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gerror: Document the disadvantages of using GError

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And move the discussion to a new subsection in the GError documentation.
Follow-up from commit 04662a8667.

https://bugzilla.gnome.org/show_bug.cgi?id=743011
This commit is contained in:
Philip Withnall 2015-09-28 13:23:29 +01:00
parent 04662a8667
commit de04fd1304
1 changed files with 24 additions and 7 deletions
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31
glib/gerror.c
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@ -33,7 +33,9 @@
* this method is both a data type (the #GError struct) and a [set of * this method is both a data type (the #GError struct) and a [set of
* rules][gerror-rules]. If you use #GError incorrectly, then your code will not * rules][gerror-rules]. If you use #GError incorrectly, then your code will not
* properly interoperate with other code that uses #GError, and users * properly interoperate with other code that uses #GError, and users
* of your API will probably get confused. * of your API will probably get confused. In most cases, [using #GError is
* preferred over numeric error codes][gerror-comparison], but there are
* situations where numeric error codes are useful for performance.
* *
* First and foremost: #GError should only be used to report recoverable * First and foremost: #GError should only be used to report recoverable
* runtime errors, never to report programming errors. If the programmer * runtime errors, never to report programming errors. If the programmer
@ -51,12 +53,6 @@
* be eliminated by fixing the bug in the program. This is why most * be eliminated by fixing the bug in the program. This is why most
* functions in GLib and GTK+ do not use the #GError facility. * functions in GLib and GTK+ do not use the #GError facility.
* *
* #GError has several advantages over numeric error codes: importantly, tools
* like [gobject-introspection](https://developer.gnome.org/gi/stable/)
* understand #GErrors and convert them to exceptions in bindings; the message
* includes more information than just a code; and use of a domain helps prevent
* misinterpretation of error codes.
*
* Functions that can fail take a return location for a #GError as their * Functions that can fail take a return location for a #GError as their
* last argument. On error, a new #GError instance will be allocated and * last argument. On error, a new #GError instance will be allocated and
* returned to the caller via this argument. For example: * returned to the caller via this argument. For example:
@ -285,6 +281,27 @@
* instead treat any unrecognized error code as equivalent to * instead treat any unrecognized error code as equivalent to
* FAILED. * FAILED.
* *
* ## Comparison of #GError and traditional error handling # {#gerror-comparison}
*
* #GError has several advantages over traditional numeric error codes:
* importantly, tools like
* [gobject-introspection](https://developer.gnome.org/gi/stable/) understand
* #GErrors and convert them to exceptions in bindings; the message includes
* more information than just a code; and use of a domain helps prevent
* misinterpretation of error codes.
*
* #GError has disadvantages though: it requires a memory allocation, and
* formatting the error message string has a performance overhead. This makes it
* unsuitable for use in retry loops where errors are a common case, rather than
* being unusual. For example, using %G_IO_ERROR_WOULD_BLOCK means hitting these
* overheads in the normal control flow. String formatting overhead can be
* eliminated by using g_set_error_literal() in some cases.
*
* These performance issues can be compounded if a function wraps the #GErrors
* returned by the functions it calls: this multiplies the number of allocations
* and string formatting operations. This can be partially mitigated by using
* g_prefix_error().
*
* ## Rules for use of #GError # {#gerror-rules} * ## Rules for use of #GError # {#gerror-rules}
* *
* Summary of rules for use of #GError: * Summary of rules for use of #GError: