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Last update Fri Jan 19 22:40:44 2007

Structs & Unions

Whereas classes are reference types, structs are value types. Any C struct can be exactly represented as a D struct. In C++ parlance, a D struct is a POD (Plain Old Data) type, with a trivial constructors and destructors. Structs and unions are meant as simple aggregations of data, or as a way to paint a data structure over hardware or an external type. External types can be defined by the operating system API, or by a file format. Object oriented features are provided with the class data type.

A struct is defined to not have an identity; that is, the implementation is free to make bit copies of the struct as convenient.

Struct, Class Comparison Table
Feature struct class C struct C++ struct C++ class
value type X   X X X
reference type   X      
data members X X X X X
hidden members   X   X X
static members X X   X X
default member initializers X X      
bit fields     X X X
non-virtual member functions X X   X X
virtual member functions   X   X X
constructors   X   X X
destructors   X   X X
RAII   X   X X
operator overloading X X   X X
inheritance   X   X X
invariants X X      
unit tests X X      
synchronizable   X      
parameterizable X X   X X
alignment control X X      
member protection X X   X X
default public X X X X  
tag name space     X X X
anonymous X   X X X
AggregateDeclaration:
	Tag { DeclDefs }
	Tag Identifier StructBody
	Tag Identifier ;

Tag:
	struct
	union

StructBody:
	{ }
	{ StructBodyDeclarations }

StructBodyDeclarations:
	StructBodyDeclaration
	StructBodyDeclaration StructBodyDeclarations

StructBodyDeclaration:
	Declaration
	StaticConstructor
	StaticDestructor
	Invariant
	UnitTest
	StructAllocator
	StructDeallocator

StructAllocator:
	ClassAllocator

StructDeallocator:
	ClassDeallocator

They work like they do in C, with the following exceptions:

Static Initialization of Structs

Static struct members are by default initialized to whatever the default initializer for the member is, and if none supplied, to the default initializer for the member's type. If a static initializer is supplied, the members are initialized by the member name, colon, expression syntax. The members may be initialized in any order. Members not specified in the initializer list are default initialized.
struct X { int a; int b; int c; int d = 7;}
static X x = { a:1, b:2};	      // c is set to 0, d to 7
static X z = { c:4, b:5, a:2 , d:5};  // z.a = 2, z.b = 5, z.c = 4, z.d = 5
C-style initialization, based on the order of the members in the struct declaration, is also supported:
static X q = { 1, 2 };	  // q.a = 1, q.b = 2, q.c = 0, q.d = 7

Static Initialization of Unions

Unions are initialized explicitly.
union U { int a; double b; }
static U u = { b : 5.0 };		// u.b = 5.0
Other members of the union that overlay the initializer, but occupy more storage, have the extra storage initialized to zero.

Dynamic Initialization of Structs

Structs can be dynamically initialized from another value of the same type:

struct S { int a; }
S t;      // default initialized
t.a = 3;
S s = t;  // s.a is set to 3

If opCall is overridden for the struct, and the struct is initialized with a value that is of a different type, then the opCall operator is called:

struct S
{   int a;

    static S opCall(int v)
    {	S s;
	s.a = v;
	return s;
    }

    static S opCall(S v)
    {	S s;
	s.a = v.a + 1;
	return s;
    }
}

S s = 3;	// sets s.a to 3
S t = s;	// sets t.a to 3, S.opCall(s) is not called

Struct Properties

.sizeof			Size in bytes of struct
.alignof		Size boundary struct needs to be aligned on
.tupleof		Gets type tuple of fields

Struct Field Properties

.offsetof		Offset in bytes of field from beginning
			of struct