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# core.attribute

This module contains UDA's (User Defined Attributes) either used in the runtime or special UDA's recognized by compiler.
License:
Boost License 1.0
Authors:
Jacob Carlborg

Source core/attribute.d

struct `selector`;
  Use this attribute to attach an Objective-C selector to a method.
  This is a special compiler recognized attribute, it has several requirements, which all will be enforced by the compiler:
  - The attribute can only be attached to methods or constructors which have Objective-C linkage. That is, a method or a constructor in a class or interface declared as

        extern(Objective-C)

    .
  ,
  - It cannot be attached to a method or constructor that is a template
  ,
  - The number of colons in the string need to match the number of arguments the method accept.
  ,
  - It can only be used once in a method declaration
  Examples:

      extern (Objective-C)
      class NSObject
      {
       this() @selector("init");
       static NSObject alloc() @selector("alloc");
       NSObject initWithUTF8String(in char* str) @selector("initWithUTF8String:");
       ObjcObject copyScriptingValue(ObjcObject value, NSString key, NSDictionary properties)
           @selector("copyScriptingValue:forKey:withProperties:");
      }

enum `optional`;
  Use this attribute to make an Objective-C interface method optional.
  An optional method is a method that does not have to be implemented in the class that implements the interface. To safely call an optional method, a runtime check should be performed to make sure the receiver implements the method.
  This is a special compiler recognized attribute, it has several requirements, which all will be enforced by the compiler:
  - The attribute can only be attached to methods which have Objective-C linkage. That is, a method inside an interface declared as extern (Objective-C)
  - It can only be used for methods that are declared inside an interface
  - It can only be used once in a method declaration
  - It cannot be attached to a method that is a template
  Examples:

      import core.attribute : optional, selector;

      extern (Objective-C):

      struct objc_selector;
      alias SEL = objc_selector*;

      SEL sel_registerName(in char* str);

      extern class NSObject
      {
          bool respondsToSelector(SEL sel) @selector("respondsToSelector:");
      }

      interface Foo
      {
          @optional void foo() @selector("foo");
          @optional void bar() @selector("bar");
      }

      class Bar : NSObject
      {
          static Bar alloc() @selector("alloc");
          Bar init() @selector("init");

          void bar() @selector("bar")
          {
          }
      }

      extern (D) void main()
      {
          auto bar = Bar.alloc.init;

          if (bar.respondsToSelector(sel_registerName("bar")))
              bar.bar();
      }

struct `gnuAbiTag`;
  Use this attribute to declare an ABI tag on a C++ symbol.
  ABI tag is an attribute introduced by the GNU C++ compiler. It modifies the mangled name of the symbol to incorporate the tag name, in order to distinguish from an earlier version with a different ABI.
  This is a special compiler recognized attribute, it has a few requirements, which all will be enforced by the compiler:
  - There can only be one such attribute per symbol.
  ,
  - The attribute can only be attached to an extern(C++) symbol (struct, class, enum, function, and their templated counterparts).
  ,
  - The attribute cannot be applied to C++ namespaces. This is to prevent confusion with the C++ semantic, which allows it to be applied to namespaces.
  ,
  - The string arguments must only contain valid characters for C++ name mangling which currently include alphanumerics and the underscore character.
  ,
  This UDA is not transitive, and inner scope do not inherit outer scopes' ABI tag. See examples below for how to translate a C++ declaration to D. Also note that entries in this UDA will be automatically sorted alphabetically, hence `gnuAbiTag`("c", "b", "a") will appear as @`gnuAbiTag`("a", "b", "c").
  See Also:
  Itanium ABI spec GCC attributes documentation.
  Examples:

      // ---- foo.cpp
      struct [[gnu::abi_tag ("tag1", "tag2")]] Tagged1_2
      {
          struct [[gnu::abi_tag ("tag3")]] Tagged3
          {
              [[gnu::abi_tag ("tag4")]]
              int Tagged4 () { return 42; }
          }
      }
      Tagged1_2 inst1;
      // ---- foo.d
      @gnuAbiTag("tag1", "tag2") struct Tagged1_2
      {
          // Notice the repetition
          @gnuAbiTag("tag1", "tag2", "tag3") struct Tagged3
          {
              @gnuAbiTag("tag1", "tag2", "tag3", "tag4") int Tagged4 ();
          }
      }
      extern __gshared Tagged1_2 inst1;

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