Add extended information to GErrors in GLib 2.67.2

Thanks to Krzesimir Nowak, a 17-year-old feature request in GLib has been implemented: it’s now possible to define GError domains which have extended information attached to their GErrors.

You could now, for example, define a GError domain for text parser errors which includes context information about a parsing failure, such as the current line and character position. Or attach the filename of a file which was being read, to the GError informing of a read failure. Define an extended error domain using G_DEFINE_EXTENDED_ERROR(). The extended information is stored in a ‘private’ struct provided by you, similarly to how it’s implemented for GObjects with G_DEFINE_TYPE_WITH_PRIVATE().

There are code examples on how to use the new APIs in the GLib documentation, so I won’t reproduce them here.

An important limitation to note is that existing GError domains which have ever been part of a stable public API cannot be extended retroactively unless you are breaking ABI. That’s because extending a GError domain increases the size of the allocated GError instances for that domain, and it’s possible that users of your API will have stack-allocated GErrors in the past.

Please don’t stack-allocate GErrors, as it makes future extensions of the API impossible, and doesn’t buy you notable extra performance, as GErrors should not be used on fast paths. By their very nature, they’re for failure reporting.

The new APIs are currently unstable, so please try them out and provide feedback now. They will be frozen with the release of GLib 2.67.3, scheduled for 11th February 2021.

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