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Structs, Unions

Whereas classes are reference types, structs are value types. Any C struct can be exactly represented as a D struct, except non-static function-nested D structs which access the context of their enclosing scope. 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
FeaturestructclassC structC++ structC++ class
value type
reference type
data members
hidden members
static members
default member initializers
bit fields
non-virtual member functions
virtual member functions
postblit/copy constructors
identity assign overload
operator overloading
unit tests
alignment control
member protection
default public
tag name space
static constructor
static destructor
inner nesting
StructDeclaration: struct Identifier ; struct Identifier AggregateBody StructTemplateDeclaration AnonStructDeclaration
AnonStructDeclaration: struct AggregateBody
UnionDeclaration: union Identifier ; union Identifier AggregateBody UnionTemplateDeclaration AnonUnionDeclaration
AnonUnionDeclaration: union AggregateBody
AggregateBody: { DeclDefsopt }

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

Opaque Structs and Unions

Opaque struct and union declarations do not have a AggregateBody:

struct S;
union U;

The members are completely hidden to the user, and so the only operations on those types are ones that do not require any knowledge of the contents of those types. For example:

struct S;
S.sizeof; // error, size is not known
S s;      // error, cannot initialize unknown contents
S* p;     // ok, knowledge of members is not necessary

They can be used to implement the PIMPL idiom.

Static Initialization of Structs

Static struct members are by default initialized to whatever the default initializer for the member is, and if none is supplied, to the default initializer for the member's type.

struct S { int a = 4; int b; }
static S x; // a is set to 4, b to 0

If a static initializer is supplied, the members are initialized by the memberName:expression syntax. The members may be initialized in any order. Initializers for statics must be evaluatable at compile time. Members not specified in the initializer list are default initialized.

struct S { int a, b, c, d = 7; }
static S x = { a:1, b:2 };           // c is set to 0, d to 7
static S 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 S q = { 1, 2 }; // q.a = 1, q.b = 2, q.c = 0, q.d = 7

The two styles can be combined:

static S q = { 1, d:3 }; // q.a = 1, q.b = 0, q.c = 0, q.d = 3

Struct literals can also be used to initialize statics, but they must be evaluable at compile time.

static S q = S( 1, 2+3 ); // q.a = 1, q.b = 5, 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

The static initializer syntax can also be used to initialize non-static variables. The initializer need not be evaluable at compile time.

struct S { int a, b, c, d = 7; }

void test(int i)
    S q = { 1, b:i }; // q.a = 1, q.b = i, q.c = 0, q.d = 7

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

Struct Literals

Struct literals consist of the name of the struct followed by a parenthesized argument list:

struct S { int x; float y; }

int foo(S s) { return s.x; }

foo( S(1, 2) ); // set field x to 1, field y to 2

Struct literals are syntactically like function calls. If a struct has a member function named opCall, then struct literals for that struct are not possible. See also opCall operator overloading for the issue workaround. It is an error if there are more arguments than fields of the struct. If there are fewer arguments than fields, the remaining fields are initialized with their respective default initializers. If there are anonymous unions in the struct, only the first member of the anonymous union can be initialized with a struct literal, and all subsequent non-overlapping fields are default initialized.

Struct Properties

Struct Properties
.sizeofSize in bytes of struct
.alignofSize boundary struct needs to be aligned on
.tupleofGets type tuple of fields

Struct Field Properties

Struct Field Properties
.offsetofOffset in bytes of field from beginning of struct

Const, Immutable and Shared Structs

A struct declaration can have a storage class of const, immutable or shared. It has an equivalent effect as declaring each member of the struct as const, immutable or shared.

const struct S { int a; int b = 2; }

void main()
    S s = S(3); // initializes s.a to 3
    S t;        // initializes t.a to 0
    t = s;      // error, t.a and t.b are const, so cannot modify them.
    t.a = 4;    // error, t.a is const

Struct Constructors

Struct constructors are used to initialize an instance of a struct. The ParameterList may not be empty. Struct instances that are not instantiated with a constructor are default initialized to their .init value.

struct S
    int x, y;

    this()  // error, cannot implement default ctor for structs

    this(int a, int b)
        x = a;
        y = b;

void main()
    S a = S(4, 5);
    auto b = S();  // same as auto b = S.init;

A constructor qualifier allows the object to be constructed with that specific qualifier.

struct S1
    int[] a;
    this(int n) { a = new int[](n); }
struct S2
    int[] a;
    this(int n) immutable { a = new int[](n); }
void main()
    // Mutable constructor creates mutable object.
    S1 m1 = S1(1);

    // Constructed mutable object is implicitly convertible to const.
    const S1 c1 = S1(1);

    // Constructed mutable object is not implicitly convertible to immutable.
    // immutable i1 = S1(1);

    // Mutable constructor cannot construct immutable object.
    // auto x1 = immutable S1(1);

    // Immutable constructor cannot construct mutable object.
    // auto x2 = S2(1);

    // Constructed immutable object is not implicitly convertible to mutable.
    // S2 m2 = immutable S2(1);

    // Constructed immutable object is implicitly convertible to const.
    const S2 c2 = immutable S2(1);

    // Immutable constructor creates immutable object.
    immutable i2 = immutable S2(1);

