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.
Expression: AssignExpression AssignExpression , Expression 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: OrOrExpression OrOrExpression ? Expression : ConditionalExpression OrOrExpression: AndAndExpression OrOrExpression || AndAndExpression AndAndExpression: OrExpression AndAndExpression && OrExpression OrExpression: XorExpression OrExpression | XorExpression XorExpression: AndExpression XorExpression ^ AndExpression AndExpression: EqualExpression AndExpression & EqualExpression EqualExpression: RelExpression EqualExpression == RelExpression EqualExpression != RelExpression EqualExpression is RelExpression RelExpression: ShiftExpression InExpression RelExpression < ShiftExpression RelExpression <= ShiftExpression RelExpression > ShiftExpression RelExpression >= ShiftExpression RelExpression !<>= ShiftExpression RelExpression !<> ShiftExpression RelExpression <> ShiftExpression RelExpression <>= ShiftExpression RelExpression !> ShiftExpression RelExpression !>= ShiftExpression RelExpression !< ShiftExpression RelExpression !<= ShiftExpression InExpression: RelExpression in ShiftExpression ShiftExpression: AddExpression ShiftExpression << AddExpression ShiftExpression >> AddExpression ShiftExpression >>> AddExpression AddExpression: MulExpression AddExpression + MulExpression AddExpression - MulExpression AddExpression ~ MulExpression MulExpression: UnaryExpression MulExpression * UnaryExpression MulExpression / UnaryExpression MulExpression % UnaryExpression UnaryExpression: PostfixExpression & UnaryExpression ++ UnaryExpression -- UnaryExpression * UnaryExpression - UnaryExpression + UnaryExpression ! UnaryExpression ~ UnaryExpression delete UnaryExpression NewExpression cast ( Type ) UnaryExpression ( Type ) . Identifier ( Expression ) PostfixExpression: PrimaryExpression PostfixExpression . Identifier PostfixExpression ++ PostfixExpression -- PostfixExpression ( ArgumentList ) IndexExpression SliceExpression IndexExpression: PostfixExpression [ ArgumentList ] SliceExpression: PostfixExpression [ AssignExpression .. AssignExpression ] PrimaryExpression: Identifier .Identifier this super null true false NumericLiteral CharacterLiteral StringLiteral FunctionLiteral AssertExpression BasicType . Identifier typeid ( Type ) AssertExpression: assert ( Expression ) ArgumentList: AssignExpression AssignExpression , ArgumentList NewExpression: new BasicType Stars [ AssignExpression ] Declarator new BasicType Stars ( ArgumentList ) new BasicType Stars new ( ArgumentList ) BasicType Stars [ AssignExpression ] Declarator new ( ArgumentList ) BasicType Stars ( ArgumentList ) new ( ArgumentList ) BasicType Stars Stars nothing * * Stars
i = ++i; c = a + (a = b); func(++i, ++i);If the compiler can determine that the result of an expression is illegally dependent on the order of evaluation, it can issue an error (but is not required to). The ability to detect these kinds of errors is a quality of implementation issue.
AssignExpression , ExpressionThe 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.
ConditionalExpression = AssignExpressionThe right operand is implicitly converted to the type of the left operand, and assigned to it. The result type is the type of the lvalue, and the result value is the value of the lvalue after the assignment.
The left operand must be an lvalue.
ConditionalExpression += AssignExpression ConditionalExpression -= AssignExpression ConditionalExpression *= AssignExpression ConditionalExpression /= AssignExpression ConditionalExpression %= AssignExpression ConditionalExpression &= AssignExpression ConditionalExpression |= AssignExpression ConditionalExpression ^= AssignExpression ConditionalExpression <<= AssignExpression ConditionalExpression >>= AssignExpression ConditionalExpression >>>= AssignExpressionAssignment operator expressions, such as:
a op= bare semantically equivalent to:
a = a op bexcept that operand a is only evaluated once.
OrOrExpression ? Expression : ConditionalExpressionThe 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 || AndAndExpressionThe result type of an OrOr expression is bool, unless the right operand has type void, when the result is type void.
The OrOr expression 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 OrOr expression 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 OrOr expression is bool then the result of the expression is the right operand converted to type bool.
