Statements

Contents

Scope Statements
Scope Block Statements
Labeled Statements
Block Statement
Expression Statement
Declaration Statement
If Statement
While Statement
Do Statement
For Statement
Foreach Statement
1. Foreach over Arrays
2. Foreach over Arrays of Characters
3. Foreach over Associative Arrays
4. Foreach over Structs and Classes with opApply
5. Foreach over Structs and Classes with Ranges
6. Foreach over Delegates
7. Foreach over Sequences
8. Foreach Ref Parameters
9. Foreach Restrictions
Foreach Range Statement
1. Break and Continue out of Foreach
Switch Statement
Final Switch Statement
Continue Statement
Break Statement
Return Statement
Goto Statement
With Statement
Synchronized Statement
Try Statement
Throw Statement
Scope Guard Statement
1. Catching C++ Class Objects
Asm Statement
Pragma Statement
Mixin Statement

The order of execution within a function is controlled by Statements. A function's body consists of a sequence of zero or more Statements. Execution occurs in lexical order, though certain statements may have deferred effects. A Statement has no value; it is executed for its effects.

Statement:
    ;
    NonEmptyStatement
    ScopeBlockStatement

NoScopeNonEmptyStatement:
    NonEmptyStatement
    BlockStatement

NoScopeStatement:
    ;
    NonEmptyStatement
    BlockStatement

NonEmptyOrScopeBlockStatement:
    NonEmptyStatement
    ScopeBlockStatement

NonEmptyStatement:
    NonEmptyStatementNoCaseNoDefault
    CaseStatement
    CaseRangeStatement
    DefaultStatement

NonEmptyStatementNoCaseNoDefault:
    LabeledStatement
    ExpressionStatement
    DeclarationStatement
    IfStatement
    WhileStatement
    DoStatement
    ForStatement
    ForeachStatement
    SwitchStatement
    FinalSwitchStatement
    ContinueStatement
    BreakStatement
    ReturnStatement
    GotoStatement
    WithStatement
    SynchronizedStatement
    TryStatement
    ScopeGuardStatement
    ThrowStatement
    AsmStatement
    MixinStatement
    ForeachRangeStatement
    PragmaStatement
    ConditionalStatement
    StaticForeachStatement
    StaticAssert
    TemplateMixin
    ImportDeclaration

Any ambiguities in the grammar between Statements and Declarations are resolved by the declarations taking precedence. Wrapping such a statement in parentheses will disambiguate it in favor of being a Statement.

Scope Statements

ScopeStatement:
    NonEmptyStatement
    BlockStatement

A new scope for local symbols is introduced for the NonEmptyStatement or BlockStatement.

Even though a new scope is introduced, local symbol declarations cannot shadow (hide) other local symbol declarations in the same function.

void func1(int x)
{
    int x;    // illegal, x shadows parameter x

    int y;

    { int y; } // illegal, y shadows enclosing scope's y

    void delegate() dg;
    dg = { int y; }; // ok, this y is not in the same function

    struct S
    {
        int y;    // ok, this y is a member, not a local
    }

    { int z; }
    { int z; }  // ok, this z is not shadowing the other z

    { int t; }
    { t++;   }  // illegal, t is undefined
}

Best Practices: Local declarations within a function should all have unique names, even if they are in non-overlapping scopes.

Scope Block Statements

ScopeBlockStatement:
    BlockStatement

A scope block statement introduces a new scope for the BlockStatement.

Labeled Statements

Statements can be labeled. A label is an identifier that precedes a statement.

LabeledStatement:
    Identifier :
    Identifier : NoScopeStatement
    Identifier : Statement

Any statement can be labeled, including empty statements, and so can serve as the target of a goto statement. Labeled statements can also serve as the target of a break or continue statement.

A label can appear without a following statement at the end of a block.

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.

Labels in one function cannot be referenced from another function.

