The object-oriented features of D all come from classes. The class hierarchy has as its root the class Object. Object defines a minimum level of functionality that each derived class has, and a default implementation for that functionality.
Classes are programmer defined types. Support for classes are what make D an object oriented language, giving it encapsulation, inheritance, and polymorphism. D classes support the single inheritance paradigm, extended by adding support for interfaces. Class objects are instantiated by reference only.
A class can be exported, which means its name and all its non-private members are exposed externally to the DLL or EXE.
A class declaration is defined:
ClassDeclaration:
class Identifier BaseClassListopt ClassBody
ClassTemplateDeclaration
BaseClassList:
: SuperClass
: SuperClass , InterfaceClasses
: InterfaceClass
SuperClass:
Identifier
InterfaceClasses:
InterfaceClass
InterfaceClass , InterfaceClasses
InterfaceClass:
Identifier
ClassBody:
{ }
{ ClassBodyDeclarations }
ClassBodyDeclarations:
ClassBodyDeclaration
ClassBodyDeclaration ClassBodyDeclarations
ClassBodyDeclaration:
DeclDef
Invariant
ClassAllocator
ClassDeallocator
Classes consist of:
class Foo {
... members ...
}
Note that there is no trailing ; after the closing } of the class
definition.
It is also not possible to declare a variable var like:
class Foo { } var;
Instead:
class Foo { }
Foo var;
Class members are always accessed with the . operator. There are no :: or -> operators as in C++.
The D compiler is free to rearrange the order of fields in a class to optimally pack them in an implementation-defined manner. Consider the fields much like the local variables in a function - the compiler assigns some to registers and shuffles others around all to get the optimal stack frame layout. This frees the code designer to organize the fields in a manner that makes the code more readable rather than being forced to organize it according to machine optimization rules. Explicit control of field layout is provided by struct/union types, not classes.
The .offsetof property gives the offset in bytes of the field from the beginning of the class instantiation. .offsetof can only be applied to expressions which produce the type of the field itself, not the class type:
class Foo {
int x;
}
...
void test(Foo foo) {
size_t o;
o = Foo.x.offsetof; // error, Foo.x needs a 'this' reference
o = foo.x.offsetof; // ok
}
The .tupleof property returns an ExpressionTuple of all the fields in the class, excluding the hidden fields and the fields in the base class.
class Foo { int x; long y; }
void test(Foo foo) {
foo.tupleof[0] = 1; // set foo.x to 1
foo.tupleof[1] = 2; // set foo.y to 2
foreach (x; foo.tupleof)
writef(x); // prints 12
}
The properties .__vptr and .__monitor give access to the class object's vtbl[] and monitor, respectively, but should not be used in user code.
Non-static member functions have an extra hidden parameter called this through which the class object's other members can be accessed.
Synchronized class member functions have the storage class synchronized. A static member function is synchronized on the classinfo object for the class, which means that one monitor is used for all static synchronized member functions for that class. For non-static synchronized functions, the monitor used is part of the class object. For example:
class Foo {
synchronized void bar() { ...statements... }
}
is equivalent to (as far as the monitors go):
class Foo {
void bar() {
synchronized (this) { ...statements... }
}
}
Structs do not have synchronized member functions.
Constructor:
this Parameters FunctionBody
TemplatedConstructor
Members are always initialized to the default initializer for their type, which is usually 0 for integer types and NAN for floating point types. This eliminates an entire class of obscure problems that come from neglecting to initialize a member in one of the constructors. In the class definition, there can be a static initializer to be used instead of the default:
class Abc {
int a; // default initializer for a is 0
long b = 7; // default initializer for b is 7
float f; // default initializer for f is NAN
}
This static initialization is done before any constructors are
called.
Constructors are defined with a function name of this and having no return value:
class Foo {
this(int x) // declare constructor for Foo
{ ...
}
this()
{ ...
}
}
Base class construction is done by calling the base class
constructor by the name super:
class A { this(int y) { } }
class B : A {
int j;
this() {
...
super(3); // call base constructor A.this(3)
...
}
}
Constructors can also call other constructors for the same class in order to share common initializations (this is called delegating constructors):
class C {
int j;
this() {
...
