What's the difference between Arduino(original) and ESP32 WString source,
and which is a better source for ARM based Arduinos
You can compare the sources 
I doubt there is a significant difference.
If you add an ARM based Arduino in the IDE, everything should be taken care of for you.
I just did a diff and there are some significant differences. For example, here's Arduino's changeBuffer:
unsigned char String::changeBuffer(unsigned int maxStrLen)
{
char *newbuffer = (char *)realloc(buffer, maxStrLen + 1);
if (newbuffer) {
buffer = newbuffer;
capacity = maxStrLen;
return 1;
}
return 0;
}
Here's ESP32:
unsigned char String::changeBuffer(unsigned int maxStrLen) {
// Can we use SSO here to avoid allocation?
if (maxStrLen < sizeof(sso_buf)) {
if (sso() || !buffer()) {
// Already using SSO, nothing to do
setSSO(true);
return 1;
} else { // if bufptr && !sso()
// Using bufptr, need to shrink into sso_buff
char temp[sizeof(sso_buf)];
memcpy(temp, buffer(), maxStrLen);
free(wbuffer());
setSSO(true);
memcpy(wbuffer(), temp, maxStrLen);
return 1;
}
}
// Fallthrough to normal allocator
size_t newSize = (maxStrLen + 16) & (~0xf);
// Make sure we can fit newsize in the buffer
if (newSize > CAPACITY_MAX) {
return false;
}
uint16_t oldLen = len();
char *newbuffer = (char *) realloc(sso() ? nullptr : wbuffer(), newSize);
if(newbuffer) {
size_t oldSize = capacity() + 1; // include NULL.
if (sso()) {
// Copy the SSO buffer into allocated space
memcpy(newbuffer, sso_buf, sizeof(sso_buf));
}
if (newSize > oldSize)
{
memset(newbuffer + oldSize, 0, newSize - oldSize);
}
setSSO(false);
setCapacity(newSize - 1);
setLen(oldLen); // Needed in case of SSO where len() never existed
setBuffer(newbuffer);
return 1;
}
return 0;
}
I see Arduino put WString.cpp in ArduinoCore-API (renamed as String.cpp):
The files in ArduinoCore-API are the non-architecture specific parts of the core library and should be able to be used for any hardware core, whether Arduino's or 3rd party. I wonder if it was a mistake to put that code in ArduinoCore-API?
some libraries (as Azure SAS or Google IoT core - MQTT password) use:
return /String/ "URL" + urlEncode(url) + "expired=" + String(exp);
and I have problems with String concat and returning Strings ... and looking for solutions or where is the problem
EDIT: 3rd party (my port, ARM based) and I use original Arduino WString source...
"URL" is not a string but a const char*. Const char* does not have concat method.
Try:
String("URL") + ...
The function return result as String
This concat make problem (return empty string)
for malloc and free I make stress-test - not have problems
My advice is to write simplified programs until you are able to create one that reproduces the problem with the minimum amount of code. For example:
void setup() {
Serial.begin(9600);
while (!Serial) {}
Serial.println("foo" + String("bar"));
}
void loop() {}
problem is elsewhere ... WString work fine
The Azure library use VLA array and and maybe need settings fo C99 - will test
I make "simple" Arduino simulator for "debug"
wow - I even like it ... I will add libraries and release it to GitHub