Block Statement

BlockStatement:
    { }
    { StatementList }

StatementList:
    Statement
    Statement StatementList

A block statement is a sequence of statements enclosed by { }. The statements are executed in lexical order, until the end of the block is reached or a statement transfers control elsewhere.

Expression Statement

ExpressionStatement:
    Expression ;

The expression is evaluated.

Expressions that have no effect, like (x + x), are illegal as expression statements unless they are cast to void.

int x;
x++;               // ok
x;                 // illegal
1+1;               // illegal
cast(void)(x + x); // ok

Declaration Statement

Declaration statements define variables, and declare types, templates, functions, imports, conditionals, static foreaches, and static asserts.

DeclarationStatement:
    StorageClassesopt Declaration

Some declaration statements:

int a;        // declare a as type int and initialize it to 0
struct S { }  // declare struct s
alias myint = int;

If Statement

If statements provide simple conditional execution of statements.

IfStatement:
    if ( IfCondition ) ThenStatement
    if ( IfCondition ) ThenStatement else ElseStatement

IfCondition:
    Expression
    auto Identifier = Expression
    TypeCtors Identifier = Expression
    TypeCtorsopt BasicType Declarator = Expression

ThenStatement:
    ScopeStatement

ElseStatement:
    ScopeStatement

Expression is evaluated and must have a type that can be converted to a boolean. If it's true the ThenStatement is transferred to, else the ElseStatement is transferred to.

The ElseStatementis associated with the innermost if statement which does not already have an associated ElseStatement.

If an auto Identifier is provided, it is declared and initialized to the value and type of the Expression. Its scope extends from when it is initialized to the end of the ThenStatement.

If a TypeCtors Identifier is provided, it is declared to be of the type specified by TypeCtors and is initialized with the value of the Expression. Its scope extends from when it is initialized to the end of the ThenStatement.

If a Declarator is provided, it is declared and initialized to the value of the Expression. Its scope extends from when it is initialized to the end of the ThenStatement.

import std.regex;
...
if (auto m = std.regex.matchFirst("abcdef", "b(c)d"))
{
    writefln("[%s]", m.pre);    // prints [a]
    writefln("[%s]", m.post);   // prints [ef]
    writefln("[%s]", m[0]);     // prints [bcd]
    writefln("[%s]", m[1]);     // prints [c]
}
else
{
    writeln(m.post); // Error: undefined identifier 'm'
}
writeln(m.pre);      // Error: undefined identifier 'm'

While Statement

WhileStatement:
    while ( IfCondition ) ScopeStatement

A While Statement implements a simple loop.

If the IfCondition is an Expression, it is evaluated and must have a type that can be converted to a boolean. If it's true the ScopeStatement is executed. After the ScopeStatement is executed, the Expression is evaluated again, and if true the ScopeStatement is executed again. This continues until the Expression evaluates to false.

int i = 0;
while (i < 10)
{
    foo(i);
    ++i;
}

If an auto Identifier is provided, it is declared and initialized to the value and type of the Expression. Its scope extends from when it is initialized to the end of the ScopeStatement.

If a Declarator is provided, it is declared and initialized to the value of the Expression. Its scope extends from when it is initialized to the end of the ScopeStatement.

A BreakStatement will exit the loop.

A ContinueStatement will transfer directly to evaluating IfCondition again.

Do Statement

DoStatement:
    do ScopeStatement  while ( Expression ) ;

Do while statements implement simple loops.

ScopeStatement 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.

int i = 0;
do
{
    foo(i);
} while (++i < 10);

A BreakStatement will exit the loop. A ContinueStatement will transfer directly to evaluating Expression again.

For Statement

For statements implement loops with initialization, test, and increment clauses.

ForStatement:
    for ( Initialize Testopt ; Incrementopt ) ScopeStatement

Initialize:
    ;
    NoScopeNonEmptyStatement

Test:
    Expression

Increment:
    Expression

Initialize 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 BreakStatement will exit the loop. A ContinueStatement will transfer directly to the Increment.

