class.dd

Ddoc

$(SPEC_S Classes,

        $(P 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.
        )

        $(P 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.
        )

        $(P A class can be exported, which means its name and all its
        non-private
        members are exposed externally to the DLL or EXE.
        )

        $(P A class declaration is defined:
        )

$(GRAMMAR
$(GNAME ClassDeclaration):
    $(D class) $(I Identifier) $(D ;)
    $(D class) $(I Identifier) $(GLINK BaseClassList)$(OPT) $(GLINK2 struct, AggregateBody)
    $(GLINK2 template, ClassTemplateDeclaration)

$(GNAME BaseClassList):
    $(D :) $(GLINK SuperClass)
    $(D :) $(GLINK SuperClass) $(D ,) $(GLINK Interfaces)
    $(D :) $(GLINK Interfaces)

$(GNAME SuperClass):
    $(GLINK2 declaration, BasicType)

$(GNAME Interfaces):
    $(GLINK Interface)
    $(GLINK Interface) $(D ,) $(I Interfaces)

$(GNAME Interface):
    $(GLINK2 declaration, BasicType)
)

Classes consist of:

$(UL
        $(LI a super class)
        $(LI interfaces)
        $(LI dynamic fields)
        $(LI static fields)
        $(LI types)
        $(LI $(RELATIVE_LINK2 synchronized-classes, an optional synchronized attribute))
        $(LI $(RELATIVE_LINK2 member-functions, member functions)
        $(UL
            $(LI static member functions)
            $(LI $(DDSUBLINK function, virtual-functions, Virtual Functions))
            $(LI $(RELATIVE_LINK2 constructors, Constructors))
            $(LI $(RELATIVE_LINK2 destructors, Destructors))
            $(LI $(RELATIVE_LINK2 static-constructor, Static Constructors))
            $(LI $(RELATIVE_LINK2 static-destructor, Static Destructors))
            $(LI $(GLINK SharedStaticConstructor)s)
            $(LI $(GLINK SharedStaticDestructor)s)
            $(LI $(RELATIVE_LINK2 invariants, Class Invariants))
            $(LI $(DDSUBLINK unittest, unittest, Unit Tests))
            $(LI $(RELATIVE_LINK2 allocators, Class Allocators))
            $(LI $(RELATIVE_LINK2 deallocators, Class Deallocators))
            $(LI $(RELATIVE_LINK2 alias-this, Alias This))
        )
        )
)

        A class is defined:

------
class Foo
{
    ... members ...
}
------

        $(P Note that there is no trailing $(D ;) after the closing $(D }) of the class
        definition.
        It is also not possible to declare a variable var like:)

------
class Foo { } var;
------

        Instead:

------
class Foo { }
Foo var;
------

$(H3 Access Control)

	$(P Access to class members is controlled using $(GLINK2 attribute, ProtectionAttribute)s.
	The default protection attribute is $(D public).
	Access control does not affect visibility.
	)

$(H3 Fields)

        $(P Class members are always accessed with the . operator.
        )

	$(P Members of a base class can be accessed by prepending the name of
	the base class followed by a dot:)

---
class A { int a; }
class B : A { int a; }

void foo(B b)
{
    b.a = 3;   // accesses field B.a
    b.A.a = 4; // accesses field A.a
}
---

        $(P 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.
        )

$(H3 Field Properties)

        $(P The $(D .offsetof) property gives the offset in bytes of the field
        from the beginning of the class instantiation.
        $(D .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
}
------

$(H3 Class Properties)

        $(P The $(D .tupleof) property returns an $(I 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)
        write(x);       // prints 12
}
---

        $(P The properties $(D .__vptr) and $(D .__monitor) give access
        to the class object's vtbl[] and monitor, respectively, but
        should not be used in user code.
        )

$(H3 Super Class)

        All classes inherit from a super class. If one is not specified,
        it inherits from Object. Object forms the root of the D class
        inheritance hierarchy.

