template-mixin.dd

Ddoc

$(SPEC_S Template Mixins,

        A $(I TemplateMixin) takes an arbitrary set of declarations from
        the body of a $(I TemplateDeclaration) and inserts them
        into the current context.

$(GRAMMAR
$(GNAME TemplateMixinDeclaration):
    $(V2 $(B mixin)) $(B template) $(TEMPLATEIDENTIFIER) $(B $(LPAREN)) $(TEMPLATEPARAMETERLIST) $(B $(RPAREN)) $(V2 $(GLINK2 template, Constraint)$(SUB $(I opt)))
	    $(B {) $(GLINK2 module, DeclDefs) $(B })

$(GNAME TemplateMixin):
    $(B mixin) $(TEMPLATEIDENTIFIER) $(B ;)
    $(B mixin) $(TEMPLATEIDENTIFIER) $(GLINK MixinIdentifier) $(B ;)
    $(B mixin) $(TEMPLATEIDENTIFIER) $(B !$(LPAREN)) $(TEMPLATEARGUMENTLIST) $(B $(RPAREN)) $(B ;)
    $(B mixin) $(TEMPLATEIDENTIFIER) $(B !$(LPAREN)) $(TEMPLATEARGUMENTLIST) $(B $(RPAREN)) $(GLINK MixinIdentifier) $(B ;)

$(GNAME MixinIdentifier):
    $(I Identifier)
)

        $(P A $(I TemplateMixin) can occur in declaration lists of modules,
        classes, structs, unions, and as a statement.
        The $(I TemplateIdentifier) refers to a $(I TemplateDeclaration).
        If the $(I TemplateDeclaration) has no parameters, the mixin
        form that has no !($(I TemplateArgumentList))
        can be used.
        )

        $(P Unlike a template instantiation, a template mixin's body is evaluated
        within the scope where the mixin appears, not where the template declaration
        is defined. It is analogous to cutting and pasting the body of
        the template into the location of the mixin. It is useful for injecting
        parameterized $(SINGLEQUOTE boilerplate) code, as well as for creating
        templated nested functions, which is not possible with
        template instantiations.
        )

------
$(V2 mixin) template Foo() {
  int x = 5;
}

$(B mixin Foo;)

struct Bar {
  $(B mixin Foo;)
}

void test() {
  writefln("x = %d", x);  // prints 5
  {
    Bar b;
    int x = 3;

    writefln("b.x = %d", b.x); // prints 5
    writefln("x = %d", x);     // prints 3
    {
      $(B mixin Foo;)
      writefln("x = %d", x);   // prints 5
      x = 4;
      writefln("x = %d", x);   // prints 4
    }
    writefln("x = %d", x);     // prints 3
  }
  writefln("x = %d", x);       // prints 5
}
------

        Mixins can be parameterized:

------
$(V2 mixin) template Foo(T) {
  T x = 5;
}

$(B mixin Foo!(int);)           // create x of type int
------

        Mixins can add virtual functions to a class:

------
$(V2 mixin) template Foo() {
  void func() { writefln("Foo.func()"); }
}

class Bar {
  $(B mixin Foo);
}

class Code : Bar {
  $(V2 override) void func() { writefln("Code.func()"); }
}

void test() {
  Bar b = new Bar();
  b.func();      // calls Foo.func()

  b = new Code();
  b.func();      // calls Code.func()
}
------

        Mixins are evaluated in the scope of where they appear, not the scope
        of the template declaration:

------
int y = 3;

$(V2 mixin) template Foo() {
  int abc() { return y; }
}

void test() {
  int y = 8;
  $(B mixin Foo;) // local y is picked up, not global y
  assert(abc() == 8);
}
------

        Mixins can parameterize symbols using alias parameters:

------
$(V2 mixin) template Foo(alias b) {
  int abc() { return b; }
}

void test() {
  int y = 8;
  $(B mixin Foo!(y);)
  assert(abc() == 8);
}
------

        This example uses a mixin to implement a generic Duff's device
        for an arbitrary statement (in this case, the arbitrary statement
        is in bold). A nested function is generated as well as a
        delegate literal, these can be inlined by the compiler:

