6.13.0 broken for non-copy-constructable String types

[mikee47]

With commit 6da6f92 both ElementProxy and MemberProxy take copies of the
TString argument instead of taking a reference.

This breaks existing code which depends upon the referencing behaviour, and also has performance/efficiency implications.

The fault addressed by the above commit is briefly outlined in #1120, caused by a dangling reference. To clarify, this is where the ElementProxy/MemberProxy take a reference to a temporary object (e.g. arduino String) which the compiler optimises out before the reference is used. This leads to unpredictable behaviour, so clearly taking proper copies is the safest (default) behaviour.

However, I have a custom FlashString class (with adapter) which is not copy-constructible and so now no longer works as intended.

Here are the details:

• FlashString
• ArduinoJson FlashStringRefAdapter

I've been unable to find a workable solution without modifying ArduinoJson so I'll propose that and see where we go.

It feels like there's room for improvement here. Can we detect when it's safe to take a reference, and do that when possible?

[bblanchon]

Hi @mikee47,

I'm sorry I break your code; I didn't see that coming.
As you understood, I had to make this change to fix #1120, and I believe it was the right fix.

Mike, why is the FlashString non-copiable?

Best Regards,
Benoit

[mikee47]

It's used to access counted strings stored in flash memory by casting as a reference. i.e. reinterpret_cast<const FlashString&>(&struc) where struc represents the in-flash data. The data is potentially very large (one use is to directly import file data at compile time) so copying it makes no sense, though obviously such strings would never be used with a JSON document.

I am looking at a revision which uses FlashString a simple wrapper class, and if that make it as easy (or easier) to use then the problem goes away.

[mikee47]

@bblanchon Just to let you know I've fixed the incompatibility problem by adding copy constructor support. Inspired by your own libraries, I've created FlashString. Thank you for setting the standard!