AndAndExpression && OrExpressionThe result type of an AndAnd expression is bool, unless the right operand has type void, when the result is type void.
The AndAnd expression 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 AndAnd expression 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 AndAnd expression is bool then the result of the expression is the right operand converted to type bool.
OrExpression | XorExpressionThe operands are OR'd together.
XorExpression ^ AndExpressionThe operands are XOR'd together.
AndExpression & EqualExpressionThe operands are AND'd together.
EqualExpression == RelExpression EqualExpression != RelExpression EqualExpression is RelExpressionEquality expressions compare the two operands for equality (==) or inequality (!=). The type of the result is bool. The operands go through the usual 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 struct objects means the bit patterns of the objects match exactly (the existence of alignment holes in the objects is accounted for, usually by setting them all to 0 upon initialization). Equality for floating point types is more complicated. -0 and +0 compare as equal. If either or both operands are NAN, then both the == and != comparisons return false. 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.imFor class objects, equality is defined as the result of calling Object.eq(). If one or the other or both objects are null, an exception is raised.
For static and dynamic arrays, equality is defined as the lengths of the arrays matching, and all the elements are equal.
EqualExpression is RelExpressionThe is compares for identity. To compare for not identity, use !(e1 is e2). The type of the result is bool. The operands go through the usual conversions to bring them to a common type before comparison.
For operand types other than class objects, static or dynamic arrays, identity is defined as being the same as equality.
For class objects, identity is defined as the object references are for the same object. Null class objects can be compared with is.
For static and dynamic arrays, identity is defined as referring to the same array elements.
The identity operator is cannot be overloaded.
RelExpression < ShiftExpression RelExpression <= ShiftExpression RelExpression > ShiftExpression RelExpression >= ShiftExpression RelExpression !<>= ShiftExpression RelExpression !<> ShiftExpression RelExpression <> ShiftExpression RelExpression <>= ShiftExpression RelExpression !> ShiftExpression RelExpression !>= ShiftExpression RelExpression !< ShiftExpression RelExpression !<= ShiftExpressionFirst, the integral promotions are done on the operands. The result type of a relational expression is bool.
For class objects, the result of Object.cmp() forms the left operand, and 0 forms the right operand. The result of the relational expression (o1 op o2) is:
(o1.cmp(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.
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.
Useful floating point operations must take into account NAN values. In particular, a relational operator can have NAN operands. The result of a relational operation on float values is less, greater, equal, or unordered (unordered means either or both of the operands is a NAN). That means there are 14 possible comparison conditions to test for:
Operator | Greater Than | Less Than | Equal | Unordered | Exception | Relation |
---|---|---|---|---|---|---|
== | F | F | T | F | no | equal |
!= | T | T | F | T | no | unordered, less, or greater |
> | T | F | F | F | yes | greater |
>= | T | F | T | F | yes | greater or equal |
< | F | T | F | F | yes | less |
<= | F | T | T | F | yes | less or equal |
!<>= | F | F | F | T | no | unordered |
<> | T | T | F | F | yes | less or greater |
<>= | T | T | T | F | yes | less, equal, or greater |
!<= | T | F | F | T | no | unordered or greater |
!< | T | F | T | T | no | unordered, greater, or equal |
!>= | F | T | F | T | no | unordered or less |
!> | F | T | T | T | no | unordered, less, or equal |
!<> | F | F | T | T | no | unordered or equal |
RelExpression in ShiftExpressionAn 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.
ShiftExpression << AddExpression ShiftExpression >> AddExpression ShiftExpression >>> AddExpressionThe operands must be integral types, and undergo the usual integral promotions. 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 more bits than the size of the quantity being shifted:
int c; c << 33; error
AddExpression + MulExpression AddExpression - MulExpression AddExpression ~ MulExpressionIf the operands are of integral types, they undergo integral promotions, 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 it is a pointer to a bit, the second operand is divided by 8 and added to the pointer. It is illegal if the second operand modulo 8 is non-zero.
bit* p; p += 1; // error, 1%8 is non-zero p += 8; // okIf 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.
Add expressions for floating point operands are not associative.
