Expressions are used to compute values with a resulting type. These values can then be assigned, tested, or ignored. Expressions can also have side effects.
Binary expressions and function arguments are evaluated in strictly left-to-right order.
import std.conv; int i = 0; (i = 2) = ++i * i++ + i; assert(i == 13); // left to right evaluation of side effects assert(text(++i, ++i) == "1415"); // left to right evaluation of arguments
Expression: CommaExpression CommaExpression: AssignExpression AssignExpression , CommaExpression
The left operand of the , is evaluated, then the right operand is evaluated. The type of the expression is the type of the right operand, and the result is the result of the right operand. Using the result of comma expressions isn't allowed.
AssignExpression: ConditionalExpression ConditionalExpression = AssignExpression ConditionalExpression += AssignExpression ConditionalExpression -= AssignExpression ConditionalExpression *= AssignExpression ConditionalExpression /= AssignExpression ConditionalExpression %= AssignExpression ConditionalExpression &= AssignExpression ConditionalExpression |= AssignExpression ConditionalExpression ^= AssignExpression ConditionalExpression ~= AssignExpression ConditionalExpression <<= AssignExpression ConditionalExpression >>= AssignExpression ConditionalExpression >>>= AssignExpression ConditionalExpression ^^= AssignExpression
For all assign expressions, the left operand must be a modifiable lvalue. The type of the assign expression is the type of the left operand, and the value is the value of the left operand after assignment occurs. The resulting expression is a modifiable lvalue.
If the operator is = then it is simple assignment. The right operand is implicitly converted to the type of the left operand, and assigned to it.
If the left and right operands are of the same struct type, and the struct type has a Postblit, then the copy operation is as described in Struct Postblit.
If the lvalue is the .length property of a dynamic array, the behavior is as described in Setting Dynamic Array Length.
If the lvalue is a static array or a slice, the behavior is as described in Array Copying and Array Setting.
If the lvalue is a user-defined property, the behavior is as described in Property Functions.
For arguments of built-in types, assignment operator expressions such as
a op= bare semantically equivalent to:
a = cast(typeof(a))(a op b)except that
For user-defined types, assignment operator expressions are overloaded separately from the binary operator. Still the left operand must be an lvalue.
ConditionalExpression: OrOrExpression OrOrExpression ? Expression : ConditionalExpression
The first expression is converted to bool, and is evaluated.
If it is true, then the second expression is evaluated, and its result is the result of the conditional expression.
If it is false, then the third expression is evaluated, and its result is the result of the conditional expression.
If either the second or third expressions are of type void, then the resulting type is void. Otherwise, the second and third expressions are implicitly converted to a common type which becomes the result type of the conditional expression.
OrOrExpression: AndAndExpression OrOrExpression || AndAndExpression
The result type of an OrOrExpression is bool, unless the right operand has type void, when the result is type void.
The OrOrExpression evaluates its left operand.
If the left operand, converted to type bool, evaluates to true, then the right operand is not evaluated. If the result type of the OrOrExpression is bool then the result of the expression is true.
If the left operand is false, then the right operand is evaluated. If the result type of the OrOrExpression is bool then the result of the expression is the right operand converted to type bool.
AndAndExpression: OrExpression AndAndExpression && OrExpression
The result type of an AndAndExpression is bool, unless the right operand has type void, when the result is type void.
The AndAndExpression evaluates its left operand.
If the left operand, converted to type bool, evaluates to false, then the right operand is not evaluated. If the result type of the AndAndExpression is bool then the result of the expression is false.
If the left operand is true, then the right operand is evaluated. If the result type of the AndAndExpression is bool then the result of the expression is the right operand converted to type bool.
Bit wise expressions perform a bitwise operation on their operands. Their operands must be integral types. First, the Usual Arithmetic Conversions are done. Then, the bitwise operation is done.
OrExpression: XorExpression OrExpression | XorExpression
The operands are OR'd together.
XorExpression: AndExpression XorExpression ^ AndExpression
The operands are XOR'd together.
AndExpression: CmpExpression AndExpression & CmpExpression
The operands are AND'd together.
CmpExpression: ShiftExpression EqualExpression IdentityExpression RelExpression InExpression
EqualExpression: ShiftExpression == ShiftExpression ShiftExpression != ShiftExpression
Equality expressions compare the two operands for equality (==) or inequality (!=). The type of the result is bool. The operands undergo the Usual Arithmetic Conversions to bring them to a common type before comparison.
If they are integral values or pointers, equality is defined as the bit pattern of the type matches exactly.
Equality for floating point types is more complicated. -0 and +0 compare as equal. If either or both operands are NAN, then both the == returns false and != returns true. Otherwise, the bit patterns are compared for equality.
