Types

D is statically typed. Every expression has a type. Types constrain the values an expression can hold, and determine the semantics of operations on those values.

Basic data types are leaf types. Derived data types build on leaf types. User defined types are aggregates of basic and derived types.

Basic Data Types

Derived Data Types

• pointer
• array
• associative array
• function
• delegate

Strings are a special case of arrays.

User-Defined Types

• enum
• struct
• union
• class

Base Types

The base type of an enum is the type it is based on:

enum E : T { ... }  // T is the base type of E

Pointer Conversions

Casting pointers to non-pointers and vice versa is allowed.

Implicit Conversions

Implicit conversions are used to automatically convert types as required.

An enum can be implicitly converted to its base type, but going the other way requires an explicit conversion. For example:

int i;

enum Foo { E }
Foo f;
i = f;           // OK
f = i;           // error
f = cast(Foo)i;  // OK
f = 0;           // error
f = Foo.E;       // OK

A derived class can be implicitly converted to its base class, but going the other way requires an explicit cast. For example:

class Base {}
class Derived : Base {}
Base bd = new Derived();              // implicit conversion
Derived db = cast(Derived)new Base(); // explicit conversion

A dynamic array, say x, of a derived class can be implicitly converted to a dynamic array, say y, of a base class iff elements of x and y are qualified as being either both const or both immutable.

class Base {}
class Derived : Base {}
const(Base)[] ca = (const(Derived)[]).init; // `const` elements
immutable(Base)[] ia = (immutable(Derived)[]).init; // `immutable` elements

A static array, say x, of a derived class can be implicitly converted to a static array, say y, of a base class iff elements of x and y are qualified as being either both const or both immutable or both mutable (neither const nor immutable).

class Base {}
class Derived : Base {}
Base[3] ma = (Derived[3]).init; // mutable elements
const(Base)[3] ca = (const(Derived)[3]).init; // `const` elements
immutable(Base)[3] ia = (immutable(Derived)[3]).init; // `immutable` elements

Integer Promotions

Integer Promotions are conversions of the following types:

If an enum has as a base type one of the types in the left column, it is converted to the type in the right column.

Usual Arithmetic Conversions

The usual arithmetic conversions convert operands of binary operators to a common type. The operands must already be of arithmetic types. The following rules are applied in order, looking at the base type:

1. If either operand is real, the other operand is converted to real.
2. Else if either operand is double, the other operand is converted to double.
3. Else if either operand is float, the other operand is converted to float.
4. Else the integer promotions are done on each operand, followed by:
   1. If both are the same type, no more conversions are done.
   2. If both are signed or both are unsigned, the smaller type is converted to the larger.
   3. If the signed type is larger than the unsigned type, the unsigned type is converted to the signed type.
   4. The signed type is converted to the unsigned type.

If one or both of the operand types is an enum after undergoing the above conversions, the result type is:

1. If the operands are the same type, the result will be of that type.
2. If one operand is an enum and the other is the base type of that enum, the result is the base type.
3. If the two operands are different enums, the result is the closest base type common to both. A base type being closer means there is a shorter sequence of conversions to base type to get there from the original type.

Integer values cannot be implicitly converted to another type that cannot represent the integer bit pattern after integral promotion. For example:

ubyte  u1 = -1;       // error, -1 cannot be represented in a ubyte
ushort u2 = -1;       // error, -1 cannot be represented in a ushort
uint   u3 = int(-1);  // ok, -1 can be represented in a uint
ulong  u4 = long(-1); // ok, -1 can be represented in a ulong

Floating point types cannot be implicitly converted to integral types. Complex or imaginary floating point types cannot be implicitly converted to non-complex floating point types. Non-complex floating point types cannot be implicitly converted to imaginary floating point types.

bool

The bool type is a byte-size type that can only hold the value true or false.

The only operators that can accept operands of type bool are: & |, ^, &=, |=, ^=, !, &&, ||, and ?:.

A bool value can be implicitly converted to any integral type, with false becoming 0 and true becoming 1.

The numeric literals 0 and 1 can be implicitly converted to the bool values false and true, respectively. Casting an expression to bool means testing for 0 or !=0 for arithmetic types, and null or !=null for pointers or references.

Delegates

Delegates are an aggregate of two pieces of data: an object reference and a pointer to a non-static member function, or a pointer to a closure and a pointer to a nested function. The object reference forms the this pointer when the function is called.

Delegates are declared similarly to function pointers:

int function(int) fp; // fp is pointer to a function
int delegate(int) dg; // dg is a delegate to a function

A delegate is initialized analogously to function pointers:

int func(int);
fp = &func;   // fp points to func
class OB
{
    int member(int);
}
OB o;
dg = &o.member; // dg is a delegate to object o and
                // member function member

Delegates cannot be initialized with static member functions or non-member functions.

Delegates are called analogously to function pointers:

fp(3);   // call func(3)
dg(3);   // call o.member(3)

The equivalent of member function pointers can be constructed using anonymous lambda functions:

class C
{
    int a;
    int foo(int i) { return i + a; }
}

// mfp is the member function pointer
auto mfp = function(C self, int i) { return self.foo(i); };
auto c = new C();  // create an instance of C
mfp(c, 1);  // and call c.foo(1)

The C style syntax for declaring pointers to functions is deprecated:

int (*fp)(int);  // fp is pointer to a function

Mixin Types

MixinType:
    mixin ( ArgumentList )

Each AssignExpression in the ArgumentList is evaluated at compile time, and the result must be representable as a string. The resulting strings are concatenated to form a string. The text contents of the string must be compilable as a valid Type, and is compiled as such.

void test(mixin("int")* p) // int* p
{
    mixin("int")[] a;      // int[] a;
    mixin("int[]") b;      // int[] b;
}

size_t

size_t is an alias to one of the unsigned integral basic types, and represents a type that is large enough to represent an offset into all addressable memory.

ptrdiff_t

ptrdiff_t is an alias to the signed integral basic type the same size as size_t.