$(H3 $(LNAME2 member-functions, Member Functions))

        $(P Non-static member functions have an extra hidden parameter
        called $(I this) through which the class object's other members
        can be accessed.
        )

	$(P Non-static member functions can have, in addition to the usual
	$(GLINK2 function, FunctionAttribute)s, the attributes
	$(D const), $(D immutable), $(D shared), or $(D inout).
	These attributes apply to the hidden $(I this) parameter.
	)
---
class C
{
    int a;
    const void foo()
    {
        a = 3; // error, 'this' is const
    }
    void foo() immutable
    {
        a = 3; // error, 'this' is immutable
    }
}
---


$(H3 $(LNAME2 synchronized-classes, Synchronized Classes))

        $(P All member functions of synchronized classes are synchronized.
        A static member function is synchronized on the $(I classinfo)
        object for the class, which means that one monitor is used
        for all static member functions for that synchronized class.
        For non-static functions of a synchronized class, the monitor
        used is part of the class object. For example:
        )

        ---
        synchronized class Foo
        {
            void bar() { ...statements... }
        }
        ---

        $(P is equivalent to (as far as the monitors go):
        )

        ---
        synchronized class Foo
        {
            void bar()
            {
                synchronized (this) { ...statements... }
            }
        }
        ---

        $(P Member functions of non-synchronized classes cannot be individually marked as synchronized.
        The synchronized attribute must be applied to the class declaration itself:
        )

        ---
        class Foo
        {
            synchronized void foo() { }  // disallowed!
        }

        synchronized class Bar
        {
            void bar() { }  // bar is synchronized
        }
        ---

        $(P Member fields of a synchronized class cannot be public:
        )

        ---
        synchronized class Foo
        {
            int foo;  // disallowed: public field
        }

        synchronized class Bar
        {
            private int bar;  // ok
        }
        ---

        $(P The $(D synchronized) attribute can only be applied to classes,
        structs cannot be marked to be synchronized.)

$(H3 $(LNAME2 constructors, Constructors))

$(GRAMMAR
$(GNAME Constructor):
    $(D this) $(GLINK2 function, Parameters) $(GLINK2 function, MemberFunctionAttributes)$(OPT) $(D ;)
    $(D this) $(GLINK2 function, Parameters) $(GLINK2 function, MemberFunctionAttributes)$(OPT) $(GLINK2 function, FunctionBody)
    $(GLINK2 template, ConstructorTemplate)
)

        $(P Members are always initialized to the
        $(LNAME2 class-default-initializer, 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.

        $(P Constructors are defined with a function name of $(D this)
        and having no return value:)

        ------
        class Foo
        {
            $(CODE_HIGHLIGHT this)(int x)  // declare constructor for Foo
            {   ...
            }
            $(CODE_HIGHLIGHT this)()
            {   ...
            }
        }
        ------

        Base class construction is done by calling the base class
        constructor by the name $(D super):

        ------
        class A { this(int y) { } }

        class B : A
        {
            int j;
            this()
            {
                ...
                $(CODE_HIGHLIGHT super)(3);  // call base constructor A.this(3)
                ...
            }
        }
        ------

        $(P Constructors can also call other constructors for the same class
        in order to share common initializations
        $(LNAME2 delegating-constructors, (this is called delegating constructors)):
        )

        ------
        class C
        {
            int j;
            this()
            {
                ...
            }
            this(int i)
            {
                $(CODE_HIGHLIGHT this)();
                j = i;
            }
        }
        ------

        If no call to constructors via $(D this) or $(D super) appear
        in a constructor, and the base class has a constructor, a call
        to $(D super)() is inserted at the beginning of the constructor.

        $(P If there is no constructor for a class, but there is a constructor
        for the base class, a default constructor of the form:)

        ------
        this() { }
        ------

        $(P is implicitly generated.)

        $(P Class object construction is very flexible, but some restrictions
        apply:)

    $(OL
        $(LI It is illegal for constructors to mutually call each other,
        although the compiler is not required to detect it. It will result
        in undefined behavior.