For complex numbers, equality is defined as equivalent to:
x.re == y.re && x.im == y.imand inequality is defined as equivalent to:
x.re != y.re || x.im != y.im
Equality for struct objects means the logical product of all equality results of the corresponding object fields. If all struct fields use bitwise equality, the whole struct equality could be optimized to one memory comparison operation (the existence of alignment holes in the objects is accounted for, usually by setting them all to 0 upon initialization).
For class and struct objects, the expression (a == b) is rewritten as a.opEquals(b), and (a != b) is rewritten as !a.opEquals(b).
For class objects, the == and != operators are intended to compare the contents of the objects, however an appropriate opEquals override must be defined for this to work. The default opEquals provided by the root Object class is equivalent to the is operator. Comparing against null is invalid, as null has no contents. Use the is and !is operators instead.
class C; C c; if (c == null) // error ... if (c is null) // ok ...
For static and dynamic arrays, equality is defined as the lengths of the arrays matching, and all the elements are equal.
IdentityExpression: ShiftExpression is ShiftExpression ShiftExpression !is ShiftExpression
The is compares for identity. To compare for nonidentity, use e1 !is e2. The type of the result is bool. The operands undergo the Usual Arithmetic Conversions to bring them to a common type before comparison.
For class objects, identity is defined as the object references are for the same object. Null class objects can be compared with is.
For struct objects and floating point values, identity is defined as the bits in the operands being identical.
For static and dynamic arrays, identity is defined as referring to the same array elements and the same number of elements.
For other operand types, identity is defined as being the same as equality.
The identity operator is cannot be overloaded.
RelExpression: ShiftExpression < ShiftExpression ShiftExpression <= ShiftExpression ShiftExpression > ShiftExpression ShiftExpression >= ShiftExpression
First, the Usual Arithmetic Conversions are done on the operands. The result type of a relational expression is bool.
For class objects, the result of Object.opCmp() forms the left operand, and 0 forms the right operand. The result of the relational expression (o1 op o2) is:
(o1.opCmp(o2) op 0)
It is an error to compare objects if one is null.
For static and dynamic arrays, the result of the relational op is the result of the operator applied to the first non-equal element of the array. If two arrays compare equal, but are of different lengths, the shorter array compares as "less" than the longer array.
Integer comparisons happen when both operands are integral types.
| Operator | Relation |
|---|---|
| < | less |
| > | greater |
| <= | less or equal |
| >= | greater or equal |
| == | equal |
| != | not equal |
It is an error to have one operand be signed and the other unsigned for a <, <=, > or >= expression. Use casts to make both operands signed or both operands unsigned.
If one or both operands are floating point, then a floating point comparison is performed.
A relational operator can have NaN operands. If either or both operands is NaN, the floating point comparison operation returns as follows:
| Operator | Relation | Returns |
|---|---|---|
| < | less | false |
| > | greater | false |
| <= | less or equal | false |
| >= | greater or equal | false |
| == | equal | false |
| != | unordered, less, or greater | true |
For class objects, the relational operators compare the contents of the objects. Therefore, comparing against null is invalid, as null has no contents.
class C; C c; if (c < null) // error ...
InExpression: ShiftExpression in ShiftExpression ShiftExpression !in ShiftExpression
An associative array can be tested to see if an element is in the array:
int foo[char[]]; ... if ("hello" in foo) ...
The in expression has the same precedence as the relational expressions <, <=, etc. The return value of the InExpression is null if the element is not in the array; if it is in the array it is a pointer to the element.
The !in expression is the logical negation of the in operation.
ShiftExpression: AddExpression ShiftExpression << AddExpression ShiftExpression >> AddExpression ShiftExpression >>> AddExpression
The operands must be integral types, and undergo the Usual Arithmetic Conversions. The result type is the type of the left operand after the promotions. The result value is the result of shifting the bits by the right operand's value.
<< is a left shift. >> is a signed right shift. >>> is an unsigned right shift.
It's illegal to shift by the same or more bits than the size of the quantity being shifted:
int c; auto x = c << 33; // error
AddExpression: MulExpression AddExpression + MulExpression AddExpression - MulExpression CatExpression
If the operands are of integral types, they undergo the Usual Arithmetic Conversions, and then are brought to a common type using the Usual Arithmetic Conversions.
If either operand is a floating point type, the other is implicitly converted to floating point and they are brought to a common type via the Usual Arithmetic Conversions.
If the operator is + or -, and the first operand is a pointer, and the second is an integral type, the resulting type is the type of the first operand, and the resulting value is the pointer plus (or minus) the second operand multiplied by the size of the type pointed to by the first operand.
If the second operand is a pointer, and the first is an integral type, and the operator is +, the operands are reversed and the pointer arithmetic just described is applied.
