size;
}