add auto_json_decref macro

[sjaeckel]

gcc and clang implement the compiler attribute cleanup() [1] which lets
you add a function call that is invoked as soon as a variable goes out
of scope.

This feature improves code where you retrieve a json_t object at one
point and you're not interested to keep it over a longer period of time.

[1] https://gcc.gnu.org/onlinedocs/gcc/Common-Variable-Attributes.html#index-g_t_0040code_007bcleanup_007d-variable-attribute-3485

[coveralls]

Coverage remained the same at 90.254% when pulling 9961e4a on sjaeckel:json-autodecref into 006638a on akheron:master.

[Mephistophiles]

Home immediately know all the dangerous moments, such as:

void blah (json_t * test) {
    auto_json_decref json_t * str = json_string ( "test");

    json_object_set_new_nocheck (test, "blah", str);
}

[sjaeckel]

@Mephistophiles as already said in the commit "...where you retrieve a json_t object at one
point and you're not interested to keep it over a longer period of time." and if you know that json_object_set_new_nocheck() takes ownership of str why would you add a auto_json_decref?

I just want to say: people who'd add a auto_json_decref there, would also just do json_decref(str)

[sjaeckel]

@akheron is there a chance this will be merged?

[akheron]

Yes, there definitely is a chance. My only concern is that this cannot be used in portable code, and that has to be communicated to users very clearly.

It's a shame actually. If all compilers had a way to do this, this should be the default and you would need to use an attribute to disable autodecref.

[sjaeckel]

My only concern is that this cannot be used in portable code...

That's for sure :-( Therefor it's also enclosed in #ifdef __GNUC__ ...

... and that has to be communicated to users very clearly.

Can I help you with anything there?

It's a shame actually. If all compilers had a way to do this, this should be the default and you would need to use an attribute to disable autodecref.

Yes it's a shame, but who uses a compiler that isn't based on or at least supports most of the functionality of gcc anyways? ...icc supports it and who cares about msvc... ;-)

[akheron]

Can I help you with anything there?

You could document it. That's what has to be done in any case :)

[akheron]

Superseded by #301