attribute.dd

Ddoc

$(SPEC_S Attributes,

$(GRAMMAR
$(GNAME AttributeSpecifier):
    $(GLINK Attribute) $(D :)
    $(GLINK Attribute) $(GLINK DeclarationBlock)

$(GNAME Attribute):
    $(RELATIVE_LINK2 linkage, $(I LinkageAttribute))
    $(RELATIVE_LINK2 align, $(I AlignAttribute))
    $(GLINK2 pragma, Pragma)
    $(RELATIVE_LINK2 deprecated, $(D deprecated))
    $(GLINK ProtectionAttribute)
    $(D static)
    $(RELATIVE_LINK2 linkage, $(D extern))
    $(D final)
    $(D synchronized)
    $(RELATIVE_LINK2 override, $(D override))
    $(RELATIVE_LINK2 abstract, $(D abstract))
    $(RELATIVE_LINK2 const, $(D const))
    $(RELATIVE_LINK2 auto, $(D auto))
    $(RELATIVE_LINK2 scope, $(D scope))
    $(RELATIVE_LINK2 gshared, $(D __gshared))
    $(RELATIVE_LINK2 shared, $(D shared))
    $(RELATIVE_LINK2 immutable, $(D immutable))
    $(RELATIVE_LINK2 inout, $(D inout))
    $(RELATIVE_LINK2 disable, $(D @disable))


$(GNAME DeclarationBlock):
    $(GLINK2 module, DeclDef)
    $(D {) $(GLINK2 module, DeclDefs)$(OPT) $(D })
)

        $(P Attributes are a way to modify one or more declarations.
        The general forms are:
        )

---
attribute declaration; // affects the declaration

attribute:     // affects all declarations until the end of
               // the current scope
  declaration;
  declaration;
  ...

attribute {    // affects all declarations in the block
  declaration;
  declaration;
  ...
}
---

$(H3 $(LNAME2 linkage, Linkage Attribute))

$(GRAMMAR
$(GNAME LinkageAttribute):
    $(D extern) $(D $(LPAREN)) $(GLINK LinkageType) $(D $(RPAREN))

$(GNAME LinkageType):
    $(D C)
    $(D C++)
    $(D D)
    $(D Windows)
    $(D Pascal)
    $(D System)
)

        $(P D provides an easy way to call C functions and operating
        system API functions, as compatibility with both is essential.
        The $(I LinkageType) is case sensitive, and is meant to be
        extensible by the implementation ($(D they are not keywords)).
        $(D C) and $(D D) must be supplied, the others are what
        makes sense for the implementation.
        $(D C++) is reserved for future use.
        $(D System) is the same as $(D Windows) on Windows platforms,
        and $(D C) on other platforms.
        $(D Implementation Note:)
        for Win32 platforms, $(D Windows) and $(D Pascal) should exist.
        )

        $(P C function calling conventions are
        specified by:
        )

---------------
extern (C):
  int foo(); // call foo() with C conventions
---------------

        $(P D conventions are:)

---------------
extern (D):
---------------

        $(P Windows API conventions are:)

---------------
extern (Windows):
  void *VirtualAlloc(
    void *lpAddress,
    uint dwSize,
    uint flAllocationType,
    uint flProtect
    );
---------------

        $(P Note that a lone $(D extern) declaration is used as a
        $(XLINK2 declaration.html#extern, storage class).)

$(H3 $(LNAME2 align, Align Attribute))

$(GRAMMAR
$(GNAME AlignAttribute):
    $(D align)
    $(D align) $(D $(LPAREN)) $(GLINK2 lex, IntegerLiteral) $(D $(RPAREN))
)


        $(P Specifies the alignment of:)

	$(OL
	$(LI variables)
	$(LI struct fields)
	$(LI union fields)
	$(LI class fields)
	$(LI struct, union, and class types)
	)

	$(P $(D align) by itself
        sets it to the default, which matches the default member alignment
        of the companion C compiler.)

