From: Lyle Johnson <ljohnson@...>
Date: 2001-03-23T23:40:02+09:00
Subject: [ruby-talk:13138] How would Ruby say this?

One group of C++ functions I'm wrapping for FXRuby have signatures like
this:

    bool getValue(double& result, const char* prompt);

and in C++ you would call them like so:

    double result = 42.0;
    if (getValue(result, "What's the frequency, Kenneth?") == true) {
        printf("The user responds: %g\n", result);
    } else {
        printf("The user cancelled without responding.\n");
    }

That is to say, it stores the primary return value in the "result" argument
(which is often done with C++ pointers or references) but it also returns a
boolean status code to indicate whether the operation succeeded or not.

The first obvious option for exposing this function in Ruby is to return
both values as a two-element array, e.g.

    initial = 42.0
    status, result = getValue(initial, "What it is?")
    if status
        puts "The user responds: #{result}"
    else
        puts "The user cancelled without responding"
    end

But this is awkward. Another option I'm considering is to have the function
raise an exception for the "false" return status:

    begin
        initial = 42.0
        result = getValue(initial, "What it is?")
        puts "The user responds: #{result}"
    rescue CancelError
        puts "The user cancelled without responding"
    end

This seems a little cleaner to me but I'm still interested in hearing what
the "Ruby" way is to handle this. A lot of the standard library classes
return "nil" on failure (e.g. attempting to access a non-existent array
index or hash key); is that the preferred way of handling this?

Any comments or suggestions appreciated,

Lyle