D Programming Language 1.0

Last update Sun Dec 30 20:34:42 2012

# Properties

Every type and expression has properties that can be queried:

Property Examples
Expression | Value
--- | ---
int.sizeof | yields 4
float.nan | yields the floating point nan (Not A Number) value
(float).nan | yields the floating point nan value
(3).sizeof | yields 4 (because 3 is an int)
2.sizeof | syntax error, since "2." is a floating point number
int.init | default initializer for int's
int.mangleof | yields the string "i"
int.stringof | yields the string "int"
(1+2).stringof | yields the string "1 + 2"

Properties for All Types
Property | Description
--- | ---
.init | initializer
.sizeof | size in bytes (equivalent to C's sizeof(type))
.alignof | alignment size
.mangleof | string representing the ‘mangled’ representation of the type
.stringof | string representing the source representation of the type

Properties for Integral Types
Property | Description
--- | ---
.init | initializer (0)
.max | maximum value
.min | minimum value

Properties for Floating Point Types
Property | Description
--- | ---
.init | initializer (NaN)
.infinity | infinity value
.nan | NaN value
.dig | number of decimal digits of precision
.epsilon | smallest increment to the value 1
.mant_dig | number of bits in mantissa
.max_10_exp | maximum int value such that 10max_10_exp is representable
.max_exp | maximum int value such that 2max_exp-1 is representable
.min_10_exp | minimum int value such that 10min_10_exp is representable as a normalized value
.min_exp | minimum int value such that 2min_exp-1 is representable as a normalized value
.max | largest representable value that's not infinity
.min | smallest representable normalized value that's not 0
.re | real part
.im | imaginary part

Properties for Class Types
Property | Description
--- | ---
.classinfo | Information about the dynamic type of the class

## .init Property

.init produces a constant expression that is the default initializer. If applied to a type, it is the default initializer for that type. If applied to a variable or field, it is the default initializer for that variable or field's type. For example:

    int a;
    int b = 1;
    typedef int t = 2;
    t c;
    t d = cast(t)3;

    int.init // is 0
    a.init   // is 0
    b.init   // is 0
    t.init   // is 2
    c.init   // is 2
    d.init   // is 2

    struct Foo {
      int a;
      int b = 7;
    }

    Foo.init.a  // is 0
    Foo.init.b  // is 7

## .stringof Property

.stringof produces a constant string that is the source representation of its prefix. If applied to a type, it is the string for that type. If applied to an expression, it is the source representation of that expression. Semantic analysis is not done for that expression. For example:

    module test;
    import std.stdio;

    struct Foo { }

    enum Enum { RED }

    typedef int myint;

    void main() {
      writeln((1+2).stringof);       // "1 + 2"
      writeln(Foo.stringof);         // "Foo"
      writeln(test.Foo.stringof);    // "Foo"
      writeln(int.stringof);         // "int"
      writeln((int*[5][]).stringof); // "int*[5u][]"
      writeln(Enum.RED.stringof);    // "cast(enum)0"
      writeln(test.myint.stringof);  // "myint"
      writeln((5).stringof);         // "5"
    }

## .sizeof Property

`e.sizeof` gives the size in bytes of the expression e.

When getting the size of a member, it is not necessary for there to be a this object:

    struct S {
      int a;
      static int foo() {
        return a.sizeof; // returns 4
      }
    }

    void test() {
      int x = S.a.sizeof; // sets x to 4
    }

`.sizeof` applied to a class object returns the size of the class reference, not the class instantiation.

## .alignof Property

`.alignof` gives the aligned size of an expression or type. For example, an aligned size of 1 means that it is aligned on a byte boundary, 4 means it is aligned on a 32 bit boundary.

## .classinfo Property

`.classinfo` provides information about the dynamic type of a class object. It returns a reference to type object.ClassInfo.

`.classinfo` applied to an interface gives the information for the interface, not the class it might be an instance of.

### User Defined Properties

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 methhod 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 simple property would be:

    struct Foo
    {
      int data() { return m_data; }	// read property

      int data(int value) { return m_data = value; } // write property

    private:
      int m_data;
    }

To use it:

    int test() {
      Foo f;

      f.data = 3;        // same as f.data(3);
      return f.data + 3; // same as return f.data() + 3;
    }

The absence of a read method means that the property is write-only. The absence of a write method means that the property is read-only. Multiple write methods can exist; the correct one is selected using the usual function overloading rules.

In all the other respects, these methods are like any other methods. They can be static, have different linkages, be overloaded with methods with multiple parameters, have their address taken, etc.

Note: Properties cannot be the lvalue of an op=, ++, or -- operator.

The built in properties `.sizeof`, `.alignof`, and `.mangleof` may not be declared as fields or methods in structs, unions, classes or enums.

If a .property is applied to a user-defined property, the .property is applied to the result of the function call.

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