}
this(int i) {
this();
j = i;
}
}
If no call to constructors via this or super appear
in a constructor, and the base class has a constructor, a call
to super() is inserted at the beginning of the constructor.
If there is no constructor for a class, but there is a constructor for the base class, a default constructor of the form:
this() { }
is implicitly generated.
Class object construction is very flexible, but some restrictions apply:
this() { this(1); }
this(int i) { this(); } // illegal, cyclic constructor calls
this() { a || super(); } // illegal
this() { (a) ? this(1) : super(); } // ok
this() {
for (...) {
super(); // illegal, inside loop
}
}
Instances of class objects are created with NewExpressions:
A a = new A(3);
The following steps happen:
Destructor:
~ this ( ) FunctionBody
The garbage collector calls the destructor function when the object
is deleted. The syntax
is:
class Foo {
~this() // destructor for Foo
{
}
}
There can be only one destructor per class, the destructor does not have any parameters, and has no attributes. It is always virtual.
The destructor is expected to release any resources held by the object.
The program can explicitly inform the garbage collector that an object is no longer referred to (with the delete expression), and then the garbage collector calls the destructor immediately, and adds the object's memory to the free storage. The destructor is guaranteed to never be called twice.
The destructor for the super class automatically gets called when the destructor ends. There is no way to call the super destructor explicitly.
The garbage collector is not guaranteed to run the destructor for all unreferenced objects. Furthermore, the order in which the garbage collector calls destructors for unreference objects is not specified. This means that when the garbage collector calls a destructor for an object of a class that has members that are references to garbage collected objects, those references may no longer be valid. This means that destructors cannot reference sub objects. This rule does not apply to auto objects or objects deleted with the DeleteExpression, as the destructor is not being run by the garbage collector, meaning all references are valid.
Objects referenced from the data segment never get collected by the gc.
StaticConstructor:
static this ( ) FunctionBody
A static constructor is defined as a function that performs
initializations before the
main() function gets control. Static constructors are used to
initialize
static class members
with values that cannot be computed at compile time.
Static constructors in other languages are built implicitly by using member initializers that can't be computed at compile time. The trouble with this stems from not having good control over exactly when the code is executed, for example:
class Foo {
static int a = b + 1;
static int b = a * 2;
}
What values do a and b end up with, what order are the initializations
executed in, what
are the values of a and b before the initializations are run, is this a
compile error, or is this
a runtime error? Additional confusion comes from it not being obvious if
an initializer is
static or dynamic.
D makes this simple. All member initializations must be determinable by the compiler at compile time, hence there is no order-of-evaluation dependency for member initializations, and it is not possible to read a value that has not been initialized. Dynamic initialization is performed by a static constructor, defined with a special syntax static this().
class Foo {
static int a; // default initialized to 0
static int b = 1;
static int c = b + a; // error, not a constant initializer
static this() // static constructor
{
a = b + 1; // a is set to 2
b = a * 2; // b is set to 4
}
}
static this() is called by the startup code before
main() is called. If it returns normally
(does not throw an exception), the static destructor is added
to the list of functions to be
called on program termination.
Static constructors have empty parameter lists.
Static constructors within a module are executed in the lexical order in which they appear. All the static constructors for modules that are directly or indirectly imported are executed before the static constructors for the importer.
The static in the static constructor declaration is not an attribute, it must appear immediately before the this:
class Foo {
static this() { ... } // a static constructor
static private this() { ... } // not a static constructor
static {
this() { ... } // not a static constructor
}
static:
this() { ... } // not a static constructor
}
StaticDestructor:
static ~ this ( ) FunctionBody
A static destructor is defined as a special static function with the
syntax static ~this().
class Foo {
static ~this() // static destructor
{
}
}
A static destructor gets called on program termination, but only if
the static constructor
completed successfully.
Static destructors have empty parameter lists.
Static destructors get called in the reverse order that the static
constructors were called in.
