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Re: [Qemu-devel] [RFC 02/15] qobject: allow NULL for qstring_get_str()


From: Eric Blake
Subject: Re: [Qemu-devel] [RFC 02/15] qobject: allow NULL for qstring_get_str()
Date: Tue, 19 Sep 2017 15:48:35 -0500
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.3.0

On 09/14/2017 02:50 AM, Peter Xu wrote:
> Then I can get NULL rather than crash when calling things like:
> 
>   qstring_get_str(qobject_to_qstring(object));
> 
> when key does not exist.

Right now, qdict_get_str() is documented as:

 * This function assumes that 'key' exists and it stores a
 * QString object.

Your code changes that, by making it now return NULL instead of crashing
on what used to be usage in violation of the contract; compared to
qdict_get_try_str() which gracefully returns NULL, but which could use
your new semantics for doing so in fewer lines of code.

I'm not necessarily opposed to the change, but I worry that it has
subtle ramifications that we haven't thought about, as well as
consistency with the rest of the QObject APIs.  It may be better to just
introduce qstring_get_try_str(), which gracefully handles NULL input,
and leave the existing function alone; and if you do introduce a new
helper, it may be worth converting existing clients (perhaps with help
from Coccinelle) to take advantage of the helper.

> +++ b/qobject/qstring.c
> @@ -125,7 +125,7 @@ QString *qobject_to_qstring(const QObject *obj)
>   */
>  const char *qstring_get_str(const QString *qstring)
>  {
> -    return qstring->string;
> +    return qstring ? qstring->string : NULL;
>  }
>  
>  /**
> 

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
Virtualization:  qemu.org | libvirt.org

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