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Every symbol, type, and expression has properties that can be queried:

Properties for All Types
.init initializer
.sizeofsize in bytes
.alignofalignment size
.mangleofstring representing the ‘mangled’ representation of the type
.stringofstring representing the source representation of the type
Expression Value
int.sizeofyields 4
(3).sizeofyields 4 (because 3 is an int)
int.init yields 0
int.mangleof yields the string "i"
int.stringof yields the string "int"
(1+2).stringof yields the string "1 + 2"

Properties for Integral Types
.maxmaximum value
.minminimum value

Properties for Floating Point Types
.infinityinfinity value
.nanNaN - Not a Number value (other NaN values can be produced)
.dignumber of decimal digits of precision
.epsilonsmallest increment to the value 1
.mant_dignumber of bits in mantissa
.max_10_expmaximum int value such that 10max_10_exp is representable
.max_expmaximum int value such that 2max_exp-1 is representable
.min_10_expminimum int value such that 10min_10_exp is representable as a normalized value
.min_expminimum int value such that 2min_exp-1 is representable as a normalized value
.maxlargest representable value that's not infinity
.min_normalsmallest representable normalized value that's not 0
.rereal part
.imimaginary part
Floating Point Examples
Expression Value
float.nan yields the floating point NaN value
(2.5F).nan yields the floating point NaN value

See also:

.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. The default values for different kinds of types are described below:

Type.init Value
Enumfirst member value
Floating PointNaN
Reference Typesnull
Structseach field's default value
Unionsfirst member value
Rationale: Where possible, the default value for a type is an invalid value.
int a;
int b = 1;

static assert(int.init == 0);
static assert(a.init == 0);
static assert(b.init == 0);

struct Foo
    int a;
    int b = 7;

static assert(Foo.init.a == 0);
static assert(Foo.init.b == 7);

.init vs Default Construction

Note that .init produces a default initialized object, not a default constructed one. If there is a default constructor for an object, it may produce a different value.

  1. If T is a nested struct, the context pointer in T.init is null.
  2. void main()
        int x;
        struct S
            void foo() { x = 1; }  // access x in enclosing scope via context pointer
        S s1;           // OK. S() correctly initialize its context pointer.
        S s2 = S();     // OK. same as s1;       // OK
        S s3 = S.init;  // Bad. the context pointer in s3 is null;       // Access violation
  3. If T is a struct which has @disable this();, T.init might return a logically incorrect object.
  4. struct S
        int x;
        @disable this();
        this(int n) { x = n; }
        invariant { assert(x > 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.x == 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. The expression will not be evaluated.

module test;
import std.stdio;

struct Dog { }

enum Color { Red }

int i = 4;

void main()
    writeln((1+2).stringof);       // "1 + 2"
    writeln(Dog.stringof);         // "Dog"
    writeln(test.Dog.stringof);    // "Dog"
    writeln(int.stringof);         // "int"
    writeln((int*[5][]).stringof); // "int*[5][]"
    writeln(Color.Red.stringof);   // "Red"
    writeln((5).stringof);         // "5"

    writeln((++i).stringof);       // "i += 1"
    writeln(i);                    // 4
Implementation Defined: The string representation for a type or expression can vary.
Best Practices: Do not use .stringof for code generation. Instead use the identifier trait, or one of the Phobos helper functions such as std.traits.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.

Implementation Defined: the actual aligned size.
Best Practices: Be particularly careful when laying out an object that must line up with an externally imposed layout. Data misalignment can result in particularly pernicious bugs. It's often worth putting in an assert to assure it is correct.

.mangleof Property

Mangling refers to how a symbol is represented in text form in the generated object file. .mangleof returns a string literal of the representation of the type or symbol it is applied to. The mangling of types and symbols with D linkage is defined by Name Mangling.

Implementation Defined:
  1. whether a leading underscore is added to a symbol
  2. the mangling of types and symbols with non-D linkage. For C and C++ linkage, this will typically match what the associated C or C++ compiler does.

.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.