Statement: LabeledStatement BlockStatement ExpressionStatement DeclarationStatement IfStatement DebugStatement VersionStatement WhileStatement DoWhileStatement ForStatement ForeachStatement SwitchStatement CaseStatement DefaultStatement ContinueStatement BreakStatement ReturnStatement GotoStatement WithStatement SynchronizeStatement TryStatement ThrowStatement VolatileStatement AsmStatement PragmaStatement
LabelledStatement: Identifier ':' StatementAny statement can be labelled, including empty statements, and so can serve as the target of a goto statement. Labelled statements can also serve as the target of a break or continue statement.
Labels are in a name space independent of declarations, variables, types, etc. Even so, labels cannot have the same name as local declarations. The label name space is the body of the function they appear in. Label name spaces do not nest, i.e. a label inside a block statement is accessible from outside that block.
BlockStatement: { } { StatementList } StatementList: Statement Statement StatementListA block statement introduces a new scope for local symbols. A local symbol's name, however, must be unique within the function.
void func1(int x) { int x; // illegal, x is multiply defined in function scope } void func2() { int x; { int x; // illegal, x is multiply defined in function scope } } void func3() { { int x; } { int x; // illegal, x is multiply defined in function scope } } void func4() { { int x; } { x++; // illegal, x is undefined } }The idea is to avoid bugs in complex functions caused by scoped declarations inadvertantly hiding previous ones. Local names should all be unique within a function.
ExpressionStatement: Expression ;Expressions that have no affect, like (x + x), are illegal in expression statements.
DeclarationStatement: Type IdentifierList ; IdentifierList: Variable Variable , IdentifierList Variable: Identifier Identifier = AssignmentExpressionIf no AssignmentExpression is there to initialize the variable, it is initialized to the default value for its type.
IfStatement: if ( Expression ) Statement if ( Expression ) Statement else StatementExpression is evaluated and must have a type that can be converted to a boolean. If it's true the if statement is transferred to, else the else statement is transferred to.
The 'dangling else' parsing problem is solved by associating the else with the nearest if statement.
WhileStatement: while ( Expression ) StatementExpression is evaluated and must have a type that can be converted to a boolean. If it's true the statement is executed. After the statement is executed, the Expression is evaluated again, and if true the statement is executed again. This continues until the Expression evaluates to false.
A break statement will exit the loop. A continue statement will transfer directly to evaluating Expression again.
DoStatement: do Statement while ( Expression )Statement is executed. Then Expression is evaluated and must have a type that can be converted to a boolean. If it's true the loop is iterated again. This continues until the Expression evaluates to false.
A break statement will exit the loop. A continue statement will transfer directly to evaluationg Expression again.
ForStatement: for (Initialize; Test; Increment) Statement Initialize: empty Expression Declaration Test: empty Expression Increment: empty ExpressionInitializer is executed. Test is evaluated and must have a type that can be converted to a boolean. If it's true the statement is executed. After the statement is executed, the Increment is executed. Then Test is evaluated again, and if true the statement is executed again. This continues until the Test evaluates to false.
A break statement will exit the loop. A continue statement will transfer directly to the Increment.
If Initializer declares a variable, that variable's scope extends through the end of Statement. For example:
for (int i = 0; i < 10; i++) foo(i);is equivalent to:
{ int i; for (i = 0; i < 10; i++) foo(i); }Function bodies cannot be empty:
for (int i = 0; i < 10; i++) ; // illegalUse instead:
for (int i = 0; i < 10; i++) { }The Initializer may be omitted. Test may also be omitted, and if so, it is treated as if it evaluated to true.
ForeachStatement: foreach (ForeachTypeList; Expression) Statement ForeachTypeList: ForeachType ForeachType , ForeachTypeList ForeachType: inout Type Identifier Type IdentifierExpression is evaluated. It must evaluate to an aggregate expression of type static array, dynamic array, associative array, struct, or class. The Statement is executed, once for each element of the aggregate expression. At the start of each iteration, the variables declared by the ForeachTypeList are set to be a copy of the contents of the aggregate. If the variable is inout, it is a reference to the contents of that aggregate.