A ForStatement creates a new scope. If Initialize declares a variable, that variable's scope extends through the end of the for 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 Initialize may be omitted (although the trailing ; is still required). Test may also be omitted, and if so, it is treated as if it evaluated to true.

Best Practices: Consider replacing ForStatements with Foreach Statements or Foreach Range Statements. Foreach loops are easier to understand, less prone to error, and easier to refactor.

Foreach Statement

A foreach statement loops over the contents of an aggregate.

AggregateForeach:
    Foreach ( ForeachTypeList ; ForeachAggregate )

ForeachStatement:
    AggregateForeach NoScopeNonEmptyStatement

Foreach:
    foreach
    foreach_reverse

ForeachTypeList:
    ForeachType
    ForeachType , ForeachTypeList

ForeachType:
    ForeachTypeAttributesopt BasicType Declarator
    ForeachTypeAttributesopt Identifier
    ForeachTypeAttributesopt alias Identifier

ForeachTypeAttributes
    ForeachTypeAttribute
    ForeachTypeAttribute ForeachTypeAttributesopt

ForeachTypeAttribute:
    ref
    TypeCtor
    enum

ForeachAggregate:
    Expression

ForeachAggregate is evaluated. It must evaluate to an expression of type static array, dynamic array, associative array, struct, class, delegate, or sequence. The NoScopeNonEmptyStatement is executed, once for each element of the aggregate. At the start of each iteration, the variables declared by the ForeachTypeList are set to be a copy of the elements of the aggregate. If the variable is ref, it is a reference to the contents of that aggregate.

The aggregate must be loop invariant, meaning that elements to the aggregate cannot be added or removed from it in the NoScopeNonEmptyStatement.

Foreach over Arrays

If the aggregate 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 type size_t for dynamic arrays. Static arrays may use any integral type that spans the length of the array. index cannot be ref. It is set to be the index of the array element.

char[] a;
...
foreach (int i, char c; a)
{
    writefln("a[%d] = '%c'", i, c);
}

For foreach, the elements for the array are iterated over starting at index 0 and continuing to the maximum of the array. For foreach_reverse, the array elements are visited in the reverse order.

Note: The ForeachTypeAttribute is implicit, and when a type is not specified, it is inferred. In that case, auto is implied, and it is not necessary (and actually forbidden) to use it.

int[] arr;
...
foreach (n; arr) // ok, n is an int
    writeln(n);

foreach (auto n; arr) // error, auto is redundant
    writeln(n);

Foreach over Arrays of Characters

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".dup;  // \u2260 encoded as 3 UTF-8 bytes

foreach (dchar c; a)
{
    writefln("a[] = %x", c); // prints 'a[] = 2260'
}

dchar[] b = "\u2260"d.dup;

foreach (char c; b)
{
    writef("%x, ", c);  // prints 'e2, 89, a0, '
}

Aggregates can be string literals, which can be accessed as char, wchar, or dchar arrays:

void test()
{
    foreach (char c; "ab")
    {
        writefln("'%s'", c);
    }
    foreach (wchar w; "xy")
    {
        writefln("'%s'", w);
    }
}

which would print:

'a'
'b'
'x'
'y'

Foreach over Associative Arrays

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 ref, and it is set to be the index of the array element. The order in which the elements of the array are iterated over is unspecified for foreach. foreach_reverse for associative arrays is illegal.

double[string] a; // index type is string, value type is double
...
foreach (string s, double d; a)
{
    writefln("a['%s'] = %g", s, d);
}

Foreach over Structs and Classes with opApply

If the aggregate expression is a struct or class object, the foreach is defined by the special opApply member function, and the foreach_reverse behavior is defined by the special opApplyReverse member function. These functions have the type:

int opApply(scope int delegate(ref Type [, ...]) dg);

int opApplyReverse(scope int delegate(ref Type [, ...]) dg);

where Type matches the Type used in the ForeachType declaration of Identifier. Multiple ForeachTypes correspond with multiple Types in the delegate type passed to opApply or opApplyReverse. There can be multiple opApply and opApplyReverse functions, one is selected by matching the type of dg to the ForeachTypes of the ForeachStatement. The body of the apply function iterates over the elements it aggregates, passing them each to the dg function. If the dg returns 0, then apply goes on to the next element. If the dg returns a nonzero value, apply must cease iterating and return that value. Otherwise, after done iterating across all the elements, apply will return 0.