--------
struct S {
  align:
     byte a;   // placed at offset 0
     int b;    // placed at offset 4
     long c;   // placed at offset 8
}

auto sz = S.sizeof;  // 16
--------

	$(P $(I IntegerLiteral) specifies the alignment
        which matches the behavior of the companion C compiler when non-default
        alignments are used. It must be a positive power of 2.
        )

        $(P A value of 1 means that no alignment is done;
        fields are packed together.
        )

--------
struct S {
  align (1):
     byte a;   // placed at offset 0
     int b;    // placed at offset 1
     long c;   // placed at offset 5
}

auto sz = S.sizeof;  // 16
--------

	$(P The alignment for the fields of an aggregate does not affect the alignment
	of the aggregate itself - that is affected by the alignment setting outside
	of the aggregate.)

--------
align (2) struct S {
  align (1):
     byte a;   // placed at offset 0
     int b;    // placed at offset 1
     long c;   // placed at offset 5
}

auto sz = S.sizeof;  // 14
--------

	$(P Setting the alignment of a field aligns it to that power of 2, regardless
	of the size of the field.)

--------
struct S {
  align (4):
     byte a;   // placed at offset 0
     byte b;   // placed at offset 4
     short c;  // placed at offset 8
}

auto sz = S.sizeof;  // 12
--------


        $(P Do not align references or pointers that were allocated
        using $(I NewExpression) on boundaries that are not
        a multiple of $(D size_t). The garbage collector assumes that pointers
        and references to gc allocated objects will be on $(D size_t)
        byte boundaries. If they are not, undefined behavior will
        result.
        )

        $(P The $(I AlignAttribute) is reset to the default when
        entering a function scope or a non-anonymous struct, union, class, and restored
        when exiting that scope.
        It is not inherited from a base class.
        )



$(H3 $(LNAME2 deprecated, Deprecated Attribute))

        $(P It is often necessary to deprecate a feature in a library,
        yet retain it for backwards compatibility. Such
        declarations can be marked as deprecated, which means
        that the compiler can be set to produce an error
        if any code refers to deprecated
        declarations:
        )

---------------
deprecated
{
  void oldFoo();
}
---------------

        $(P $(D Implementation Note:) The compiler should have a switch
        specifying if deprecated declarations should be compiled with
        out complaint or not.
        )


$(H3 Protection Attribute)

$(GRAMMAR
$(GNAME ProtectionAttribute):
    $(D private)
    $(D package)
    $(D protected)
    $(D public)
    $(D export)
)

        $(P Protection is an attribute that is one of
        $(D private), $(D package), $(D protected),
        $(D public) or $(D export).
        )

        $(P Private means that only members of the enclosing class can access
        the member, or members and functions in the same module as the
        enclosing class.
        Private members cannot be overridden.
        Private module members are equivalent to $(D static) declarations
        in C programs.
        )

        $(P Package extends private so that package members can be accessed
        from code in other modules that are in the same package.
        This applies to the innermost package only, if a module is in
        nested packages.
        )

        $(P Protected means that only members of the enclosing class or any
        classes derived from that class,
        or members and functions in the same module
        as the enclosing class, can access the member.
        If accessing a protected instance member through a derived class member
        function,
        that member can only be accessed for the object instance
        which can be implicitly cast to the same type as $(SINGLEQUOTE this).
        Protected module members are illegal.
        )

        $(P Public means that any code within the executable can access the member.
        )

        $(P Export means that any code outside the executable can access the
        member. Export
        is analogous to exporting definitions from a DLL.
        )

        $(P Protection does not participate in name lookup.
        In particular, if two symbols with the same name are in scope,
        and that name is used unqualified then the lookup will be ambiguous,
        even if one of the symbols is inaccessible due to protection.
        For example:
        )

---------------
module A;
private class Foo {}
---------------

---------------
module B;
public class Foo {}
---------------

---------------
import A;
import B;

