Re: [Jsoncpp-devel] Backwards incompatible change in jsoncpp 64-bit support

[Stephan B. <sg...@go...>]

On Wed, May 25, 2011 at 4:56 PM, Baptiste Lepilleur <
bap...@gm...> wrote:

>
>> This is that feature that allows the following usage:
>
> Json::Value array;
> array.append( 1234 );
>

That means a null value is effectively mutable, and only arrays/objects are
mutable in JS (JSON doesn't describe mutability, and it has no operators
which would allow mutation).


> Maybe we should introduce a distinction between "default" initialization
> and "null", but I'm not sure how to go about that...
>

Absolutely. In the 2 JSON libs i've implemented (one in C++, one in C), the
following values all have a corresponding static/constant Value instance
(mainly as a memory allocation optimization):

true
false
null
"" (empty string)
undefined (not used by JSON, but as a placeholder for default-initialized
values)
double 0.0
integer 0

(the last two are internal optimizations, and not generally all that useful)

A default-constructed Value has the undefined value:

Value v; // v.isUndef() will return true
v = Object(); // now v.isObject() will return true

If someone wants a Null value, they get a reference to the shared instance:

Value v = Value::Null();

This requires no allocation (the object instances are static/const), and
because values are all immutable (except array/object, of course), this is
safe in terms of "nobody can change the value of Null by assigning over a
reference to it."

i don't know if the jsoncpp code is structured such that such constant
values would be immediately implementable, though.

-- 
----- stephan beal
http://wanderinghorse.net/home/stephan/