The question is quite abstract, but I'll try to answer.

The concept you might need is called “Promise” (or “Future”). The idea is that instead of returning chosen value, get_dialogue_choice immediately returns an object, namely “promise object”. Think of it as a box with a value (instead of the value).

The trick is that the box is empty when the function is returned - the player haven't chosen the value yet. The value actually appears when the choice is made.

How the value is delivered there; how the script is notified about that and how it actually behaves - is up to the implementation. Usually, promise objects can be queried for the existence of the value (.is_ready() - “box is ready, and has a value inside?” ); and queried for the value itself (.get() – “unpack the box”).

Now you can see how a promise-compatible “if (x == “constant”) { return A; } else { return B; }” function would look like and how. It would accept a boxed object (x); return a boxed object (containing either A or B), but which one exactly is not decided upon return. Once x is resolved; then this box can be resolved as well, becoming A or B (and propagating resolution chain further).

This pattern lets your code execute immediately (storing a chain of “what to do next” list) and actually perform the code later.

Implementing it is a nice exercise, but I suggest you not to try to apply all above “as is” in C++. I would advice you using an existing scripting system that supports what you want (if still want to be fancy: wrangle JS engine, for example V8, to do that: promises are common in JS, for example your trusty fetch returns one)

I recommend looking into aspromise that romanpunia implemented.

But if you don't want to use that, a simple implementation can be done like this:

// In AngelScript:
int get_dialogue_choice()
{
	suspend();
	return retrieve_dialogue_choice();
}

The get_dialogue_choice script function can be implemented by default by your application so the script writer doesn't have to know it is a script function it is calling.

The functions suspend and retrieve_dialogue_choice would be implemented in your application and registered with the script engine.

suspend simply calls asGetActiveContext()->Suspend() after which the script execution would be suspended and the asIScriptContext::Execute returns. Your application then continues the control loop and once the user makes a choice in the dialogue the choice is written to a variable, which can get read and returned by the retrieve_dialog_choice function (alternatively you can just register the variable with the engine directly).

Regards,
Andreas