If both operands are pointers, and the operator is +, then it is illegal.
If both operands are pointers, and the operator is -, the pointers are subtracted and the result is divided by the size of the type pointed to by the operands. It is an error if the pointers point to different types. The type of the result is ptrdiff_t.
If both operands are of integral types and an overflow or underflow occurs in the computation, wrapping will happen. That is, uint.max + 1 == uint.min and uint.min - 1 == uint.max.
Add expressions for floating point operands are not associative.
CatExpression: AddExpression ~ MulExpression
A CatExpression concatenates arrays, producing a dynamic array with the result. The arrays must be arrays of the same element type. If one operand is an array and the other is of that array's element type, that element is converted to an array of length 1 of that element, and then the concatenation is performed.
MulExpression: UnaryExpression MulExpression * UnaryExpression MulExpression / UnaryExpression MulExpression % UnaryExpression
The operands must be arithmetic types. They undergo the Usual Arithmetic Conversions.
For integral operands, the *, /, and % correspond to multiply, divide, and modulus operations. For multiply, overflows are ignored and simply chopped to fit into the integral type.
For integral operands of the / and % operators, the quotient rounds towards zero and the remainder has the same sign as the dividend. If the divisor is zero, an Exception is thrown.
For floating point operands, the * and / operations correspond to the IEEE 754 floating point equivalents. % is not the same as the IEEE 754 remainder. For example, 15.0 % 10.0 == 5.0, whereas for IEEE 754, remainder(15.0,10.0) == -5.0.
Mul expressions for floating point operands are not associative.
UnaryExpression: & UnaryExpression ++ UnaryExpression -- UnaryExpression * UnaryExpression - UnaryExpression + UnaryExpression ! UnaryExpression ComplementExpression ( Type ) . Identifier ( Type ) . TemplateInstance DeleteExpression CastExpression PowExpression
ComplementExpression: ~ UnaryExpression
ComplementExpressions work on integral types (except bool). All the bits in the value are complemented.
Note: the usual Integer Promotions are not performed prior to the complement operation.
NewExpression: new AllocatorArgumentsopt Type NewExpressionWithArgs NewExpressionWithArgs: new AllocatorArgumentsopt Type [ AssignExpression ] new AllocatorArgumentsopt Type ( ArgumentListopt ) NewAnonClassExpression AllocatorArguments: ( ArgumentListopt ) ArgumentList: AssignExpression AssignExpression , AssignExpression , ArgumentList
NewExpressions are used to allocate memory on the garbage collected heap (default) or using a class or struct specific allocator.
To allocate multidimensional arrays, the declaration reads in the same order as the prefix array declaration order.
char[][] foo; // dynamic array of strings ... foo = new char[][30]; // allocate array of 30 strings
The above allocation can also be written as:
foo = new char[][](30); // allocate array of 30 strings
To allocate the nested arrays, multiple arguments can be used:
int[][][] bar; ... bar = new int[][][](5, 20, 30);The code above is equivalent to:
bar = new int[][][5]; foreach (ref a; bar) { a = new int[][20]; foreach (ref b; a) { b = new int[30]; } }
If there is a new ( ArgumentList ), then those arguments are passed to the class or struct specific allocator function after the size argument.
If a NewExpression is used as an initializer for a function local variable with scope storage class, and the ArgumentList to new is empty, then the instance is allocated on the stack rather than the heap or using the class specific allocator.
DeleteExpression: delete UnaryExpression
NOTE: delete has been deprecated. Instead, please use destroy if feasible, or core.memory.__delete as a last resort.
If the UnaryExpression is a class object reference, and there is a destructor for that class, the destructor is called for that object instance.
Next, if the UnaryExpression is a class object reference, or a pointer to a struct instance, and the class or struct has overloaded operator delete, then that operator delete is called for that class object instance or struct instance.
Otherwise, the garbage collector is called to immediately free the memory allocated for the class instance or struct instance. If the garbage collector was not used to allocate the memory for the instance, undefined behavior will result.
If the UnaryExpression is a pointer or a dynamic array, the garbage collector is called to immediately release the memory. If the garbage collector was not used to allocate the memory for the instance, undefined behavior will result.
The pointer, dynamic array, or reference is set to null after the delete is performed. Any attempt to reference the data after the deletion via another reference to it will result in undefined behavior.
If UnaryExpression is a variable allocated on the stack, the class destructor (if any) is called for that instance. Neither the garbage collector nor any class deallocator is called.