For example, consider a class that is a container for two elements:

class Foo
{
    uint[2] array;

    int opApply(scope int delegate(ref 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)
    {
        writefln("%d", u);
    }
}

which would print:

73
82

The scope storage class on the dg parameter means that the parameter's value does not escape the scope of the opApply function (an example would be assigning dg to a global). If it cannot be statically guaranteed that dg does not escape, a closure may be allocated for it on the heap instead of the stack. Best practice is to annotate delegate parameters with scope when possible.

opApply can also be a templated function, which will infer the types of parameters based on the ForeachStatement.

For example:

struct S
{
    import std.traits : ParameterTypeTuple;  // introspection template

    int opApply(Dg)(scope Dg dg)
    if (ParameterTypeTuple!Dg.length == 2) // foreach with 2 parameters
    {
        return 0;
    }

    int opApply(Dg)(scope Dg dg)
    if (ParameterTypeTuple!Dg.length == 3) // foreach with takes 3 parameters
    {
        return 0;
    }
}

void main()
{
    foreach (int a, int b; S()) { }  // calls first opApply function
    foreach (int a, int b, float c; S()) { }  // calls second opApply function
}

It is important to make sure that, if opApply catches any exceptions, that those exceptions did not originate from the delegate passed to opApply. The user would expect exceptions thrown from a foreach body to both terminate the loop, and propagate outside the foreach body.

Foreach over Structs and Classes with Ranges

If the aggregate expression is a struct or class object, but the opApply for foreach, or opApplyReverse foreach_reverse do not exist, then iteration over struct and class objects can be done with range primitives. For foreach, this means the following properties and methods must be defined:

Foreach Range Properties
Property	Purpose
`.empty`	returns true if no more elements
`.front`	return the leftmost element of the range

Foreach Range Methods
Method	Purpose
`.popFront()`	move the left edge of the range right by one

Meaning:

foreach (e; range) { ... }

translates to:

for (auto __r = range; !__r.empty; __r.popFront())
{
    auto e = __r.front;
    ...
}

Similarly, for foreach_reverse, the following properties and methods must be defined:

Foreach_reverse Range Properties
Property	Purpose
`.empty`	returns true if no more elements
`.back`	return the rightmost element of the range

Foreach_reverse Range Methods
Method	Purpose
`.popBack()`	move the right edge of the range left by one

Meaning:

foreach_reverse (e; range) { ... }

translates to:

for (auto __r = range; !__r.empty; __r.popBack())
{
    auto e = __r.back;
    ...
}

Foreach over Delegates

If ForeachAggregate is a delegate, the type signature of the delegate is of the same as for opApply. This enables many different named looping strategies to coexist in the same class or struct.

For example:

void main()
{
    // Custom loop implementation, that iterates over powers of 2 with
    // alternating sign. The loop body is passed in dg.
    int myLoop(int delegate(ref int) dg)
    {
        for (int z = 1; z < 128; z *= -2)
        {
            auto ret = dg(z);

            // If the loop body contains a break, ret will be non-zero.
            if (ret != 0)
                return ret;
        }
        return 0;
    }

    // This example loop simply collects the loop index values into an array.
    int[] result;
    foreach (ref x; &myLoop)
    {
        result ~= x;
    }
    assert(result == [1, -2, 4, -8, 16, -32, 64, -128]);
}

Note: When ForeachAggregate is a delegate, the compiler does not try to implement reverse traversal of the results returned by the delegate when foreach_reverse is used. This may result in code that is confusing to read. Therefore, using foreach_reverse with a delegate is now deprecated, and will be rejected in the future.