MulExpression * UnaryExpression MulExpression / UnaryExpression MulExpression % UnaryExpressionThe operands must be arithmetic types. They undergo integral promotions, and then are brought to a common type using 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. If the right operand of divide or modulus operators is 0, a DivideByZeroException is thrown.
For floating point operands, the operations correspond to the IEEE 754 floating point equivalents. The modulus operator only works with reals, it is illegal to use it with imaginary or complex operands.
Mul expressions for floating point operands are not associative.
& UnaryExpression ++ UnaryExpression -- UnaryExpression * UnaryExpression - UnaryExpression + UnaryExpression ! UnaryExpression ~ UnaryExpression delete UnaryExpression NewExpression cast ( Type ) UnaryExpression ( Type ) . Identifier ( Expression )
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 30 arrays of strings
(type) unaryexpressionThere is an ambiguity in the grammar, however. Consider:
(foo) - p;Is this a cast of a dereference of negated p to type foo, or is it p being subtracted from foo? This cannot be resolved without looking up foo in the symbol table to see if it is a type or a variable. But D's design goal is to have the syntax be context free - it needs to be able to parse the syntax without reference to the symbol table. So, in order to distinguish a cast from a parenthesized subexpression, a different syntax is necessary.
C++ does this by introducing:
dynamic_cast<type>(expression)which is ugly and clumsy to type. D introduces the cast keyword:
cast(foo) -p; // cast (-p) to type foo (foo) - p; // subtract p from foocast has the nice characteristic that it is easy to do a textual search for it, and takes some of the burden off of the relentlessly overloaded () operator.
D differs from C/C++ in another aspect of casts. Any casting of a class reference to a derived class reference is done with a runtime check to make sure it really is a proper downcast. This means that it is equivalent to the behavior of the dynamic_cast operator in C++.
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 }
PostfixExpression . Identifier PostfixExpression -> Identifier PostfixExpression ++ PostfixExpression -- PostfixExpression ( ArgumentList ) PostfixExpression [ ArgumentList ] PostfixExpression [ AssignExpression .. AssignExpression ]
PostfixExpression [ ArgumentList ]PostfixExpression is evaluated. if PostfixExpression is an expression of type static array or dynamic array, the variable length is declared and set to be the length of the array. A new declaration scope is created for the evaluation of the ArgumentList and length appears in that scope only.
PostfixExpression [ AssignExpression .. AssignExpression ]PostfixExpression is evaluated. if PostfixExpression is an expression of type static array or dynamic array, the variable length 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 length 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.
Identifier .Identifier this super null true false NumericLiteral CharacterLiteral StringLiteral FunctionLiteral AssertExpression BasicType . Identifier typeid ( Type )
class A { char get() { return 'A'; } char foo() { return typeof(this).get(); } char bar() { return this.get(); } } class B : A { char get() { return 'B'; } } void main() { B b = new B(); b.foo(); // returns 'A' b.bar(); // returns 'B' }
FunctionLiteral function FunctionBody function ( ParameterList ) FunctionBody function Type ( ParameterList ) FunctionBody delegate FunctionBody delegate ( ParameterList ) FunctionBody delegate Type ( ParameterList ) FunctionBodyFunctionLiterals enable embedding anonymous functions and anonymous delegates directly into expressions. Type is the return type of the function or delegate, if omitted it defaults to void. ( ParameterList ) forms the parameters to the function. If omitted it defaults to the empty parameter list (). The type of a function literal is pointer to function or pointer to delegate.
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;} ; }And:
int abc(int delegate(long i)); void test() { int b = 3; int foo(long c) { return 6 + b; } abc(&foo); }is exactly equivalent to:
int abc(int delegate(long i)); void test() { int b = 3; abc( delegate int(long c) { return 6 + b; } ); }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, delegate { d += 1; } ); loop(3, 10, delegate { f += 1; } ); 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.
AssertExpression: assert ( Expression )Asserts evaluate the expression. If the result is false, an AssertError is thrown. If the result is true, then no exception is thrown. It is an error if the expression contains any side effects that the program depends on. The compiler may optionally not evaluate assert expressions at all. The result type of an assert expression is void. Asserts are a fundamental part of the Contract Programming support in D.
TypeidExpression: typeid ( Type )Returns an instance of class TypeInfo corresponding to Type.