CastExpression: cast ( Type ) UnaryExpression cast ( TypeCtorsopt ) UnaryExpression
A CastExpression converts the UnaryExpression to Type.
cast(foo) -p; // cast (-p) to type foo (foo) - p; // subtract p from foo
Any casting of a class reference to a derived class reference is done with a runtime check to make sure it really is a downcast. null is the result if it isn't.
class A { ... } class B : A { ... } void test(A a, B b) { B bx = a; // error, need cast B bx = cast(B) a; // bx is null if a is not a B A ax = b; // no cast needed A ax = cast(A) b; // no runtime check needed for upcast }
In order to determine if an object o is an instance of a class B use a cast:
if (cast(B) o) { // o is an instance of B } else { // o is not an instance of B }
Casting a pointer type to and from a class type is done as a type paint (i.e. a reinterpret cast).
Casting a dynamic array to another dynamic array is done only if the array lengths multiplied by the element sizes match. The cast is done as a type paint, with the array length adjusted to match any change in element size. If there's not a match, a runtime error is generated.
import std.stdio; int main() { byte[] a = [1,2,3]; auto b = cast(int[])a; // runtime array cast misalignment int[] c = [1, 2, 3]; auto d = cast(byte[])c; // ok // prints: // [1, 0, 0, 0, 2, 0, 0, 0, 3, 0, 0, 0] writeln(d); return 0; }
Casting a floating point literal from one type to another changes its type, but internally it is retained at full precision for the purposes of constant folding.
void test() { real a = 3.40483L; real b; b = 3.40483; // literal is not truncated to double precision assert(a == b); assert(a == 3.40483); assert(a == 3.40483L); assert(a == 3.40483F); double d = 3.40483; // truncate literal when assigned to variable assert(d != a); // so it is no longer the same const double x = 3.40483; // assignment to const is not assert(x == a); // truncated if the initializer is visible }
Casting a floating point value to an integral type is the equivalent of converting to an integer using truncation.
void main() { int a = cast(int) 0.8f; assert(a == 0); long b = cast(long) 1.5; assert(b == 1L); long c = cast(long) -1.5; assert(c == -1); }
Casting a value v to a struct S, when value is not a struct of the same type, is equivalent to:
S(v)
Casting to a CastQual replaces the qualifiers to the type of the UnaryExpression.
shared int x; assert(is(typeof(cast(const)x) == const int));
Casting with no Type or CastQual removes any top level const, immutable, shared or inout type modifiers from the type of the UnaryExpression.
shared int x; assert(is(typeof(cast()x) == int));
Casting an expression to void type is allowed to mark that the result is unused. On ExpressionStatement, it could be used properly to avoid "has no effect" error.
void foo(lazy void exp) {} void main() { foo(10); // NG - has no effect in expression '10' foo(cast(void)10); // OK }
PowExpression: PostfixExpression PostfixExpression ^^ UnaryExpression
PowExpression raises its left operand to the power of its right operand.
PostfixExpression: PrimaryExpression PostfixExpression . Identifier PostfixExpression . TemplateInstance PostfixExpression . NewExpression PostfixExpression ++ PostfixExpression -- PostfixExpression ( ArgumentListopt ) TypeCtorsopt BasicType ( ArgumentListopt ) IndexExpression SliceExpression
IndexExpression: PostfixExpression [ ArgumentList ]
PostfixExpression is evaluated. If PostfixExpression is an expression of type static array or dynamic array, the symbol $ is set to be the number of elements in the array. If PostfixExpression is a ValueSeq, the symbol $ is set to be the number of elements in the sequence. A new declaration scope is created for the evaluation of the ArgumentList and $ appears in that scope only.
If PostfixExpression is a ValueSeq, then the ArgumentList must consist of only one argument, and that must be statically evaluatable to an integral constant. That integral constant n then selects the nth expression in the ValueSeq, which is the result of the IndexExpression. It is an error if n is out of bounds of the ValueSeq.
SliceExpression: PostfixExpression [ ] PostfixExpression [ Slice ,opt ] Slice: AssignExpression AssignExpression , Slice AssignExpression .. AssignExpression AssignExpression .. AssignExpression , Slice
PostfixExpression is evaluated. if PostfixExpression is an expression of type static array or dynamic array, the special variable $ is declared and set to be the length of the array. A new declaration scope is created for the evaluation of the AssignExpression..AssignExpression and $ appears in that scope only.
The first AssignExpression is taken to be the inclusive lower bound of the slice, and the second AssignExpression is the exclusive upper bound. The result of the expression is a slice of the PostfixExpression array.
If the [ ] form is used, the slice is of the entire array.
The type of the slice is a dynamic array of the element type of the PostfixExpression.
A SliceExpression is not a modifiable lvalue.
