D Programming Language 2.0

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Last update Tue Jun 17 20:13:35 2008

# Expressions

C and C++ programmers will find the D expressions very familiar, with a few interesting additions.

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.

## Order Of Evaluation

The following binary expressions are evaluated in strictly left-to-right order:

OrExpression, XorExpression, AndExpression, CmpExpression, ShiftExpression, AddExpression, CatExpression, MulExpression, CommaExpression, OrOrExpression, AndAndExpression

The following binary expressions are evaluated in an implementation-defined order:

AssignExpression, function parameters

It is an error to depend on order of evaluation when it is not specified. For example, the following are illegal:

    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.

## Expressions

    Expression:
    	AssignExpression
    	AssignExpression , Expression

The left operand of the , is evaluated, then the right operand is evaluated. The type of the expression is the type of the right operand, and the result is the result of the right operand.

## Assign Expressions

    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

The 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.

### Assignment Operator Expressions

Assignment operator expressions, such as:

    a op= b

are semantically equivalent to:

    a = a op b

except that operand a is only evaluated once.

## Conditional Expressions

    ConditionalExpression:
    	OrOrExpression
    	OrOrExpression ? Expression : ConditionalExpression

The first expression is converted to bool, and is evaluated. If it is true, then the second expression is evaluated, and its result is the result of the conditional expression. If it is false, then the third expression is evaluated, and its result is the result of the conditional expression. If either the second or third expressions are of type void, then the resulting type is void. Otherwise, the second and third expressions are implicitly converted to a common type which becomes the result type of the conditional expression.

## OrOr Expressions

    OrOrExpression:
    	AndAndExpression
    	OrOrExpression || AndAndExpression

The result type of an OrOrExpression is bool, unless the right operand has type void, when the result is type void.

The OrOrExpression evaluates its left operand. If the left operand, converted to type bool, evaluates to true, then the right operand is not evaluated. If the result type of the OrOrExpression is bool then the result of the expression is true. If the left operand is false, then the right operand is evaluated. If the result type of the OrOrExpression is bool then the result of the expression is the right operand converted to type bool.

## AndAnd Expressions

    AndAndExpression:
    	OrExpression
    	AndAndExpression && OrExpression

The result type of an AndAndExpression is bool, unless the right operand has type void, when the result is type void.

The AndAndExpression evaluates its left operand.

If the left operand, converted to type bool, evaluates to false, then the right operand is not evaluated. If the result type of the AndAndExpression is bool then the result of the expression is false.

If the left operand is true, then the right operand is evaluated. If the result type of the AndAndExpression is bool then the result of the expression is the right operand converted to type bool.

## Bitwise Expressions

Bit wise expressions perform a bitwise operation on their operands. Their operands must be integral types. First, the default integral promotions are done. Then, the bitwise operation is done.

### Or Expressions

    OrExpression:
    	XorExpression
    	OrExpression | XorExpression

The operands are OR'd together.

### Xor Expressions

    XorExpression:
    	AndExpression
    	XorExpression ^ AndExpression

The operands are XOR'd together.

### And Expressions

    AndExpression:
    	CmpExpression
    	AndExpression & CmpExpression

The operands are AND'd together.

## Compare Expressions

    CmpExpression:
    	EqualExpression
    	IdentityExpression
    	RelExpression
    	InExpression

## Equality Expressions

    EqualExpression:
    	ShiftExpression
    	ShiftExpression == ShiftExpression
    	ShiftExpression != ShiftExpression
    	ShiftExpression is ShiftExpression
    	ShiftExpression !is ShiftExpression

Equality 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 == returns false and != returns true. Otherwise, the bit patterns are compared for equality.

For complex numbers, equality is defined as equivalent to:

    x.re == y.re && x.im == y.im

and inequality is defined as equivalent to:

    x.re != y.re || x.im != y.im

For class and struct objects, the expression `(a == b)` is rewritten as `a.opEquals(b)`, and `(a != b)` is rewritten as `!a.opEquals(b)`.

For class objects, the `==` and `!=` operators compare the contents of the objects. Therefore, comparing against `null` is invalid, as `null` has no contents. Use the `is` and `!is` operators instead.

    class C;
    C c;
    if (c == null)  // error
      ...
    if (c is null)  // ok
      ...