Foo f1; // error, could be either A.Foo or B.Foo
B.Foo f2; // ok
---------------

$(H3 $(LNAME2 const, Const Attribute))

        $(P The $(D const) attribute declares constants that can be
        evaluated at compile time. For example:
        )

---------------
const int foo = 7;

const {
  double bar = foo + 6;
}
---------------



$(H3 $(LNAME2 immutable, immutable Attribute))

$(H3 $(LNAME2 gshared, $(D __gshared Attribute)))

        $(P By default, non-immutable global declarations reside in thread local
        storage. When a global variable is marked with the $(D __gshared)
        attribute, its value is shared across all threads.)

---
int foo;            // Each thread has its own exclusive copy of foo.
__gshared int bar;  // bar is shared by all threads.
---

        $(P $(D __gshared) may also be applied to member variables and local
        variables. In these cases, $(D __gshared) is equivalent to $(D static),
        except that the variable is shared by all threads rather than being
        thread local.)

---
class Foo {
  __gshared int bar;
}

int foo() {
  __gshared int bar = 0;
  return bar++; // Not thread safe.
}
---

        $(P Unlike the $(D shared) attribute, $(D __gshared) provides no
        safe-guards against data races or other multi-threaded synchronization
        issues. It is the responsibility of the programmer to ensure that
        access to variables marked $(D __gshared) is synchronized correctly.)

        $(P $(D __gshared) is disallowed in safe mode.)

$(H3 $(LNAME2 shared, shared Attribute))

$(H3 $(LNAME2 inout, inout Attribute))

$(H3 $(LNAME2 disable, @disable Attribute))

        $(P A reference to a declaration marked with the $(CODE @disable) attribute causes
        a compile time error.
        This can be used to explicitly disallow certain operations or overloads
        at compile time rather than relying on generating a runtime error.
        )

---
struct T {
  @disable this(this) { } // disabling this makes T not copyable
}

struct S {
  T t;   // uncopyable member makes S also not copyable
}

@disable void foo() { }

void main() {
  S s;
  S t = s; // error, S is not copyable
  foo();   // error, foo is disabled
}
---


$(H3 $(LNAME2 override, Override Attribute))

        $(P The $(D override) attribute applies to virtual functions.
        It means that the function must override a function with the
        same name and parameters in a base class. The override attribute
        is useful for catching errors when a base class's member function
        gets its parameters changed, and all derived classes need to have
        their overriding functions updated.
        )

---------------
class Foo {
  int bar();
  int abc(int x);
}

class Foo2 : Foo {
  override {
    int bar(char c); // error, no bar(char) in Foo
    int abc(int x);  // ok
  }
}
---------------

$(H3 Static Attribute)

        $(P The $(D static) attribute applies to functions and data.
        It means that the declaration does not apply to a particular
        instance of an object, but to the type of the object. In
        other words, it means there is no $(D this) reference.
        $(D static) is ignored when applied to other declarations.
        )

---------------
class Foo {
  static int bar() { return 6; }
  int foobar() { return 7; }
}

...

Foo f = new Foo;
Foo.bar();      // produces 6
Foo.foobar();   // error, no instance of Foo
f.bar();        // produces 6;
f.foobar();     // produces 7;
---------------

$(P
        Static functions are never virtual.
)
$(P
        Static data has only one instance for the entire program,
        not once per object.
)
$(P
        Static does not have the additional C meaning of being local
        to a file. Use the $(D private) attribute in D to achieve that.
        For example:
)

---------------
module foo;
int x = 3;         // x is global
private int y = 4; // y is local to module foo
---------------


$(H3 $(LNAME2 auto, Auto Attribute))

        $(P The $(D auto) attribute is used when there are no other attributes
        and type inference is desired.
        )

---
auto i = 6.8;   // declare i as a double
---

$(H3 $(LNAME2 scope, Scope Attribute))

