Missing 'const'-ness of 'root' in json_unpack() about jansson HOT 5 CLOSED

Hi and thanks for the report.

The 'o' format allows you to extract a raw json_t *, which can then possibly be changed by the user (by using e.g. json_string_set_value()). When using the 'O' format, the reference count of the extracted value is increased, so even json_unpack() itself may modify root or value contained in it. For these reasons, root cannot be const.

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Oh, I got you. Nevertheless, that is a subtle constraint because simple tree reading couldn't be achieved in const-methods if you wanted to use json_t* as a member of a class (i.e. a permanent state handler). Only temporary usage would be possible (my case btw). You could consider using some kind of 'mutable' or 'const_cast' analog for reference couters to avoid that. Anyway, thanks again!
P.S. Can't wait for a version with 'json_dump_callback' to come out!:)

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You could consider using some kind of 'mutable' or 'const_cast' analog for reference couters to avoid that.

What do you mean? AFAICS, the only possibility would be to have a separate json_unpack_const() that doesn't have the O format, and would extract o items as const json_t *. The latter is subtle, of course, as there's no way to determine the types of variable arguments in the vararg function.

Can't wait for a version with 'json_dump_callback' to come out!:)

I'll try to hurry with that :)

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Well, it would be enough to have two signatures of just one json_unpack() function if it were C++.:) Sad. However, json_unpack_const() looks pretty good to me anyway. The main goal would be to give some suitable way to extract data from constant states as the json_dump() does that while encoding. Up to you, of course.:) Thanks for the great work! I am on the way to give it a try under really heavy load...

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I think the best would be to document why the argument is not const, and state it explicitly that (json_t *)const_variable is safe as long as the o and espcially O formats are not used.

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