------
$(V2 mixin) template duffs_device(alias id1, alias id2, alias s)
{
  void duff_loop()
  {
    if (id1 < id2)
    {
      typeof(id1) n = (id2 - id1 + 7) / 8;
      switch ((id2 - id1) % 8)
      {
        case 0: do { s(); $(V2 goto case;)
        case 7:      s(); $(V2 goto case;)
        case 6:      s(); $(V2 goto case;)
        case 5:      s(); $(V2 goto case;)
        case 4:      s(); $(V2 goto case;)
        case 3:      s(); $(V2 goto case;)
        case 2:      s(); $(V2 goto case;)
        case 1:      s(); continue;
        default:     assert(0, "Impossible");
                } while (--n > 0);
      }
    }
  }
}

void foo() { writefln("foo"); }

void test() {
  int i = 1;
  int j = 11;

  mixin duffs_device!(i, j, $(B delegate { foo(); }) );
  duff_loop();  // executes foo() 10 times
}
------

<h2>Mixin Scope</h2>

        The declarations in a mixin are $(SINGLEQUOTE imported) into the surrounding
        scope. If the name of a declaration in a mixin is the same
        as a declaration in the surrounding scope, the surrounding declaration
        overrides the mixin one:

------
int x = 3;

$(V2 mixin) template Foo() {
  int x = 5;
  int y = 5;
}

$(B mixin Foo;)
int y = 3;

void test() {
  writefln("x = %d", x);  // prints 3
  writefln("y = %d", y);  // prints 3
}
------

        If two different mixins are put in the same scope, and each
        define a declaration with the same name, there is an ambiguity
        error when the declaration is referenced:

------
$(V2 mixin) template Foo() {
  int x = 5;
  void func(int x) { }
}

$(V2 mixin) template Bar() {
  int x = 4;
  void func() { }
}

$(B mixin Foo;)
$(B mixin Bar;)

void test() {
  writefln("x = %d", x); // error, x is ambiguous
  func();             // error, func is ambiguous
}
------
        $(P The call to $(B func()) is ambiguous because
        Foo.func and Bar.func are in different scopes.
        )

        $(P If a mixin has a $(I MixinIdentifier), it can be used to
        disambiguate:
        )
------
int x = 6;

$(V2 mixin) template Foo() {
  int x = 5;
  int y = 7;
  void func() { }
}

$(V2 mixin) template Bar() {
  int x = 4;
  void func() { }
}

$(B mixin Foo F;)
$(B mixin Bar B;)

void test() {
  writefln("y = %d", y);     // prints 7
  writefln("x = %d", x);     // prints 6
  writefln("F.x = %d", F.x); // prints 5
  writefln("B.x = %d", B.x); // prints 4
  F.func();                  // calls Foo.func
  B.func();                  // calls Bar.func
}
------
        $(P Alias declarations can be used to overload together
        functions declared in different mixins:)

-----
$(V2 mixin) template Foo() {
  void func(int x) {  }
}

$(V2 mixin) template Bar() {
  void func() {  }
}

mixin Foo!() F;
mixin Bar!() B;

$(B alias F.func func;)
$(B alias B.func func;)

void main() {
  func();  // calls B.func
  func(1); // calls F.func
}
-----


        $(P A mixin has its own scope, even if a declaration is overridden
        by the enclosing one:)

------
int x = 4;

$(V2 mixin) template Foo() {
  int x = 5;
  int bar() { return x; }
}

$(B mixin Foo;)

void test() {
  writefln("x = %d", x);         // prints 4
  writefln("bar() = %d", bar()); // prints 5
}
------

)

Macros:
        TITLE=Mixins
        WIKI=Mixin
        CATEGORY_SPEC=$0
        TEMPLATEIDENTIFIER=$(GLINK2 template, TemplateIdentifier)
        TEMPLATEPARAMETERLIST=$(GLINK2 template, TemplateParameterList)
        TEMPLATEARGUMENTLIST=$(GLINK2 template, TemplateArgumentList)
        FOO=