Marking Features as Deprecated

In the API headers, a feature (a class, method, function, enum, typedef, etc) can be marked as deprecated by using the XAPIAN_DEPRECATED() or XAPIAN_DEPRECATED_CLASS macros. Note that you can’t deprecate a preprocessor macro.

For compilers with a suitable mechanism (such as GCC, clang and MSVC) this causes compile-time warning messages to be emitted for any use of the deprecated feature. For compilers without support, the macro just expands to its argument.

Sometimes a deprecated feature will also be removed from the library itself (particularly something like a typedef), but if the feature is still used inside the library (for example, so we can define class methods), then use XAPIAN_DEPRECATED_EX() or XAPIAN_DEPRECATED_CLASS_EX instead, which will only issue a warning in user code (this relies on user code including xapian.h and library code including individual headers)

You must add this line to any API header which uses XAPIAN_DEPRECATED() or XAPIAN_DEPRECATED_CLASS:

#include <xapian/deprecated.h>

When marking a feature as deprecated, document the deprecation in docs/deprecation.rst. When actually removing deprecated features, please tidy up by removing the inclusion of <xapian/deprecated.h> from any file which no longer marks any features as deprecated.

The XAPIAN_DEPRECATED() macro should wrap the whole declaration except for the semicolon and any “definition” part, for example:

XAPIAN_DEPRECATED(int old_function(double arg));

class Foo {
    XAPIAN_DEPRECATED(int old_method());

    XAPIAN_DEPRECATED(int old_const_method() const);

    XAPIAN_DEPRECATED(virtual int old_virt_method()) = 0;

    XAPIAN_DEPRECATED(static int old_static_method());

    XAPIAN_DEPRECATED(static const int OLD_CONSTANT) = 42;

Mark a class as deprecated by inserting XAPIAN_DEPRECATED_CLASS after the class keyword like so:

    Foo() { }

    // ...

You can simply mark a method defined inline in a class with XAPIAN_DEPRECATED() like so:

class Foo {
    // This failed to compile with GCC 3.3.5.
    XAPIAN_DEPRECATED(int old_inline_method()) { return 42; }

Implementing Deprecation Warnings for the Bindings

Currently we don’t have an equivalent of the C++ XAPIAN_DEPRECATED() macro for the bindings, but it would be good to have. Here are some notes on how this could be achieved for various languages we support:

  • PHP now has a E_USER_DEPRECATED error level - in a deprecated method we could do:

        'World::hi() is deprecated, use World::hello() instead',
  • Python has DeprecationWarning, which we were using in 1.2.x a bit:

        'World::hi() is deprecated, use World::hello() instead',
  • Ruby - there are external libraries to handle deprecation warnings, but the simplest option without external dependencies seems to be:

    warn "[DEPRECATION] World::hi() is deprecated, use World::hello() instead"
  • Perl:

    use warnings;
    warnings::warnif('deprecated', 'World::hi() is deprecated, use World::hello() instead');
  • Java has an annotation, @Deprecated, which can be used to indicate deprecation.

It would be great (but probably hard) to reuse the XAPIAN_DEPRECATION() markers. Perhaps parsing the doxygen XML for @deprecated markers would be simpler?

Also, it would be good if the warnings could be turned off easily, as runtime deprecation warnings can be annoying for end users.