Foreach over Sequences

If the aggregate expression is a sequence, there can be one or two iteration symbols declared. If one, then the symbol is an element alias of each element in the sequence in turn.

If the sequence is a TypeSeq, then the foreach statement is executed once for each type, and the element alias is set to each type.

When the sequence is a ValueSeq, the element alias is a variable and is set to each value in the sequence. If the type of the variable is given, it must match the type of the sequence contents. If no type is given, the type of the variable is set to the type of the sequence element, which may change from iteration to iteration.

If there are two symbols declared, the first is the index variable and the second is the element alias. The index must be of int, uint, long or ulong type, it cannot be ref, and it is set to the index of each sequence element.

Example:

import std.meta : AliasSeq;

void main()
{
    alias Seq = AliasSeq!(int, "literal", main);

    foreach (sym; Seq)
    {
        pragma(msg, sym.stringof);
    }
}

Output:

int
"literal"
main()

Foreach Ref Parameters

ref can be used to update the original elements:

void test()
{
    static uint[2] a = [7, 8];

    foreach (ref uint u; a)
    {
        u++;
    }
    foreach (uint u; a)
    {
        writefln("%d", u);
    }
}

which would print:

8
9

ref can not be applied to the index values.

If not specified, the Types in the ForeachType can be inferred from the type of the ForeachAggregate.

Foreach Restrictions

The 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;         // ok

Foreach Range Statement

A foreach range statement loops over the specified range.

RangeForeach:
    Foreach ( ForeachType ; LwrExpression .. UprExpression )

LwrExpression:
    Expression

UprExpression:
    Expression

ForeachRangeStatement:
    RangeForeach ScopeStatement

ForeachType declares a variable with either an explicit type, or a type inferred from LwrExpression and UprExpression. The ScopeStatement is then executed n times, where n is the result of UprExpression - LwrExpression. If UprExpression is less than or equal to LwrExpression, the ScopeStatement is executed zero times. If Foreach is foreach, then the variable is set to LwrExpression, then incremented at the end of each iteration. If Foreach is foreach_reverse, then the variable is set to UprExpression, then decremented before each iteration. LwrExpression and UprExpression are each evaluated exactly once, regardless of how many times the ScopeStatement is executed.

import std.stdio;

int foo()
{
    write("foo");
    return 10;
}

void main()
{
    foreach (i; 0 .. foo())
    {
        write(i);
    }
}

prints:

foo0123456789

Break and Continue out of Foreach

A BreakStatement in the body of the foreach will exit the foreach, a ContinueStatement will immediately start the next iteration.

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 ) ScopeStatement

CaseStatement:
    case ArgumentList : ScopeStatementList

CaseRangeStatement:
    case FirstExp : .. case LastExp : ScopeStatementList

FirstExp:
    AssignExpression

LastExp:
    AssignExpression

DefaultStatement:
    default : ScopeStatementList

ScopeStatementList:
    StatementListNoCaseNoDefault

StatementListNoCaseNoDefault:
    StatementNoCaseNoDefault
    StatementNoCaseNoDefault StatementListNoCaseNoDefault

StatementNoCaseNoDefault:
    ;
    NonEmptyStatementNoCaseNoDefault
    ScopeBlockStatement

Expression is evaluated. The result type T must be of integral type or char[], wchar[] or dchar[]. 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, ArgumentList, are a comma separated list of expressions.

A CaseRangeStatement is a shorthand for listing a series of case statements from FirstExp to LastExp.

If none of the case expressions match, and there is a default statement, the default statement is transferred to.

A switch statement must have a default statement.

The case expressions must all evaluate to a constant value or array, or a runtime initialized const or immutable variable of integral type. They must be implicitly convertible to the type of the switch Expression.

Case expressions must all evaluate to distinct values. Const or immutable variables must all have different names. If they share a value, the first case statement with that value gets control. There must be exactly one default statement.

The ScopeStatementList introduces a new scope.

