In plugin CTOR:

gistfile1.txt

  if (sInstances == 0 && sSettings == nullptr)
  {
    WDL_String settingsFolder;
    AppSupportPath(settingsFolder, false);

 #ifdef OS_MAC
    settingsFolder.AppendFormatted(MAX_PATH, "/%s", BUNDLE_NAME);
#elif defined(OS_WIN)
    settingsFolder.AppendFormatted(MAX_PATH, "\\%s", BUNDLE_NAME);
#endif

    sSettings = new NAMPersistance(settingsFolder.Get());
  }
  
  sInstances++;

NeuralAmpModelerPersistance.h

#pragma once

#include "IPlugPlatform.h"

#if defined OS_MAC || defined OS_IOS || defined OS_LINUX
  #include "IPlugSWELL.h"
#endif
#include "wdlstring.h"
#include "mutex.h"


class NAMPersistance
{
public:
  enum EThingsToPersist
  {
    kModelDirectory,
    kIRDirectory,
    kNumThingsToPersist,
  };

  const char* tags[2] = {"ModelDirectory", "kIRDirectory"};
  
  NAMPersistance(const char* settingsFolder);
  void Init();
  void UpdateINI();
  WDL_String* Get(EThingsToPersist thing);
  void Set(EThingsToPersist thing, const char* str);
  
private:
  WDL_String mDefaults[kNumThingsToPersist];
  WDL_String mItems[kNumThingsToPersist];
  WDL_String mINIPath;
  WDL_Mutex mMutex;
};

NeuralAmpModlerPersistance.cpp

#include "IPlugLogger.h"
#include "NeuralAmpModelerPersistance.h"

#include <sys/stat.h>

static const int BUFFER_SIZE = 256;

NAMPersistance::NAMPersistance(const char* settingsFolder)
{
  WDL_String defaultModelPath, defaultIRPath;

#if defined OS_MAC
  defaultModelPath.Set(settingsFolder);
  defaultModelPath.AppendFormatted(MAX_PATH, "/Models");
//  defaultIRPath = ...
#elif defined OS_IOS
  defaultModelPath.Set(settingsFolder);
  defaultModelPath.AppendFormatted(MAX_PATH, "/Models");
#elif defined OS_WIN
  char strPath[MAX_PATH];
  ExpandEnvironmentStrings("%ProgramFiles%", strPath, ARRAYSIZE(strPath));
  defaultModelPath.AppendFormatted(MAX_PATH, "%s\\NeuralAmpModeler\\Models", strPath);
#endif
  auto PREF_FILE_NAME = "NeuralAmpModeler-settings.ini";

  mDefaults[kModelDirectory].Set(&defaultModelPath);
  mItems[kModelDirectory].Set(mDefaults[kModelDirectory].Get(), BUFFER_SIZE);
  mDefaults[kIRDirectory].Set(&defaultIRPath);
  mItems[kIRDirectory].Set(mDefaults[kIRDirectory].Get(), BUFFER_SIZE);

  mINIPath.Set(settingsFolder);
#if defined OS_MAC || defined OS_IOS
  mINIPath.AppendFormatted(MAX_PATH, "/%s", PREF_FILE_NAME);
#elif defined OS_WIN
  mINIPath.AppendFormatted(MAX_PATH, "\\%s", PREF_FILE_NAME);
#endif
  
  struct stat st;
  if (stat(settingsFolder, &st) == 0) // if NAM directory exists
  {
    Init();
  }
  else // folder doesn't exist - make folder
  {
    if (CreateDirectory(settingsFolder, NULL) == 0)
    {
      DBGMSG("error - couldn't create settings folder %s\n", settingsFolder);
      return;
    }
  }
  
  UpdateINI(); // will write file if doesn't exist
}

void NAMPersistance::Init()
{
  WDL_MutexLock lock(&mMutex);

  char buf[BUFFER_SIZE]; // temp buffer for reading profile strings

  struct stat st;
  if (stat(mINIPath.Get(), &st) == 0) // if settings file exists read values into state
  {
    //CHECK PATHS
    for (int i = 0; i < kNumThingsToPersist; i++)
    {
      GetPrivateProfileString("paths", tags[i], mDefaults[i].Get(), buf, BUFFER_SIZE, mINIPath.Get());

      //take off trailing slash
#if defined OS_MAC
      if (buf[strlen(buf)-1] == '/')
      {
        buf[strlen(buf)-1] = 0;
      }
#elif defined OS_WIN
      if (buf[strlen(buf)-1] == '\\')
      {
        buf[strlen(buf)-1] = 0;
      }
#endif

      if (stat(buf, &st) == 0)
      {
        mItems[i].Set(buf);
      }
      else
      {
        DBGMSG("invalid %s value in ini, setting to default\n", tags[i]);
        mItems[i].Set(mDefaults[i].Get());
      }
    }
  }

  UpdateINI();
}

void NAMPersistance::UpdateINI()
{  
  WDL_MutexLock lock(&mMutex);
  for (int i = 0; i< kNumThingsToPersist; i++)
  {
    WritePrivateProfileString("paths", tags[i], mItems[i].Get(), mINIPath.Get());
  }
}

WDL_String* NAMPersistance::Get(EThingsToPersist thing)
{
  WDL_MutexLock lock(&mMutex);
  char buf[BUFFER_SIZE]; // temp buffer for reading profile strings
  GetPrivateProfileString("paths", tags[thing], mDefaults[thing].Get(), buf, BUFFER_SIZE, mINIPath.Get());
  
  mItems[thing].Set(buf);

  return &mItems[thing];
}

void NAMPersistance::Set(EThingsToPersist thing, const char* str)
{
  WDL_MutexLock lock(&mMutex);

  mItems[thing].Set(str);

  WritePrivateProfileString("paths", tags[thing], mItems[thing].Get(), mINIPath.Get());
}