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.
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:
- no bit fields
- alignment can be explicitly specified
- no separate tag name space - tag names go into the current scope
- declarations like:
struct ABC x;
are not allowed, replace with:ABC x;
- anonymous structs/unions are allowed as members of other structs/unions
- Default initializers for members can be supplied.
- Member functions and static members are allowed.
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 = 5C-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 = 5.0Other 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