If the slice bounds can be known at compile time, the slice expression is implicitly convertible to an lvalue of static array. For example:
arr[a .. b] // typed T[]
If both a and b are integers (may be constant-folded),
the slice expression can be converted to a static array type
T[b - a].
void foo(int[2] a) { assert(a == [2, 3]); } void bar(ref int[2] a) { assert(a == [2, 3]); a[0] = 4; a[1] = 5; assert(a == [4, 5]); } void baz(int[3] a) {} void main() { int[] arr = [1, 2, 3]; foo(arr[1 .. 3]); assert(arr == [1, 2, 3]); bar(arr[1 .. 3]); assert(arr == [1, 4, 5]); //baz(arr[1 .. 3]); // cannot match length }
The following forms of slice expression can be convertible to a static array type:
| Form | The length calculated at compile time |
|---|---|
| arr[] | The compile time length of arr if it's known. |
| arr[a .. b] | b - a |
| arr[e-a .. e] | a |
| arr[e .. e+b] | b |
| arr[e-a .. e+b] | a + b |
| arr[e+a .. e+b] | b - a if a <= b |
| arr[e-a .. e-b] | a - b if a >= b |
If PostfixExpression is a ValueSeq, then the result of the slice is a new ValueSeq formed from the upper and lower bounds, which must statically evaluate to integral constants. It is an error if those bounds are out of range.
PrimaryExpression: Identifier . Identifier TemplateInstance . TemplateInstance this super null true false $ IntegerLiteral FloatLiteral CharacterLiteral StringLiterals ArrayLiteral AssocArrayLiteral FunctionLiteral AssertExpression MixinExpression ImportExpression NewExpressionWithArgs BasicTypeX . Identifier BasicTypeX ( ArgumentListopt ) TypeCtor ( Type ) . Identifier TypeCtor ( Type ) ( ArgumentListopt ) Typeof TypeidExpression IsExpression ( Expression ) SpecialKeyword TraitsExpression
Identifier is looked up at module scope, rather than the current lexically nested scope.
Within a non-static member function, this resolves to a reference to the object for which the function was called. If the object is an instance of a struct, this will be a pointer to that instance. If a member function is called with an explicit reference to typeof(this), a non-virtual call is made:
class A { char get() { return 'A'; } char foo() { return typeof(this).get(); } char bar() { return this.get(); } } class B : A { override char get() { return 'B'; } } void main() { B b = new B(); assert(b.foo() == 'A'); assert(b.bar() == 'B'); }
Assignment to this is not allowed.
super is identical to this, except that it is cast to this's base class. It is an error if there is no base class. It is an error to use super within a struct member function. (Only class Object has no base class.) If a member function is called with an explicit reference to super, a non-virtual call is made.
Assignment to super is not allowed.
null represents the null value for pointers, pointers to functions, delegates, dynamic arrays, associative arrays, and class objects. If it has not already been cast to a type, it is given the singular type typeof(null) and it is an exact conversion to convert it to the null value for pointers, pointers to functions, delegates, etc. After it is cast to a type, such conversions are implicit, but no longer exact.
These are of type bool and when cast to another integral type become the values 1 and 0, respectively.
Character literals are single characters and resolve to one of type char, wchar, or dchar. If the literal is a \u escape sequence, it resolves to type wchar. If the literal is a \U escape sequence, it resolves to type dchar. Otherwise, it resolves to the type with the smallest size it will fit into.
StringLiterals: StringLiteral StringLiterals StringLiteral
String literals can implicitly convert to any of the following types, they have equal weight:
| immutable(char)* |
| immutable(wchar)* |
| immutable(dchar)* |
| immutable(char)[] |
| immutable(wchar)[] |
| immutable(dchar)[] |
By default, a string literal is typed as a dynamic array, but the element count is known at compile time. So all string literals can be implicitly converted to static array types.
void foo(char[2] a) { assert(a == "bc"); } void bar(ref const char[2] a) { assert(a == "bc"); } void baz(const char[3] a) {} void main() { string str = "abc"; foo(str[1 .. 3]); bar(str[1 .. 3]); //baz(str[1 .. 3]); // cannot match length }
String literals have a 0 appended to them, which makes them easy to pass to C or C++ functions expecting a const char* string. The 0 is not included in the .length property of the string literal.
ArrayLiteral: [ ArgumentListopt ]
Array literals are a comma-separated list of AssignExpressions between square brackets [ and ]. The AssignExpressions form the elements of a dynamic array, the length of the array is the number of elements. The common type of the all elements is taken to be the type of the array element, and all elements are implicitly converted to that type.
auto a1 = [1,2,3]; // type is int[], with elements 1, 2 and 3 auto a2 = [1u,2,3]; // type is uint[], with elements 1u, 2u, and 3u
By default, an array literal is typed as a dynamic array, but the element count is known at compile time. So all array literals can be implicitly converted to static array types.
void foo(long[2] a) { assert(a == [2, 3]); } void bar(ref long[2] a) { assert(a == [2, 3]); a[0] = 4; a[1] = 5; assert(a == [4, 5]); } void baz(const char[3] a) {} void main() { long[] arr = [1, 2, 3]; foo(arr[1 .. 3]); assert(arr == [1, 2, 3]); bar(arr[1 .. 3]); assert(arr == [1, 4, 5]); //baz(arr[1 .. 3]); // cannot match length }
If any of the arguments in the ArgumentList are a ValueSeq, then the elements of the ValueSeq are inserted as arguments in place of the sequence.