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;
}

A ScopeStatementList must either be empty, or be ended with a ContinueStatement, BreakStatement, ReturnStatement, GotoStatement, ThrowStatement or assert(0) expression unless this is the last case. This is to set apart with C's error-prone implicit fall-through behavior. goto case; could be used for explicit fall-through:

int number;
string message;
switch (number)
{
    default:    // valid: ends with 'throw'
        throw new Exception("unknown number");

    case 3:     // valid: ends with 'break' (break out of the 'switch' only)
        message ~= "three ";
        break;

    case 4:     // valid: ends with 'continue' (continue the enclosing loop)
        message ~= "four ";
        continue;

    case 5:     // valid: ends with 'goto' (explicit fall-through to next case.)
        message ~= "five ";
        goto case;

    case 6:     // ERROR: implicit fall-through
        message ~= "six ";

    case 1:     // valid: the body is empty
    case 2:     // valid: this is the last case in the switch statement.
        message = "one or two";
}

A break statement will exit the switch BlockStatement.

Strings can be used in switch expressions. For example:

string 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. char, wchar and dchar 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.

Final Switch Statement

FinalSwitchStatement:
    final switch ( Expression ) ScopeStatement

A final switch statement is just like a switch statement, except that:

No DefaultStatement is allowed.
No CaseRangeStatements are allowed.
If the switch Expression is of enum type, all the enum members must appear in the CaseStatements.
The case expressions cannot evaluate to a run time initialized value.

Implementation Defined: If the Expression value does not match any of the CaseRangeStatements, whether that is diagnosed at compile time or at runtime.

Continue Statement

ContinueStatement:
    continue Identifieropt ;

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

continue executes the next iteration of its innermost enclosing while, for, foreach, 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 throw out of the finally clause, the continue target is never reached.

for (i = 0; i < 10; i++)
{
    if (foo(i))
        continue;
    bar();
}

Break Statement

BreakStatement:
    break Identifieropt ;

break exits the innermost enclosing while, for, foreach, 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 throw out of the finally clause, the break target is never reached.

for (i = 0; i < 10; i++)
{
    if (foo(i))
        break;
}

Return Statement

ReturnStatement:
    return Expressionopt ;

return exits the current function and supplies its return value.

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, throw statement, or assert(0) expression is required if the function specifies a return type that is not void, unless the function contains inline assembler code.

Before the function actually returns, any objects with scope storage duration are destroyed, any enclosing finally clauses are executed, any scope(exit) statements are executed, any scope(success) statements 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 Contract Programming), that postcondition is executed after the Expression is evaluated and before the function actually returns.

int foo(int x)
{
    return x + 3;
}

Goto Statement

GotoStatement:
    goto Identifier ;
    goto default ;
    goto case ;
    goto case Expression ;

goto transfers to the statement labeled with Identifier.

    if (foo)
        goto L1;
    x = 3;
L1:
    x++;

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.

switch (x)
{
    case 3:
        goto case;
    case 4:
        goto default;
    case 5:
        goto case 4;
    default:
        x = 4;
        break;
}

Any intervening finally clauses are executed, along with releasing any intervening synchronization mutexes.

It is illegal for a GotoStatement 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 ) ScopeStatement
    with ( Symbol ) ScopeStatement
    with ( TemplateInstance ) ScopeStatement

where Expression evaluates to a class reference or struct instance. Within the with body the referenced object is searched first for identifier symbols.

The WithStatement

with (expression)
{
    ...
    ident;
}

is semantically equivalent to:

{
    Object tmp;
    tmp = expression;
    ...
    tmp.ident;
}

Note that Expression only gets evaluated once and is not copied. The with statement does not change what this or super refer to.