If the aggregate expression is a static or dynamic array, there can be one or two variables declared. If one, then the variable is said to be the value set to the elements of the array, one by one. The type of the variable must match the type of the array contents, except for the special cases outlined below. If there are two variables declared, the first is said to be the index and the second is said to be the value. The index must be of int or uint type, it cannot be inout, and it is set to be the index of the array element.
char[] a; ... foreach (int i, char c; a) { printf("a[%d] = '%c'\n", i, c); }If the aggregate expression is a static or dynamic array of chars, wchars, or dchars, then the Type of the value can be any of char, wchar, or dchar. In this manner any UTF array can be decoded into any UTF type:
char[] a = "\xE2\x89\xA0"; // \u2260 encoded as 3 UTF-8 bytes foreach (dchar c; a) { printf("a[] = %x\n", c); // prints 'a[] = 2260' } dchar[] b = "\u2260"; foreach (char c; b) { printf("%x, ", c); // prints 'e2, 89, a0' }If the aggregate expression is an associative array, there can be one or two variables declared. If one, then the variable is said to be the value set to the elements of the array, one by one. The type of the variable must match the type of the array contents. If there are two variables declared, the first is said to be the index and the second is said to be the value. The index must be of the same type as the indexing type of the associative array. It cannot be inout, and it is set to be the index of the array element.
double[char[]] a; // index type is char[], value type is double ... foreach (char[] s, double d; a) { printf("a['%.*s'] = %g\n", s, d); }If the aggregate expression is a static or dynamic array, the elements are iterated over starting at index 0 and continuing to the maximum of the array. If it is an associative array, the order of the elements is undefined. If it is a struct or class object, it is defined by the special opApply member function.
If the aggregate is a struct or a class object, that struct or class must have an opApply function with the type:
int opApply(int delegate(inout Type [, ...]) dg);where Type matches the Type used in the foreach declaration of Identifier. Multiple ForeachTypes correspond with multiple Type's in the delegate type passed to opApply. There can be multiple opApply functions, one is selected by matching the type of dg to the ForeachTypes of the ForeachStatement. The body of the opApply function iterates over the elements it aggregates, passing them each to the dg function. If the dg returns 0, then opApply goes on to the next element. If the dg returns a nonzero value, opApply must cease iterating and return that value. Otherwise, after done iterating across all the elements, opApply will return 0.
For example, consider a class that is a container for two elements:
class Foo { uint array[2]; int opApply(int delegate(inout uint) dg) { int result = 0; for (int i = 0; i < array.length; i++) { result = dg(array[i]); if (result) break; } return result; } }An example using this might be:
void test() { Foo a = new Foo(); a.array[0] = 73; a.array[1] = 82; foreach (uint u; a) { printf("%d\n", u); } }which would print:
73 82Aggregates can be string literals, which can be accessed as char, wchar, or dchar arrays:
void test() { foreach (char c; "ab") { printf("'%c'\n", c); } foreach (wchar w; "xy") { wprintf("'%c'\n", w); } }which would print:
'a' 'b' 'x' 'y'inout can be used to update the original elements:
void test() { static uint[2] a = [7, 8]; foreach (inout uint u; a) { u++; } foreach (uint u; a) { printf("%d\n", u); } }which would print:
8 9The aggregate itself must not be resized, reallocated, free'd, reassigned or destructed while the foreach is iterating over the elements.
int[] a; int[] b; foreach (int i; a) { a = null; // error a.length += 10; // error a = b; // error } a = null; // okA BreakStatement in the body of the foreach will exit the foreach, a ContinueStatement will immediately start the next iteration.
SwitchStatement: switch ( Expression ) BlockStatement CaseStatement: case ExpressionList : Statement DefaultStatement: default: StatementExpression is evaluated. The result type T must be of integral type or char[] or wchar[]. The result is compared against each of the case expressions. If there is a match, the corresponding case statement is transferred to.
The case expressions, ExpressionList, are a comma separated list of expressions.
If none of the case expressions match, and there is a default statement, the default statement is transferred to.
If none of the case expressions match, and there is not a default statement, a SwitchError is thrown. The reason for this is to catch the common programming error of adding a new value to an enum, but failing to account for the extra value in switch statements. This behavior is unlike C or C++.
The case expressions must all evaluate to a constant value or array, and be implicitly convertible to the type T of the switch Expression.
Case expressions must all evaluate to distinct values. There may not be two or more default statements.
Case statements and default statements associated with the switch can be nested within block statements; they do not have to be in the outermost block. For example, this is allowed:
switch (i) { case 1: { case 2: } break; }Like in C and C++, case statements 'fall through' to subsequent case values. A break statement will exit the switch BlockStatement. For example:
switch (i) { case 1: x = 3; case 2: x = 4; break; case 3,4,5: x = 5; break; }will set x to 4 if i is 1.
Note: Unlike C and C++, strings can be used in switch expressions. For example:
char[] name; ... switch (name) { case "fred": case "sally": ... }For applications like command line switch processing, this can lead to much more straightforward code, being clearer and less error prone. Both ascii and wchar strings are allowed.
Implementation Note: The compiler's code generator may assume that the case statements are sorted by frequency of use, with the most frequent appearing first and the least frequent last. Although this is irrelevant as far as program correctness is concerned, it is of performance interest.