If struct constructor is annotated with @disable and has empty parameter, the struct is disabled construction without calling other constructor.

struct S
    int x;

    // Disables default construction, function body can be empty.
    @disable this();

    this(int v) { x = v; }
void main()
    //S s;        // default construction is disabled
    //S s = S();  // also disabled
    S s = S(1);   // construction with calling constructor

Struct Postblits

    this ( this ) MemberFunctionAttributesopt ;
    this ( this ) MemberFunctionAttributesopt FunctionBody

Copy construction is defined as initializing a struct instance from another struct of the same type. Copy construction is divided into two parts:

  1. blitting the fields, i.e. copying the bits
  2. running postblit on the result

The first part is done automatically by the language, the second part is done if a postblit function is defined for the struct. The postblit has access only to the destination struct object, not the source. Its job is to ‘fix up’ the destination as necessary, such as making copies of referenced data, incrementing reference counts, etc. For example:

struct S
    int[] a;    // array is privately owned by this instance
        a = a.dup;

Disabling struct postblit makes the object not copyable.

struct T
    @disable this(this);  // disabling makes T not copyable
struct S
    T t;   // uncopyable member makes S also not copyable

void main()
    S s;
    S t = s; // error, S is not copyable

Unions may not have fields that have postblits.

Struct Destructors

Destructors are called when an object goes out of scope. Their purpose is to free up resources owned by the struct object.

Unions may not have fields that have destructors.

Struct Invariants

    invariant ( ) BlockStatement
    invariant BlockStatement

StructInvariants specify the relationships among the members of a struct instance. Those relationships must hold for any interactions with the instance from its public interface.

The invariant is in the form of a const member function. The invariant is defined to hold if all the AssertExpressions within the invariant that are executed succeed.

If the invariant does not hold, then the program enters an invalid state.

Any invariants for fields are applied before the struct invariant.

There may be multiple invariants in a struct. They are applied in lexical order.

StructInvariants must hold at the exit of the struct constructor (if any), and at the entry of the struct destructor (if any).

StructInvariants must hold at the entry and exit of all public or exported non-static member functions. The order of application of invariants is:

  1. preconditions
  2. invariant
  3. function body
  4. invariant
  5. postconditions

The invariant need not hold if the struct instance is implicitly constructed using the default .init value.

struct Date
    this(int d, int h)
        day = d;    // days are 1..31
        hour = h;   // hours are 0..23

        assert(1 <= day && day <= 31);
        assert(0 <= hour && hour < 24);

    int day;
    int hour;

Public or exported non-static member functions cannot be called from within an invariant.

struct Foo
    public void f() { }
    private void g() { }

        f();  // error, cannot call public member function from invariant
        g();  // ok, g() is not public

Undefined Behavior: happens if the invariant does not hold and execution continues.

Implementation Defined:

  1. Whether the StructInvariant is executed at runtime or not. This is typically controlled with a compiler switch.
  2. The behavior when the invariant does not hold is typically the same as for when AssertExpressions fail.

Best Practices:

  1. Do not indirectly call exported or public member functions within a struct invariant, as this can result in infinite recursion.
  2. Avoid reliance on side effects in the invariant. as the invariant may or may not be executed.
  3. Avoid having mutable public fields of structs with invariants, as then the invariant cannot verify the public interface.

Identity Assignment Overload

While copy construction takes care of initializing an object from another object of the same type, or elaborate destruction is needed for the type, assignment is defined as copying the contents of one object over another, already initialized, type:

struct S { ... }  // S has postblit or destructor
S s;      // default construction of s
S t = s;  // t is copy-constructed from s
t = s;    // t is assigned from s

Struct assignment t=s is defined to be semantically equivalent to:


where opAssign is a member function of S:

ref S opAssign(ref S s)
    S tmp = this;   // bitcopy this into tmp
    this = s;       // bitcopy s into this
    tmp.__dtor();   // call destructor on tmp
    return this;

An identity assignment overload is required for a struct if one or more of these conditions hold:

If an identity assignment overload is required and does not exist, an identity assignment overload function of the type ref S opAssign(ref S) will be automatically generated.

A user-defined one can implement the equivalent semantics, but can be more efficient.

One reason a custom opAssign might be more efficient is if the struct has a reference to a local buffer:

struct S
    int[] buf;
    int a;

    ref S opAssign(ref const S s)
        a = s.a;
        return this;

        buf = buf.dup;

Here, S has a temporary workspace buf[]. The normal postblit will pointlessly free and reallocate it. The custom opAssign will reuse the existing storage.

Nested Structs

A nested struct is a struct that is declared inside the scope of a function or a templated struct that has aliases to local functions as a template argument. Nested structs have member functions. It has access to the context of its enclosing scope (via an added hidden field).

void foo()
    int i = 7;
    struct SS
        int x,y;
        int bar() { return x + i + 1; }
    SS s;
    s.x = 3;; // returns 11

A struct can be prevented from being nested by using the static attribute, but then of course it will not be able to access variables from its enclosing scope.

void foo()
    int i = 7;
    static struct SS
        int x, y;
        int bar()
            return i; // error, SS is not a nested struct

Unions and Special Member Functions

Unions may not have postblits, destructors, or invariants.

Associative Arrays