        ------
        this() { this(1); }
        this(int i) { this(); } // illegal, cyclic constructor calls
        ------
        )

        $(LI If any constructor call appears inside a constructor, any
        path through the constructor must make exactly one constructor
        call:

        ------
        this() { a || super(); }       // illegal

        this() { (a) ? this(1) : super(); }     // ok

        this()
        {
            for (...)
            {
                super();  // illegal, inside loop
            }
        }
        ------
        )

        $(LI It is illegal to refer to $(D this) implicitly or explicitly
        prior to making a constructor call.)

        $(LI Constructor calls cannot appear after labels (in order to make
        it easy to check for the previous conditions in the presence of goto's).)
    )

        $(P Instances of class objects are created with $(I NewExpression)s:)

        ------
        A a = new A(3);
        ------

        $(P The following steps happen:)

    $(OL
        $(LI Storage is allocated for the object.
        If this fails, rather than return $(D null), an
        $(D OutOfMemoryError) is thrown.
        Thus, tedious checks for null references are unnecessary.
        )

        $(LI The raw data is statically initialized using the values provided
        in the class definition.
        The pointer to the vtbl[] (the array of pointers to virtual functions)
        is assigned.
        This ensures that constructors are
        passed fully formed objects for which virtual functions can be called.
        This operation is equivalent to doing a memory copy of a static
        version of the object onto the newly allocated one,
        although more advanced compilers
        may be able to optimize much of this away.
        )

        $(LI If there is a constructor defined for the class,
        the constructor matching the
        argument list is called.
        )

        $(LI If class invariant checking is turned on, the class invariant
        is called at the end of the constructor.
        )
    )

        $(P Constructors can have one of these member function attributes:
        $(D const), $(D immutable), and $(D shared). Construction of qualified
        objects will then be restricted to the implemented qualified constructors.
        )
        ------
        class C
        {
            this();   // non-shared mutable constructor
        }

        // create mutable object
        C m = new C();

        // create const object using by mutable constructor
        const C c2 = new const C();

        // a mutable constructor cannot create an immutable object
        // immutable C i = new immutable C();

        // a mutable constructor cannot create a shared object
        // shared C s = new shared C();
        ------

        $(P Constructors can be overloaded with different attributes.
        )
        ------
        class C
        {
            this();               // non-shared mutable constructor
            this() shared;        // shared mutable constructor
            this() immutable;     // immutable constructor
        }

        C m = new C();
        shared s = new shared C();
        immutable i = new immutable C();
        ------

        $(P If the constructor can create unique object (e.g. if it is pure),
        the object can be implicitly convertible to any qualifiers.
        )
        ------
        class C
        {
            this() pure;
            // Based on the definition, this creates a mutable object. But the
            // created object cannot contain any mutable global data.
            // Then compiler can guarantee that the created object is unique.

            this(int[] arr) immutable pure;
            // Based on the definition, this creates an immutable object. But
            // the argument int[] never appears in the created object so it
            // isn't implicitly convertible to immutable. Also, it cannot store
            // any immutable global data.
            // Therefore the compiler can guarantee that the created object is
            // unique.
        }

        immutable i = new immutable C();           // this() pure is called
        shared s = new shared C();                 // this() pure is called
        C m = new C([1,2,3]);       // this(int[]) immutable pure is called
        ------

$(H3 $(LNAME2 field-init, Field initialization inside constructor))

    $(P Inside constructor, the first instance field assignment is specially
        handled for its initialization.
    )

        ------
        class C
        {
            int num;
            this()
            {
                num = 1;  // initialize
                num = 2;  // assignment
            }
        }
        ------

    $(P If the field type has opAssign method, it won't be used for initialization.)

        ------
        struct A
        {
            this(int n) {}
            void opAssign(A rhs) {}
        }
        class C
        {
            A val;
            this()
            {
                val = A(1);  // A(1) is moved in this.val for initializing
                val = A(2);  // rewritten to val.opAssign(A(2))
            }
        }
        ------

    $(P If the field type is not modifiable, multiple initialization will be rejected.)