Array literals are allocated on the memory managed heap. Thus, they can be returned safely from functions:
int[] foo() { return [1, 2, 3]; }
When array literals are cast to another array type, each element of the array is cast to the new element type. When arrays that are not literals are cast, the array is reinterpreted as the new type, and the length is recomputed:
import std.stdio; void main() { // cast array literal const short[] ct = cast(short[]) [cast(byte)1, 1]; // this is equivalent with: // const short[] ct = [cast(short)1, cast(short)1]; writeln(ct); // writes [1, 1] // cast other array expression // --> normal behavior of CastExpression byte[] arr = [cast(byte)1, cast(byte)1]; short[] rt = cast(short[]) arr; writeln(rt); // writes [257] }In other words, casting literal expression will change the literal type.
AssocArrayLiteral: [ KeyValuePairs ] KeyValuePairs: KeyValuePair KeyValuePair , KeyValuePairs KeyValuePair: KeyExpression : ValueExpression KeyExpression: AssignExpression ValueExpression: AssignExpression
Associative array literals are a comma-separated list of key:value pairs between square brackets [ and ]. The list cannot be empty. The common type of the all keys is taken to be the key type of the associative array, and all keys are implicitly converted to that type. The common type of the all values is taken to be the value type of the associative array, and all values are implicitly converted to that type. An AssocArrayLiteral cannot be used to statically initialize anything.
[21u:"he", 38:"ho", 2:"hi"]; // type is string[uint], // with keys 21u, 38u and 2u // and values "he", "ho", and "hi"
If any of the keys or values in the KeyValuePairs are a ValueSeq, then the elements of the ValueSeq are inserted as arguments in place of the sequence.
FunctionLiteral: function Typeopt ParameterAttributes opt FunctionLiteralBody delegate Typeopt ParameterMemberAttributes opt FunctionLiteralBody ParameterMemberAttributes FunctionLiteralBody FunctionLiteralBody Lambda ParameterAttributes: Parameters FunctionAttributesopt ParameterMemberAttributes: Parameters MemberFunctionAttributesopt FunctionLiteralBody: BlockStatement FunctionContractsopt BodyStatement
FunctionLiterals enable embedding anonymous functions and anonymous delegates directly into expressions. Type is the return type of the function or delegate, if omitted it is inferred from any ReturnStatements in the FunctionLiteralBody. ( ArgumentList ) forms the arguments to the function. If omitted it defaults to the empty argument list ( ). The type of a function literal is pointer to function or pointer to delegate. If the keywords function or delegate are omitted, it is inferred from whether FunctionLiteralBody is actually accessing to the outer context.
For example:
int function(char c) fp; // declare pointer to a function void test() { static int foo(char c) { return 6; } fp = &foo; }is exactly equivalent to:
int function(char c) fp; void test() { fp = function int(char c) { return 6;} ; }
Also:
int abc(int delegate(int i)); void test() { int b = 3; int foo(int c) { return 6 + b; } abc(&foo); }is exactly equivalent to:
int abc(int delegate(int i)); void test() { int b = 3; abc( delegate int(int c) { return 6 + b; } ); }
and the following where the return type int and function/delegate are inferred:
int abc(int delegate(int i)); int def(int function(int s)); void test() { int b = 3; abc( (int c) { return 6 + b; } ); // inferred to delegate def( (int c) { return c * 2; } ); // inferred to function //def( (int c) { return c * b; } ); // error! // Because the FunctionLiteralBody accesses b, then the function literal type // is inferred to delegate. But def cannot receive delegate. }
If the type of a function literal can be uniquely determined from its context, the parameter type inference is possible.
void foo(int function(int) fp); void test() { int function(int) fp = (n) { return n * 2; }; // The type of parameter n is inferred to int. foo((n) { return n * 2; }); // The type of parameter n is inferred to int. }
Anonymous delegates can behave like arbitrary statement literals. For example, here an arbitrary statement is executed by a loop:
double test() { double d = 7.6; float f = 2.3; void loop(int k, int j, void delegate() statement) { for (int i = k; i < j; i++) { statement(); } } loop(5, 100, { d += 1; }); loop(3, 10, { f += 3; }); return d + f; }
When comparing with nested functions, the function form is analogous to static or non-nested functions, and the delegate form is analogous to non-static nested functions. In other words, a delegate literal can access stack variables in its enclosing function, a function literal cannot.