For Symbol which is a scope or TemplateInstance, the corresponding scope is searched when looking up symbols. For example:

struct Foo
{
    alias Y = int;
}
...
Y y;        // error, Y undefined
with (Foo)
{
    Y y;    // same as Foo.Y y;
}

Use of with object symbols that shadow local symbols with the same identifier are not allowed. This is to reduce the risk of inadvertent breakage of with statements when new members are added to the object declaration.

struct S
{
    float x;
}

void main()
{
    int x;
    S s;
    with (s)
    {
        x++;  // error, shadows the int x declaration
    }
}

In nested WithStatements, the inner-most scope takes precedence. If a symbol cannot be resolved at the inner-most scope, resolution is forwarded incrementally up the scope hierarchy.

import std.stdio;

struct Foo
{
    void f() { writeln("Foo.f"); }
}

struct Bar
{
    void f() { writeln("Bar.f"); }
}

struct Baz
{
    // f() is not implemented
}

void f()
{
    writeln("f");
}

void main()
{
    Foo foo;
    Bar bar;
    Baz baz;

    f();               // prints "f"

    with(foo)
    {
        f();           // prints "Foo.f"

        with(bar)
        {
            f();       // prints "Bar.f"

            with(baz)
            {
                f();   // prints "Bar.f".  `Baz` does not implement `f()` so
                       // resolution is forwarded to `with(bar)`'s scope
            }
        }
        with(baz)
        {
            f();       // prints "Foo.f".  `Baz` does not implement `f()` so
                       // resolution is forwarded to `with(foo)`'s scope
        }
    }
    with(baz)
    {
        f();           // prints "f".  `Baz` does not implement `f()` so
                       // resolution is forwarded to `main`'s scope. `f()` is
                       // not implemented in `main`'s scope, so resolution is
                       // subsequently forward to module scope.
    }
}

Synchronized Statement

The synchronized statement wraps a statement with a mutex to synchronize access among multiple threads.

SynchronizedStatement:
    synchronized ScopeStatement
    synchronized ( Expression ) ScopeStatement

Synchronized allows only one thread at a time to execute ScopeStatement by using a mutex.

What mutex is used is determined by the Expression. If there is no Expression, then a global mutex is created, one per such synchronized statement. Different synchronized statements will have different global mutexes.

If there is an Expression, it must evaluate to either an Object or an instance of an Interface, in which case it is cast to the Object instance that implemented that Interface. The mutex used is specific to that Object instance, and is shared by all synchronized statements referring to that instance.

The synchronization gets released even if ScopeStatement terminates with an exception, goto, or return.

Example:

synchronized { ... }

This implements a standard critical section.

Synchronized statements support recursive locking; that is, a function wrapped in synchronized is allowed to recursively call itself and the behavior will be as expected: The mutex will be locked and unlocked as many times as there is recursion.

Try Statement

Exception handling is done with the try-catch-finally statement.

TryStatement:
    try ScopeStatement Catches
    try ScopeStatement Catches FinallyStatement
    try ScopeStatement FinallyStatement

Catches:
    Catch
    Catch Catches

Catch:
    catch ( CatchParameter ) NoScopeNonEmptyStatement

CatchParameter:
    BasicType Identifieropt

FinallyStatement:
    finally NoScopeNonEmptyStatement

CatchParameter declares a variable v of type T, where T is Throwable or derived from Throwable. 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 CatchParameter 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.

The FinallyStatement is always executed, whether the try ScopeStatement exits with a goto, break, continue, return, exception, or fall-through.

If an exception is raised in the FinallyStatement and is not caught before the original exception is caught, it is chained to the previous exception via the next member of Throwable. Note that, in contrast to most other programming languages, the new exception does not replace the original exception. Instead, later exceptions are regarded as 'collateral damage' caused by the first exception. The original exception must be caught, and this results in the capture of the entire chain.

Thrown objects derived from class $Error are treated differently. They bypass the normal chaining mechanism, such that the chain can only be caught by catching the first Error. In addition to the list of subsequent exceptions, Error also contains a pointer that points to the original exception (the head of the chain) if a bypass occurred, so that the entire exception history is retained.

import std.stdio;

int main()
{
    try
    {
        try
        {
            throw new Exception("first");
        }
        finally
        {
            writeln("finally");
            throw new Exception("second");
        }
    }
    catch (Exception e)
    {
        writeln("catch %s", e.msg);
    }
    writeln("done");
    return 0;
}

prints:

finally
catch first
done

A FinallyStatement may not exit with a goto, break, continue, or return; nor may it be entered with a goto.