        ------
        class C
        {
            immutable int num;
            this()
            {
                num = 1;  // OK
                num = 2;  // Error: multiple field initialization
            }
        }
        ------

    $(P If the assignment expression for the field initialization may be invoked
        multiple times, it would als be rejected.
    )

        ------
        class C
        {
            immutable int num;
            immutable string str;
            this()
            {
                foreach (i; 0..2)
                {
                    num = 1;    // Error: field initialization not allowed in loops
                }
                size_t i = 0;
            Label:
                str = "hello";  // Error: field initialization not allowed after labels
                if (i++ < 2)
                    goto Label;
            }
        }
        ------

$(H3 $(LNAME2 destructors, Destructors))

$(GRAMMAR
$(GNAME Destructor):
    $(D ~ this ( )) $(GLINK2 function, MemberFunctionAttributes)$(OPT) $(D ;)
    $(D ~ this ( )) $(GLINK2 function, MemberFunctionAttributes)$(OPT) $(GLINK2 function, FunctionBody)
)

        The garbage collector calls the destructor function when the object
        is deleted. The syntax
        is:

        ------
        class Foo
        {
            ~this() // destructor for Foo
            {
            }
        }
        ------

        $(P There can be only one destructor per class, the destructor
        does not have any parameters,
        and has no attributes. It is always virtual.
        )

        $(P The destructor is expected to release any resources held by the
        object.
        )

        $(P 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.
        )

        $(P The destructor for the super class automatically gets called when
        the destructor ends. There is no way to call the super destructor
        explicitly.
        )

        $(P 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 $(I DeleteExpression), as the destructor is not being run
        by the garbage collector, meaning all references are valid.
        )

        $(P Objects referenced from the data segment never get collected
        by the gc.
        )

$(H3 Static Constructors)

$(GRAMMAR
$(GNAME StaticConstructor):
    $(D static this ( )) $(D ;)
    $(D static this ( )) $(GLINK2 function, FunctionBody)
)

	$(P


        A static constructor is a function that performs
        initializations of thread local data before the $(D main()) function gets control for
	the main thread, and upon thread startup.

	Static constructors are used to initialize
        static class members
        with values that cannot be computed at compile time.
        )

        $(P 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.

        $(P 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 $(D 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

    $(CODE_HIGHLIGHT static this)()    // static constructor
    {
        a = b + 1;          // a is set to 2
        b = a * 2;          // b is set to 4
    }
}
------

	$(P

		If $(D main()) or the thread returns normally,
        (does not throw an exception), the static destructor is added
        to the list of functions to be
        called on thread termination.

        Static constructors have empty parameter lists.
        )

	$(P
        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.
        )

	$(P
        The $(D static) in the static constructor declaration is not
        an attribute, it must appear immediately before the $(D 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
}
------

$(H3 Static Destructors)

$(GRAMMAR
$(GNAME StaticDestructor):
    $(D static ~ this ( )) $(GLINK2 function, MemberFunctionAttributes)$(OPT) $(D ;)
    $(D static ~ this ( )) $(GLINK2 function, MemberFunctionAttributes)$(OPT) $(GLINK2 function, FunctionBody)
)

        A static destructor is defined as a special static function with the
        syntax $(D static ~this()).

------
class Foo
{
    static ~this() // static destructor
    {
    }
}
------

	$(P
        A static destructor gets called on thread 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.
        )

	$(P
        The $(D static) in the static destructor declaration is not
        an attribute, it must appear immediately before the $(D ~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
}
------

$(H3 Shared Static Constructors)

$(GRAMMAR
$(GNAME SharedStaticConstructor):
    $(D shared static this ( )) $(D ;)
    $(D shared static this ( )) $(GLINK2 function, FunctionBody)
)

        $(P Shared static constructors are executed before any $(GLINK StaticConstructor)s,
        and are intended for initializing any shared global data.
        )

$(H3 Shared Static Destructors)

$(GRAMMAR
$(GNAME SharedStaticDestructor):
    $(D shared static ~ this ( )) $(GLINK2 function, MemberFunctionAttributes)$(OPT) $(D ;)
    $(D shared static ~ this ( )) $(GLINK2 function, MemberFunctionAttributes)$(OPT) $(GLINK2 function, FunctionBody)
)