Lambda: function Typeopt ParameterAttributes => AssignExpression delegate Typeopt ParameterMemberAttributes => AssignExpression ParameterMemberAttributes => AssignExpression Identifier => AssignExpression
Lambdas are a shorthand syntax for FunctionLiterals.
Just one Identifier is rewritten to Parameters:
( Identifier )The following part => AssignExpression is rewritten to FunctionLiteralBody:
{ return AssignExpression ; }Example usage:
import std.stdio; void main() { auto i = 3; auto twice = function (int x) => x * 2; auto square = delegate (int x) => x * x; auto n = 5; auto mul_n = (int x) => x * n; writeln(twice(i)); // prints 6 writeln(square(i)); // prints 9 writeln(mul_n(i)); // prints 15 }
The implicit conversions of built-in scalar types can be explicitly represented by using function call syntax. For example:
auto a = short(1); // implicitly convert an integer literal '1' to short auto b = double(a); // implicitly convert a short variable 'a' to double auto c = byte(128); // error, 128 cannot be represented in a byte
If the argument is omitted, it means default construction of the scalar type:
auto a = ushort(); // same as: ushort.init auto b = wchar(); // same as: wchar.init auto c = creal(); // same as: creal.init
AssertExpression: assert ( AssignExpression ,opt ) assert ( AssignExpression , AssignExpression ,opt )
The first AssignExpression must evaluate to true. If it does not, an Assert Failure has occurred and the program enters an Invalid State.
If the first AssignExpression consists entirely of compile time constants, and evaluates to false, it is a special case; it signifies that it is unreachable code. Compile Time Function Execution (CTFE) is not attempted.
AssertExpression has different semantics if it is in a unittest or in contract.
The second AssignExpression, if present, must be implicitly convertible to type const(char)[].
If the first AssignExpression is a reference to a class instance for which a Class Invariant exists, the Class Invariant must hold.
If the first AssignExpression is a pointer to a struct instance for which a Struct Invariant exists, the Struct Invariant must hold.
The type of an AssertExpression is void.
void main() { assert(0, "an" ~ " error message"); }
When compiled and run, it will produce the message:
[email protected](3) an error message
The implementation may handle the case of the first AssignExpression evaluating at compile time to false differently in that in release mode it may simply generate a HLT instruction or equivalent.
MixinExpression: mixin ( AssignExpression )
The AssignExpression must evaluate at compile time to a constant string. The text contents of the string must be compilable as a valid Expression, and is compiled as such.
int foo(int x) { return mixin("x + 1") * 7; // same as ((x + 1) * 7) }
ImportExpression: import ( AssignExpression )
The AssignExpression must evaluate at compile time to a constant string. The text contents of the string are interpreted as a file name. The file is read, and the exact contents of the file become a string literal.
Implementations may restrict the file name in order to avoid directory traversal security vulnerabilities. A possible restriction might be to disallow any path components in the file name.
Note that by default an import expression will not compile unless you pass one or more paths via the -J switch. This tells the compiler where it should look for the files to import. This is a security feature.
void foo() { // Prints contents of file foo.txt writeln(import("foo.txt")); }
TypeidExpression: typeid ( Type ) typeid ( Expression )
If Type, returns an instance of class TypeInfo corresponding to Type.
If Expression, returns an instance of class TypeInfo corresponding to the type of the Expression. If the type is a class, it returns the TypeInfo of the dynamic type (i.e. the most derived type). The Expression is always executed.
class A { } class B : A { } void main() { writeln(typeid(int)); // int uint i; writeln(typeid(i++)); // uint writeln(i); // 1 A a = new B(); writeln(typeid(a)); // B writeln(typeid(typeof(a))); // A }
IsExpression: is ( Type ) is ( Type : TypeSpecialization ) is ( Type == TypeSpecialization ) is ( Type : TypeSpecialization , TemplateParameterList ) is ( Type == TypeSpecialization , TemplateParameterList ) is ( Type Identifier ) is ( Type Identifier : TypeSpecialization ) is ( Type Identifier == TypeSpecialization ) is ( Type Identifier : TypeSpecialization , TemplateParameterList ) is ( Type Identifier == TypeSpecialization , TemplateParameterList ) TypeSpecialization: Type struct union class interface enum function delegate super const immutable inout shared return __parameters
IsExpressions are evaluated at compile time and are used for checking for valid types, comparing types for equivalence, determining if one type can be implicitly converted to another, and deducing the subtypes of a type. The result of an IsExpression is a boolean of value true if the condition is satisfied. If the condition is not satisfied, the result is a boolean of value false.
Type is the type being tested. It must be syntactically correct, but it need not be semantically correct. If it is not semantically correct, the condition is not satisfied.