$(P
        The $(D scope) attribute is used for local variables and for class
        declarations. For class declarations, the $(D scope) attribute creates
        a $(I scope) class.
        For local declarations, $(D scope) implements the RAII (Resource
        Acquisition Is Initialization) protocol. This means that the
        destructor for an object is automatically called when the
        reference to it goes out of scope. The destructor is called even
        if the scope is exited via a thrown exception, thus $(D scope)
        is used to guarantee cleanup.
)
$(P
        If there is more than one $(D scope) variable going out of scope
        at the same point, then the destructors are called in the reverse
        order that the variables were constructed.
)
$(P
        $(D scope) cannot be applied to globals, statics, data members, ref
        or out parameters. Arrays of $(D scope)s are not allowed, and $(D scope)
        function return values are not allowed. Assignment to a $(D scope),
        other than initialization, is not allowed.
        $(D Rationale:) These restrictions may get relaxed in the future
        if a compelling reason to appears.
)

$(H3 $(LNAME2 abstract, Abstract Attribute))

$(P
        If a class is abstract, it cannot be instantiated
        directly. It can only be instantiated as a base class of
        another, non-abstract, class.
)
$(P
        Classes become abstract if they are defined within an
        abstract attribute, or if any of the virtual member functions
        within it are declared as abstract.
)
$(P
        Non-virtual functions cannot be declared as abstract.
)
$(P
        Functions declared as abstract can still have function
        bodies. This is so that even though they must be overridden,
        they can still provide $(SINGLEQUOTE base class functionality.)
)

$(SECTION3 $(LNAME2 uda, User Defined Attributes),

        $(P
            User Defined Attributes (UDA) are compile time expressions that can be attached
            to a declaration. These attributes can then be queried, extracted, and manipulated
            at compile time. There is no runtime component to them.
        )

        $(P
            Grammatically, a UDA is a StorageClass:
        )

$(GRAMMAR
$(GNAME StorageClass):
    $(GLINK UserDefinedAttribute)

$(GNAME UserDefinedAttribute):
    @(ArgumentList)
    @CallExpression
)
        $(P
            And looks like:
        )

---
@(3) int a;
@("string", 7) int b;

enum Foo;
@Foo int c;

struct Bar
{
    int x;
}

@Bar(3) int d;
---

        $(P
            If there are multiple UDAs in scope for a declaration, they are concatenated:
        )

---
@(1) {
    @(2) int a;         // has UDA's (1, 2)
    @("string") int b;  // has UDA's (1, "string")
}
---

        $(P
            UDA's can be extracted into an expression tuple using $(D __traits):
        )

---
@('c') string s;
pragma(msg, __traits(getAttributes, s)); // prints tuple('c')
---
)

        $(P
            If there are no user defined attributes for the symbol, an empty tuple is returned.
            The expression tuple can be turned into a manipulatable tuple:
        )

---
template Tuple (T...)
{
    alias T Tuple;
}

enum EEE = 7;
@("hello") struct SSS { }
@(3) { @(4) @EEE @SSS int foo; }

alias Tuple!(__traits(getAttributes, foo)) TP;

pragma(msg, TP); // prints tuple(3, 4, 7, (SSS))
pragma(msg, TP[2]); // prints 7
---

        $(P
            Of course the tuple types can be used to declare things:
        )

---
TP[3] a; // a is declared as an SSS
---

        $(P
            The attribute of the type name is not the same as the attribute of the variable:
        )

---
pragma(msg, __traits(getAttributes, typeof(a))); // prints tuple("hello")
---

        $(P
            Of course, the real value of UDA's is to be able to create user defined types with
            specific values. Having attribute values of basic types does not scale.
            The attribute tuples can be manipulated like any other tuple, and can be passed as
            the argument list to a template.
        )

        $(P
            Whether the attributes are values or types is up to the user, and whether later
            attributes accumulate or override earlier ones is also up to how the user
            interprets them.
        )
)

Macros:
        TITLE=Attributes
        WIKI=Attribute
        CATEGORY_SPEC=$0