        $(P Shared static destructors are executed at program termination
        in the reverse order that
        $(GLINK SharedStaticConstructor)s were executed.
        )


$(H3 $(LNAME2 invariants, Class Invariants))

$(GRAMMAR
$(GNAME Invariant):
    $(D invariant ( )) $(GLINK2 statement, BlockStatement)
    $(D invariant) $(GLINK2 statement, BlockStatement)
)

    Class invariants are used to specify characteristics of a class that always
    must be true (except while executing a member function).
    They are described in $(GLINK2 contracts, Invariants).

$(H3 $(LNAME2 allocators, Class Allocators))
$(B Note): Class allocators are deprecated in D2.
$(GRAMMAR
$(GNAME Allocator):
    $(D new) $(GLINK2 function, Parameters) $(D ;)
    $(D new) $(GLINK2 function, Parameters) $(GLINK2 function, 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
        $(D void*).
        If the allocator fails, it must not return a $(D 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 $(D new) in the $(I NewExpression):

------
class Foo
{
    this(char[] a) { ... }

    new(uint size, int x, int y)
    {
        ...
    }
}

...

new(1,2) Foo(a);        // calls new(Foo.sizeof,1,2)
------

        $(P Derived classes inherit any allocator from their base class,
        if one is not specified.
        )

        $(P The class allocator is not called if the instance is created
        on the stack.
        )

        $(P See also
        $(DDLINK1 memory, newdelete, Explicit Class Instance Allocation).
        )

$(H3 $(LNAME2 deallocators, Class Deallocators))
$(B Note): Class deallocators and the delete operator are deprecated in D2.
Use the $(D destroy) function to finalize an object by calling its destructor.
The memory of the object is $(B not) immediately deallocated, instead the GC
will collect the memory of the object at an undetermined point after finalization:

------
class Foo { int x; this() { x = 1; } }
Foo foo = new Foo;
destroy(foo);
assert(foo.x == int.init);  // object is still accessible
------

$(GRAMMAR
$(GNAME Deallocator):
    $(D delete) $(GLINK2 function, Parameters) $(D ;)
    $(D delete) $(GLINK2 function, Parameters) $(GLINK2 function, 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 $(D void*).
        Only one can be specified for a class.
        When a delete expression:

------
delete f;
------

        $(P 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.
        )

        $(P Derived classes inherit any deallocator from their base class,
        if one is not specified.
        )

        $(P The class allocator is not called if the instance is created
        on the stack.
        )

        $(P See also
        $(DDSUBLINK memory, newdelete, Explicit Class Instance Allocation).
        )

$(H3 $(LEGACY_LNAME2 AliasThis, alias-this, Alias This))

$(GRAMMAR
$(GNAME AliasThis):
    $(D alias) $(I Identifier) $(D this ;)
)

        $(P An $(I AliasThis) declaration names a member to subtype.
        The $(I Identifier) names that member.
        )

        $(P A class or struct can be implicitly converted to the $(I AliasThis)
        member.
        )

---
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
}
---

        $(P If the member is a class or struct, undefined lookups will
        be forwarded to the $(I AliasThis) member.
        )

---
struct Foo
{
    int baz = 4;
    int get() { return 7; }
}

class Bar
{
    Foo foo;
    alias foo this;
}

void test()
{
    auto bar = new Bar;
    int i = bar.baz; // i == 4
    i = bar.get(); // i == 7
}
---

        $(P If the $(I Identifier) refers to a property member
        function with no parameters, conversions and undefined
        lookups are forwarded to the return value of the function.
        )

---
struct S
{
    int x;
    @property int get()
    {
        return x * 2;
    }
    alias get this;
}

void test()
{
    S s;
    s.x = 2;
    int i = s; // i == 4
}
---

        $(P Multiple $(I AliasThis) are allowed. For implicit conversions
        and forwarded lookups, all $(I AliasThis) declarations are attempted;
        if more than one $(I AliasThis) is eligible, the ambiguity is
        disallowed by raising an error.
        Note: Multiple $(I AliasThis) is currently unimplemented.
        )


$(H3 $(LNAME2 auto, Scope Classes))
$(B Note): Scope classes have been $(DDSUBLINK deprecate, scope for allocating classes on the stack, recommended for deprecation).

