www.digitalmars.com [Home] [Search] [D]
Last modified Feb 23, 2003.

Statements

C and C++ programmers will find the D statements very familiar, with a few interesting additions.
	Statement:
		LabeledStatement
		BlockStatement
		ExpressionStatement
		DeclarationStatement
		IfStatement
		DebugStatement
		VersionStatement
		WhileStatement
		DoWhileStatement
		ForStatement
		SwitchStatement
		CaseStatement
		DefaultStatement
		ContinueStatement
		BreakStatement
		ReturnStatement
		GotoStatement
		WithStatement
		SynchronizeStatement
		TryStatement
		ThrowStatement
		VolatileStatement
		AsmStatement

Labelled Statements

Statements can be labelled. A label is an identifier that precedes a statement.
	LabelledStatement:
		Identifier ':' Statement
	
Any 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.


Block Statement

A block statement is a sequence of statements enclosed by { }. The statements are executed in lexical order.
	BlockStatement:
		{ }
		{ StatementList }

	StatementList:
		Statement
		Statement StatementList
	
A 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.

Expression Statement

The expression is evaluated.
	ExpressionStatement:
		Expression ;
	
Expressions that have no affect, like (x + x), are illegal in expression statements.

Declaration Statement

Declaration statements declare and initialize variables.
	DeclarationStatement:
		Type IdentifierList ;

	IdentifierList:
		Variable
		Variable , IdentifierList

	Variable:
		Identifier
		Identifier = AssignmentExpression
	
If no AssignmentExpression is there to initialize the variable, it is initialized to the default value for its type.

If Statement

If statements provide simple conditional execution of statements.
	IfStatement:
		if ( Expression ) Statement
		if ( Expression ) Statement else Statement
	
Expression 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.


While Statement

While statements implement simple loops.
	WhileStatement:
		while ( Expression ) Statement
	
Expression 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 evaluationg Expression again.


Do-While Statement

Do-While statements implement simple loops.
	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.


For Statement

For statements implement loops with initialization, test, and increment clauses.
	ForStatement:
		for (Initialize; Test; Increment) Statement

	Initialize:
		empty
		Expression
		Declaration

	Test:
		empty
		Expression

	Increment:
		empty
		Expression
	
Initializer 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++)
		;	// illegal
	
Use 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.

Switch Statement

A switch statement goes to one of a collection of case statements depending on the value of the switch expression.
	SwitchStatement:
		switch ( Expression ) BlockStatement

	CaseStatement:
		case Expression : Statement

	DefaultStatement:
		default: Statement
	
Expression 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.

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 SwitchException 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.

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:
			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.


Continue Statement

A continue aborts the current iteration of its enclosing loop statement, and starts the next iteration.
	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.


Break Statement

A break exits the enclosing statement.
	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.


Return Statement

A return exits the current function and supplies its return value.
	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.


Goto Statement

A goto transfers to the statement labelled with Identifier.
	GotoStatement:
		goto Identifier ;
	
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.


With Statement

The with statement is a way to simplify repeated references to the same object.
	WithStatement:
		with ( Expression ) BlockStatement
		with ( TemplateInstance ) BlockStatement
	
where 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.

Synchronize Statement

The synchronize statement wraps a statement with critical section to synchronize access among multiple threads.
	SynchronizeStatement:
		synchronized Statement
		synchronized ( Expression ) Statement
	
synchronized 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.

Try Statement

Exception handling is done with the try-catch-finally statement.
	TryStatement:
		try BlockStatement Catches
		try BlockStatement Catches finally BlockStatement
		try BlockStatement finally BlockStatement

	Catches:
		LastCatch
		Catch
		Catch Catches

	LastCatch:
		catch BlockStatement

	Catch:
		catch ( Parameter ) BlockStatement
	
Parameter 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.


Throw Statement

Throw an exception.
	ThrowStatement:
		throw Expression ;
	
Expression is evaluated and must be an Object reference. The Object reference is thrown as an exception.

Volatile Statement

Do not cache values across volatile statement boundaries.
	VolatileStatement:
		volatile Statement
	
Statement 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.


Asm Statement

Inline assembler is supported with the asm statement:
	AsmStatement:
		asm { }
		asm { AsmInstructionList }

	AsmInstructionList:
		AsmInstruction ;
		AsmInstruction ; AsmInstructionList
	
An 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:

These rules exist to ensure that D source code can be tokenized independently of syntactic or semantic analysis.

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 ...
	}
	

Copyright (c) 1999-2002 by Digital Mars, All Rights Reserved