Identifier is declared to be an alias of the resulting type if the condition is satisfied. The Identifier forms can only be used if the IsExpression appears in a StaticIfCondition.
TypeSpecialization is the type that Type is being compared against.
The forms of the IsExpression are:
alias int func(int); // func is a alias to a function type void foo() { if (is(func[])) // not satisfied because arrays of // functions are not allowed writeln("satisfied"); else writeln("not satisfied"); if (is([][])) // error, [][] is not a syntactically valid type ... }
alias Bar = short; void foo() { if (is(Bar : int)) // satisfied because short can be // implicitly converted to int writeln("satisfied"); else writeln("not satisfied"); }
alias Bar = short; void foo() { if (is(Bar == int)) // not satisfied because short is not // the same type as int writeln("satisfied"); else writeln("not satisfied"); }
alias Bar = short; void foo() { static if (is(Bar T)) alias S = T; else alias S = long; writeln(typeid(S)); // prints "short" if (is(Bar T)) // error, Identifier T form can // only be in StaticIfConditions ... }
alias Bar = int; alias Abc = long*; void foo() { static if (is(Bar T : int)) alias S = T; else alias S = long; writeln(typeid(S)); // prints "int" static if (is(Abc U : U*)) { U u; writeln(typeid(typeof(u))); // prints "long" } }The way the type of Identifier is determined is analogous to the way template parameter types are determined by TemplateTypeParameterSpecialization.
| keyword | alias type for Identifier |
|---|---|
| struct | Type |
| union | Type |
| class | Type |
| interface | Type |
| super | TypeSeq of base classes and interfaces |
| enum | the base type of the enum |
| function | TypeSeq of the function parameter types. For C- and D-style variadic functions, only the non-variadic parameters are included. For typesafe variadic functions, the ... is ignored. |
| delegate | the function type of the delegate |
| return | the return type of the function, delegate, or function pointer |
| __parameters | the parameter sequence of a function, delegate, or function pointer. This includes the parameter types, names, and default values. |
| const | Type |
| immutable | Type |
| shared | Type |
alias Bar = short; enum E : byte { Emember } void foo() { static if (is(Bar T == int)) // not satisfied, short is not int alias S = T; alias U = T; // error, T is not defined static if (is(E V == enum)) // satisified, E is an enum V v; // v is declared to be a byte }
import std.stdio, std.typecons; void main() { alias Tup = Tuple!(int, string); alias AA = long[char[]]; static if (is(Tup : Template!Args, alias Template, Args...)) { writeln(__traits(isSame, Template, Tuple)); // true writeln(is(Template!(int, long) == Tup)); // true writeln(typeid(Args[0])); // int writeln(typeid(Args[1])); // immutable(char)[] } static if (is(AA T : T[U], U : const char[])) { writeln(typeid(T)); // long writeln(typeid(U)); // const char[] } static if (is(AA A : A[B], B : int)) { assert(0); // should not match, as B is not an int } static if (is(int[10] W : W[len], int len)) { writeln(typeid(W)); // int writeln(len); // 10 } static if (is(int[10] X : X[len], int len : 5)) { assert(0); // should not match, len should be 10 } }
SpecialKeyword: __FILE__ __FILE_FULL_PATH__ __MODULE__ __LINE__ __FUNCTION__ __PRETTY_FUNCTION__
__FILE__ and __LINE__ expand to the source file name and line number at the point of instantiation. The path of the source file is left up to the compiler.
__FILE_FULL_PATH__ expands to the absolute source file name at the point of instantiation.
__MODULE__ expands to the module name at the point of instantiation.
__FUNCTION__ expands to the fully qualified name of the function at the point of instantiation.
__PRETTY_FUNCTION__ is similar to __FUNCTION__, but also expands the function return type, its parameter types, and its attributes.
Example:
module test; import std.stdio; void test(string file = __FILE__, size_t line = __LINE__, string mod = __MODULE__, string func = __FUNCTION__, string pretty = __PRETTY_FUNCTION__, string fileFullPath = __FILE_FULL_PATH__) { writefln("file: '%s', line: '%s', module: '%s',\nfunction: '%s', " ~ "pretty function: '%s',\nfile full path: '%s'", file, line, mod, func, pretty, fileFullPath); } int main(string[] args) { test(); return 0; }
Assuming the file was at /example/test.d, this will output:
file: 'test.d', line: '13', module: 'test', function: 'test.main', pretty function: 'int test.main(string[] args)', file full path: '/example/test.d'
An implementation may rearrange the evaluation of expressions according to arithmetic associativity and commutativity rules as long as, within that thread of execution, no observable difference is possible.
This rule precludes any associative or commutative reordering of floating point expressions.