A FinallyStatement may not contain any Catches. This restriction may be relaxed in future versions.

Throw Statement

Throws an exception.

ThrowStatement:
    throw Expression ;

Expression is evaluated and must be a Throwable reference. The Throwable reference is thrown as an exception.

throw new Exception("message");

Best Practices: Use Assert Expressions rather than Error to report program bugs and abort the program.

Scope Guard Statement

ScopeGuardStatement:
    scope(exit) NonEmptyOrScopeBlockStatement
    scope(success) NonEmptyOrScopeBlockStatement
    scope(failure) NonEmptyOrScopeBlockStatement

The ScopeGuardStatement executes NonEmptyOrScopeBlockStatement at the close of the current scope, rather than at the point where the ScopeGuardStatement appears. scope(exit) executes NonEmptyOrScopeBlockStatement when the scope exits normally or when it exits due to exception unwinding. scope(failure) executes NonEmptyOrScopeBlockStatement when the scope exits due to exception unwinding. scope(success) executes NonEmptyOrScopeBlockStatement when the scope exits normally.

If there are multiple ScopeGuardStatements in a scope, they will be executed in the reverse lexical order in which they appear. If any scope instances are to be destroyed upon the close of the scope, their destructions will be interleaved with the ScopeGuardStatements in the reverse lexical order in which they appear.

write("1");
{
    write("2");
    scope(exit) write("3");
    scope(exit) write("4");
    write("5");
}
writeln();

writes:

{
    scope(exit) write("1");
    scope(success) write("2");
    scope(exit) write("3");
    scope(success) write("4");
}
writeln();

writes:

struct Foo
{
    this(string s) { write(s); }
    ~this() { write("1"); }
}

try
{
    scope(exit) write("2");
    scope(success) write("3");
    Foo f = Foo("0");
    scope(failure) write("4");
    throw new Exception("msg");
    scope(exit) write("5");
    scope(success) write("6");
    scope(failure) write("7");
}
catch (Exception e)
{
}
writeln();

writes:

A scope(exit) or scope(success) statement may not exit with a throw, goto, break, continue, or return; nor may it be entered with a goto.

Catching C++ Class Objects

On many platforms, catching C++ class objects is supported. Catching C++ objects and D objects cannot both be done in the same TryStatement. Upon exit from the Catch, any destructors for the C++ object will be run and the storage used for it reclaimed. C++ objects cannot be caught in @safe code.

Asm Statement

Inline assembler is supported with the asm statement:

AsmStatement:
    asm FunctionAttributesopt { AsmInstructionListopt }

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:

It must use the same tokens as the D language uses.
The comment form must match the D language comments.
Asm instructions are terminated by a ;, not by an end of line.

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 (D_InlineAsm_X86)
{
    asm
    {
        ...
    }
}
else
{
    /* ... some workaround ... */
}

Semantically consecutive AsmStatements shall not have any other instructions (such as register save or restores) inserted between them by the compiler.

Pragma Statement

PragmaStatement

Mixin Statement

MixinStatement:
    mixin ( ArgumentList ) ;

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

import std.stdio;

void main()
{
    int j;
    mixin("
        int x = 3;
        for (int i = 0; i < 3; i++)
            writeln(x + i, ++j);
        ");    // ok

    string s = "int y;";
    mixin(s);  // ok
    y = 4;     // ok, mixin declared y

    string t = "y = 3;";
    mixin(t);  // error, t is not evaluatable at compile time

    mixin("y =") 4; // error, string must be complete statement

    mixin("y =" ~ "4;");  // ok

    mixin("y =", 2+2, ";");  // ok
}