        $(P A scope class is a class with the $(D scope) attribute, as in:)

------
scope class Foo { ... }
------

        The scope characteristic is inherited, so any classes derived
        from a scope class are also scope.

        $(P A scope class reference can only appear as a function local variable.
        It must be declared as being $(D scope):)

------
scope class Foo { ... }

void func()
{
    Foo f;    // error, reference to scope class must be scope
    scope Foo g = new Foo(); // correct
}
------

        When a 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.

$(H3 $(LNAME2 final, Final Classes))

        $(P Final classes cannot be subclassed:)

---
final class A { }
class B : A { }  // error, class A is final
---

$(H2 $(LNAME2 nested, Nested Classes))

        A $(I nested class) is a class that is declared inside the scope
        of a function or another class.
        A nested class has access to the variables and other symbols
        of the classes and functions it is nested inside:

------
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 $(D static) attribute, then it can
        not access variables of the enclosing scope that are local to the
        stack or need a $(D 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 $(D this) of the enclosing class's instance
        if it is nested inside a class.

        $(P 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
}
------



        $(P A $(I this) can be supplied to the creation of an
        inner class instance by prefixing it to the $(I NewExpression):
        )

---------
class Outer
{
    int a;

    class Inner
    {
        int foo()
        {
            return a;
        }
    }
}

int bar()
{
    Outer o = new Outer;
    o.a = 3;
    Outer.Inner oi = $(CODE_HIGHLIGHT o).new Inner;
    return oi.foo();    // returns 3
}
---------

        $(P Here $(D o) supplies the $(I this) to the outer class
        instance of $(D Outer).
        )

        $(P The property $(D .outer) used in a nested class gives the
        $(D this) pointer to its enclosing class. If the enclosing
        context is not a class, the $(D .outer) will give the pointer
        to it as a $(D void*) type.
        )

----
class Outer
{
    class Inner
    {
        Outer foo()
        {
            return this.$(CODE_HIGHLIGHT outer);
        }
    }

    void bar()
    {
        Inner i = new Inner;
        assert(this == i.foo());
    }
}

void test()
{
    Outer o = new Outer;
    o.bar();
}
----

$(H3 $(LNAME2 anonymous, Anonymous Nested Classes))

        $(P An anonymous nested class is both defined and instantiated with
        a $(I NewAnonClassExpression):
        )

$(GRAMMAR
$(GNAME NewAnonClassExpression):
    $(D new) $(GLINK2 expression, AllocatorArguments)$(OPT) $(D class) $(I ClassArguments)$(OPT) $(GLINK SuperClass)$(OPT) $(GLINK Interfaces)$(OPT) $(GLINK2 struct, AggregateBody)

$(GNAME ClassArguments):
    $(D $(LPAREN)) $(GLINK2 expression, ArgumentList)$(OPT) $(D $(RPAREN))
)

        $(P which is equivalent to:
        )

----
$(D class) $(I Identifier) $(D :) $(I SuperClass) $(I Interfaces) $(I AggregateBody)

$(D new) $(D $(LPAREN))$(I ArgumentList)$(D $(RPAREN)) $(I Identifier) $(D $(LPAREN))$(I ArgumentList)$(D $(RPAREN));
----

        $(P where $(I Identifier) is the name generated for the anonymous
        nested class.
        )

$(SECTION3 $(LEGACY_LNAME2 ConstClass, const-class, Const, Immutable and Shared Classes),

    $(P If a $(I ClassDeclaration) has a $(D const), $(D immutable)
        or $(D 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.
    )
)
)

Macros:
        TITLE=Classes
        WIKI=Class
        CATEGORY_SPEC=$0
        FOO=