Properties - D Programming Language
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Keywords: D programming language

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# 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)
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_normal | 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

Note: .init produces a default initialized object, not default constructed. That means using .init is sometimes incorrect.

1. If T is a nested struct, the context pointer in T.init is null.

    void main()
    {
        int a;
        struct S
        {
            void foo() { a = 1; }  // access a variable in enclosing scope
        }
        S s1;           // OK. S() correctly initialize its frame pointer.
        S s2 = S();     // OK. same as s1
        S s3 = S.init;  // Bad. the frame pointer in s3 is null
        s3.foo();       // Access violation
    }

2. If T is a struct which has @disable this();, T.init might return a logically incorrect object.

    struct S
    {
        int a;
        @disable this();
        this(int n) { a = n; }
        invariant { assert(a > 0); }
        void check() {}
    }
    void main()
    {
      //S s1;           // Error: variable s1 initializer required for type S
      //S s2 = S();     // Error: constructor S.this is not callable
                        // because it is annotated with @disable
        S s3 = S.init;  // Bad. s3.a == 0, and it violates the invariant of S.
        s3.check();     // Assertion failure.
    }

### .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"
    }

Note: Using .stringof for code generation is not recommended, as the internal representation of a type or expression can change between different compiler versions.

Instead you should prefer to use the identifier trait, or one of the Phobos helper functions such as fullyQualifiedName.

### .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.TypeInfo_Class.

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

### User Defined Properties

User defined properties can be created using Property Functions.

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