Attributes

Functions can be attributed in various ways in D. Let's have a look at two built-in attributes as well as user-defined attributes. There are also the built-ins @safe, @system and @trusted mentioned in the first chapter.

@property

Properties are functions that can be syntactically treated as if they were fields or variables. Properties can be read from or written to. A property is read by calling a method or function with no arguments; a property is written by calling a method or function with its argument being the value it is set to.

A function marked as @property looks like a normal member to the outside world:

struct Foo {
    @property bar() { return 10; }
    @property bar(int x) { writeln(x); }
}

Foo foo;
writeln(foo.bar); // actually calls foo.bar()
foo.bar = 10; // calls foo.bar(10);

@nogc

When the D compiler encounters a function that is marked as @nogc it will make sure that no memory allocations are done within the context of that function. A @nogc function is just allowed to call other @nogc functions.

void foo() @nogc {
  // ERROR:
    auto a = new A;
}

User-defined attributes (UDAs)

Any function or type in D can be attributed with user-defined types:

struct Bar { this(int x) {} }

struct Foo {
  @("Hello") {
      @Bar(10) void foo() {
        ...
      }
  }
}

Any type, built-in or user-defined, can be attributed to functions. The function foo() in this example will have the attributes "Hello" (type string) and Bar (type Bar with value 10). To get the attributes of a function (or type) use the built-in compiler traits __traits(getAttributes, Foo) which returns an AliasSeq.

UDAs allow generic code to be enhanced by giving user-defined types another dimension that helps compile time generators to adapt to that specific type.

In-depth

• User Defined Attributes in Programming in D
• Attributes in D

rdmd playground.d