The static in the static destructor declaration is not an attribute, it must appear immediately before the ~this:
class Foo {
static ~this() { ... } // a static destructor
static private ~this() { ... } // not a static destructor
static
{
~this() { ... } // not a static destructor
}
static:
~this() { ... } // not a static destructor
}
SharedStaticConstructor:
shared static this ( ) FunctionBody
Shared static constructors are executed before any StaticConstructors, and are intended for initializing any shared global data.
SharedStaticDestructor:
shared static ~ this ( ) FunctionBody
Shared static destructors are executed at program termination in the reverse order that SharedStaticConstructors were executed.
Invariant:
invariant ( ) BlockStatement
Class invariants are used to specify characteristics of a class that always
must be true (except while executing a member function). For example, a
class representing a date might have an invariant that the day must be 1..31
and the hour must be 0..23:
class Date {
int day;
int hour;
invariant() {
assert(1 <= day && day <= 31);
assert(0 <= hour && hour < 24);
}
}
The class invariant is a contract saying that the asserts must hold true. The invariant is checked when a class constructor completes, at the start of the class destructor, before a public or exported member is run, and after a public or exported function finishes.
The code in the invariant may not call any public non-static members of the class, either directly or indirectly. Doing so will result in a stack overflow, as the invariant will wind up being called in an infinitely recursive manner.
Since the invariant is called at the start of public or exported members, such members should not be called from constructors.
class Foo {
public void f() { }
private void g() { }
invariant() {
f(); // error, cannot call public member function from invariant
g(); // ok, g() is not public
}
}
The invariant
can be checked when a class object is the argument to an
assert()
expression, as:
Date mydate;
...
assert(mydate); // check that class Date invariant holds
Invariants contain assert expressions, and so when they fail,
they throw a AssertErrors.
Class invariants are inherited, that is,
any class invariant is implicitly anded with the invariants of its base
classes.
There can be only one Invariant per class.
When compiling for release, the invariant code is not generated, and the compiled program runs at maximum speed.
ClassAllocator:
new Parameters FunctionBody
A class member function of the form:
new(uint size) {
...
}
is called a class allocator.
The class allocator can have any number of parameters, provided
the first one is of type uint.
Any number can be defined for a class, the correct one is
determined by the usual function overloading rules.
When a new expression:
new Foo;
is executed, and Foo is a class that has
an allocator, the allocator is called with the first argument
set to the size in bytes of the memory to be allocated for the
instance.
The allocator must allocate the memory and return it as a
void*.
If the allocator fails, it must not return a null, but
must throw an exception.
If there is more than one parameter to the allocator, the
additional arguments are specified within parentheses after
the new in the NewExpression:
class Foo {
this(char[] a) { ... }
new(uint size, int x, int y) {
...
}
}
...
new(1,2) Foo(a); // calls new(Foo.sizeof,1,2)
Derived classes inherit any allocator from their base class, if one is not specified.
The class allocator is not called if the instance is created on the stack.
See also Explicit Class Instance Allocation.
ClassDeallocator:
delete Parameters FunctionBody
A class member function of the form:
delete(void *p) {
...
}
is called a class deallocator.
The deallocator must have exactly one parameter of type void*.
Only one can be specified for a class.
When a delete expression:
delete f;
is executed, and f is a reference to a class instance that has a deallocator, the deallocator is called with a pointer to the class instance after the destructor (if any) for the class is called. It is the responsibility of the deallocator to free the memory.
Derived classes inherit any deallocator from their base class, if one is not specified.
The class allocator is not called if the instance is created on the stack.
See also Explicit Class Instance Allocation.
AliasThis:
alias Identifier this;
An AliasThis declaration names another class or struct member to which any undefined lookups will be forwarded. The Identifier names that member.
A class or struct can be implicitly converted to the AliasThis member.
There is only one AliasThis allowed per class or struct.
struct S {
int x;
alias x this;
}
int foo(int i) { return i * 2; }
void test() {
S s;
s.x = 7;
int i = -s; // i == -7
i = s + 8; // i == 15
i = s + s; // i == 14
i = 9 + s; // i == 16
i = foo(s); // implicit conversion to int
}
scope class Foo { ... }
The scope characteristic is inherited, so if any classes derived
from a scope class are also scope.