ContinueStatement: continue; continue Identifier ;continue executes the next iteration of its innermost enclosing while, for, or do loop. The increment clause is executed.
If continue is followed by Identifier, the Identifier must be the label of an enclosing while, for, or do loop, and the next iteration of that loop is executed. It is an error if there is no such statement.
Any intervening finally clauses are executed, and any intervening synchronization objects are released.
Note: If a finally clause executes a return, throw, or goto out of the finally clause, the continue target is never reached.
BreakStatement: break; break Identifier ;break exits the innermost enclosing while, for, do, or switch statement, resuming execution at the statement following it.
If break is followed by Identifier, the Identifier must be the label of an enclosing while, for, do or switch statement, and that statement is exited. It is an error if there is no such statement.
Any intervening finally clauses are executed, and any intervening synchronization objects are released.
Note: If a finally clause executes a return, throw, or goto out of the finally clause, the break target is never reached.
ReturnStatement: return; return Expression ;Expression is required if the function specifies a return type that is not void. The Expression is implicitly converted to the function return type.
At least one return statement is required if the function specifies a return type that is not void.
Expression is illegal if the function specifies a void return type.
Before the function actually returns, any enclosing finally clauses are executed, and any enclosing synchronization objects are released.
The function will not return if any enclosing finally clause does a return, goto or throw that exits the finally clause.
If there is an out postcondition (see design by contract), that postcondition is executed after the Expression is evaluated and before the function actually returns.
GotoStatement: goto Identifier ; goto default ; goto case ; goto case Expression ;The second form, goto default;, transfers to the innermost DefaultStatement of an enclosing SwitchStatement.
The third form, goto case;, transfers to the next CaseStatement of the innermost enclosing SwitchStatement.
The fourth form, goto case Expression;, transfers to the CaseStatement of the innermost enclosing SwitchStatement with a matching Expression.
Any intervening finally clauses are executed, along with releasing any intervening synchronization mutexes.
It is illegal for a goto to be used to skip initializations.
WithStatement: with ( Expression ) BlockStatement with ( TemplateInstance ) BlockStatementwhere Expression evaluates to an Object reference. Within the with body the referenced Object is searched first for identifier symbols. The with statement
with (expression) { ... ident; }is semantically equivalent to:
{ Object tmp; tmp = expression; ... tmp.ident; }Note that expression only gets evaluated once. The with statement does not change what this or super refer to.
SynchronizeStatement: synchronized Statement synchronized ( Expression ) Statementsynchronized allows only one thread at a time to execute Statement.
synchronized (Expression), where Expression evaluates to an Object reference, allows only one thread at a time to use that Object to execute the Statement.
The synchronization gets released even if Statement terminates with an exception, goto, or return.
Example:
synchronized { ... }This implements a standard critical section.
TryStatement: try BlockStatement Catches try BlockStatement Catches finally BlockStatement try BlockStatement finally BlockStatement Catches: LastCatch Catch Catch Catches LastCatch: catch BlockStatement Catch: catch ( Parameter ) BlockStatementParameter declares a variable v of type T, where T is Object or derived from Object. v is initialized by the throw expression if T is of the same type or a base class of the throw expression. The catch clause will be executed if the exception object is of type T or derived from T.
If just type T is given and no variable v, then the catch clause is still executed.
It is an error if any Catch Parameter type T1 hides a subsequent Catch with type T2, i.e. it is an error if T1 is the same type as or a base class of T2.
LastCatch catches all exceptions.
ThrowStatement: throw Expression ;Expression is evaluated and must be an Object reference. The Object reference is thrown as an exception.
VolatileStatement: volatile StatementStatement is evaluated, and no common subexpressions or memory references cached in registers are propagated either into it or out of it. This is useful for accessing memory that can change asynchronously, such as memory mapped I/O or memory accessed by multiple threads.
A volatile statement does not guarantee atomicity. For that, use synchronized statements.
AsmStatement: asm { } asm { AsmInstructionList } AsmInstructionList: AsmInstruction ; AsmInstruction ; AsmInstructionListAn asm statement enables the direct use of assembly language instructions. This makes it easy to obtain direct access to special CPU features without resorting to an external assembler. The D compiler will take care of the function calling conventions, stack setup, etc.
The format of the instructions is, of course, highly dependent on the native instruction set of the target CPU, and so is implementation defined. But, the format will follow the following conventions:
For example, for the Intel Pentium:
int x = 3; asm { mov EAX,x; // load x and put it in register EAX }Inline assembler can be used to access hardware directly:
int gethardware() { asm { mov EAX, dword ptr 0x1234; } }For some D implementations, such as a translator from D to C, an inline assembler makes no sense, and need not be implemented. The version statement can be used to account for this:
version (InlineAsm) { asm { ... } } else { ... some workaround ... }