ExpressionEvaluator | Simple Math and Pseudo C | Script Programming library

after a long time debugging with this pain in the ass. I finally found the solution to this. (But I do not clearly understand why it works :) ). I just want to post my answer here so if anyone has faced this issue like me can find a solution.

As you can see I have a function called ExpressionEvaluator::evaluate, this function will be involved by multithreaded. Everything is working fine with Qt 5.9.6 but when I upgraded to Qt 5.15.2 the application often crash and the stack trace is pointed to around QJsEngine with the line d->engine.globalObject() For my research, maybe this is the issue:

• QJsEngine have a function called gc for garbage collection
• gc will run with the thread the owned the engine.
• Whenever another thread is trying the access globalObject() or any function (like evaluate()) of QJsEngine and if gc is called at the same time => crash

And yeah like I said before, I do not truly understand the issue but with this guess, I was able to create a solution and it works!

Here is my solution:

• I create another thread (event loop thread ) and QJsEngine will live in that thread (also all of its function will be executed on this thread)
• When another thread invoked evaluate() it will emit a signal and that signal is connected to the slot (the slot will do the work - and of course, this slot is executed in only one thread)
• This solution makes sure: gc and all the functions will be executed on the same thread.

P/s: Sorry if my explanation is hard to understand but this issue is kind of specific and it is stick with my application so this is the best I can do. But for the summary: make sure all the function of QJsEngine is called in the same thread

You could use a simple recursive function.

• If the current object has OR key, then check if some of the items in the array are truthy.
• If AND, check if every item is truthy.
• If one of the item in the array is an object, recursively call the function on the object to get its value

There are many approaches, but since you want a generic approach, I advise you to use MiscUtil it's a generic library which has generic operators (Add, Subtract..etc). You can use it in your code, and it will ease things for you.

For operators, there is no need to create a class for each operator, as their functionalities are one-to-one. For instance, Addition class, will only contain addition operator (a + b). There is no complex math to it. So, what I suggest is to use method for these operators instead. and focus on the main class. The main class can contain many operators as needed. Some are simple (like addition, subtract ..etc.) others might be complex. For the simple operators, keep it simple, while for the complex operators, you might need to implement some classes for them to simplify them.

For the implementation design itself, I think Fluent API would be more beneficial in your case. So, imagine the library usage would be something like this :

The error says "source is null for getProperty(null, "roleID")" which means the role is null.

Adding a null check should fix the problem.