An scope class reference can only appear as a function local variable. It must be declared as being scope:
scope class Foo { ... }
void func() {
Foo f; // error, reference to scope class must be scope
scope Foo g = new Foo(); // correct
}
When an scope class reference goes out of scope, the destructor
(if any) for it is automatically called. This holds true even if
the scope was exited via a thrown exception.
Final classes cannot be subclassed:
final class A { }
class B : A { } // error, class A is final
class Outer {
int m;
class Inner {
int foo() {
return m; // Ok to access member of Outer
}
}
}
void func() {
int m;
class Inner {
int foo() {
return m; // Ok to access local variable m of func()
}
}
}
If a nested class has the static attribute, then it can
not access variables of the enclosing scope that are local to the
stack or need a this:
class Outer {
int m;
static int n;
static class Inner {
int foo() {
return m; // Error, Inner is static and m needs a this
return n; // Ok, n is static
}
}
}
void func() {
int m;
static int n;
static class Inner {
int foo() {
return m; // Error, Inner is static and m is local to the stack
return n; // Ok, n is static
}
}
}
Non-static nested classes work by containing an extra hidden member
(called the context pointer)
that is the frame pointer of the enclosing function if it is nested
inside a function, or the this of the enclosing class's instance
if it is nested inside a class.
When a non-static nested class is instantiated, the context pointer is assigned before the class's constructor is called, therefore the constructor has full access to the enclosing variables. A non-static nested class can only be instantiated when the necessary context pointer information is available:
class Outer {
class Inner { }
static class SInner { }
}
void func() {
class Nested { }
Outer o = new Outer; // Ok
Outer.Inner oi = new Outer.Inner; // Error, no 'this' for Outer
Outer.SInner os = new Outer.SInner; // Ok
Nested n = new Nested; // Ok
}
While a non-static nested class can access the stack variables
of its enclosing function, that access becomes invalid once
the enclosing function exits:
class Base {
int foo() { return 1; }
}
Base func() {
int m = 3;
class Nested : Base {
int foo() { return m; }
}
Base b = new Nested;
assert(b.foo() == 3); // Ok, func() is still active
return b;
}
int test() {
Base b = func();
return b.foo(); // Error, func().m is undefined
}
If this kind of functionality is needed, the way to make it work
is to make copies of the needed variables within the nested class's
constructor:
class Base {
int foo() { return 1; }
}
Base func() {
int m = 3;
class Nested : Base {
int m_;
this() { m_ = m; }
int foo() { return m_; }
}
Base b = new Nested;
assert(b.foo() == 3); // Ok, func() is still active
return b;
}
int test() {
Base b = func();
return b.foo(); // Ok, using cached copy of func().m
}
A this can be supplied to the creation of an inner class instance by prefixing it to the NewExpression:
class Outer {
int a;
class Inner {
int foo() {
return a;
}
}
}
int bar() {
Outer o = new Outer;
o.a = 3;
Outer.Inner oi = o.new Inner;
return oi.foo(); // returns 3
}
Here o supplies the this to the outer class instance of Outer.
The property .outer used in a nested class gives the this pointer to its enclosing class. If the enclosing context is not a class, the .outer will give the pointer to it as a void* type.
class Outer {
class Inner {
Outer foo() {
return this.outer;
}
}
void bar() {
Inner i = new Inner;
assert(this == i.foo());
}
}
void test() {
Outer o = new Outer;
o.bar();
}
An anonymous nested class is both defined and instantiated with a NewAnonClassExpression:
NewAnonClassExpression:
new AllocatorArgumentsopt class ClassArgumentsopt SuperClassopt InterfaceClassesopt ClassBody
ClassArguments:
( ArgumentListopt )
which is equivalent to:
class Identifier : SuperClass InterfaceClasses
ClassBody
new (ArgumentList) Identifier (ArgumentList);
where Identifier is the name generated for the anonymous
nested class.
Const
Immutable and Shared Classes,
If a ClassDeclaration has a const, immutable
or shared
storage class, then it is as if each member of the class
was declared with that storage class.
If a base class is const, immutable or shared, then all classes derived
from it are also const, immutable or shared.