For static and dynamic arrays, equality is defined as the lengths of the arrays matching, and all the elements are equal.

### Identity Expressions

    IdentityExpression:
    	ShiftExpression is ShiftExpression
    	ShiftExpression !is ShiftExpression

The 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 class objects, identity is defined as the object references are for the same object. Null class objects can be compared with is.

For struct objects, identity is defined as the bits in the struct being identical.

For static and dynamic arrays, identity is defined as referring to the same array elements and the same number of elements.

For other operand types, identity is defined as being the same as equality.

The identity operator is cannot be overloaded.

## Relational Expressions

    RelExpression:
    	ShiftExpression
    	ShiftExpression < ShiftExpression
    	ShiftExpression <= ShiftExpression
    	ShiftExpression > ShiftExpression
    	ShiftExpression >= ShiftExpression
    	ShiftExpression !<>= ShiftExpression
    	ShiftExpression !<> ShiftExpression
    	ShiftExpression <> ShiftExpression
    	ShiftExpression <>= ShiftExpression
    	ShiftExpression !> ShiftExpression
    	ShiftExpression !>= ShiftExpression
    	ShiftExpression !< ShiftExpression
    	ShiftExpression !<= ShiftExpression

First, the integral promotions are done on the operands. The result type of a relational expression is bool.

For class objects, the result of Object.opCmp() forms the left operand, and 0 forms the right operand. The result of the relational expression (o1 op o2) is:

    (o1.opCmp(o2) op 0)

It is an error to compare objects if one is null.

For static and dynamic arrays, the result of the relational op is the result of the operator applied to the first non-equal element of the array. If two arrays compare equal, but are of different lengths, the shorter array compares as "less" than the longer array.

### Integer comparisons

Integer comparisons happen when both operands are integral types.

Integer comparison operators
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.

### Floating point comparisons

If one or both operands are floating point, then a floating point comparison is performed.

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:

Floating point comparison operators
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

#### Notes:

1. For floating point comparison operators, `(a !op b)` is not the same as `!(a op b)`.
2. "Unordered" means one or both of the operands is a NAN.
3. "Exception" means the Invalid Exception is raised if one of the operands is a NAN. It does not mean an exception is thrown. The Invalid Exception can be checked using the functions in std.c.fenv.

### Class comparisons

For class objects, the relational operators compare the contents of the objects. Therefore, comparing against `null` is invalid, as `null` has no contents.

    class C;
    C c;
    if (c < null)  // error
      ...

## In Expressions

    InExpression:
    	ShiftExpression in ShiftExpression

An associative array can be tested to see if an element is in the array:

    int foo[char[]];
    ...
    if ("hello" in foo)
    	...

The in expression has the same precedence as the relational expressions <, <=, etc. The return value of the InExpression is null if the element is not in the array; if it is in the array it is a pointer to the element.

## Shift Expressions

    ShiftExpression:
    	AddExpression
    	ShiftExpression << AddExpression
    	ShiftExpression >> AddExpression
    	ShiftExpression >>> AddExpression

The 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

## Add Expressions

    AddExpression:
    	MulExpression
    	AddExpression + MulExpression
    	AddExpression - MulExpression
    	CatExpression

If 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 the second operand is a pointer, and the first is an integral type, and the operator is +, the operands are reversed and the pointer arithmetic just described is applied.

If both operands are pointers, and the operator is +, then it is illegal. For -, the pointers are subtracted and the result is divided by the size of the type pointed to by the operands. It is an error if the pointers point to different types.

Add expressions for floating point operands are not associative.

## Cat Expressions

    CatExpression:
    	AddExpression ~ MulExpression

A CatExpression concatenates arrays, producing a dynmaic array with the result. The arrays must be arrays of the same element type. If one operand is an array and the other is of that array's element type, that element is converted to an array of length 1 of that element, and then the concatenation is performed.

## Mul Expressions

    MulExpression:
    	UnaryExpression
    	MulExpression * UnaryExpression
    	MulExpression / UnaryExpression
    	MulExpression % UnaryExpression

The 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, an Exception is thrown.

For integral operands of the % operator, the sign of the result is positive if the operands are positive, otherwise the sign of the result is implementation defined.

For floating point operands, the operations correspond to the IEEE 754 floating point equivalents.

Mul expressions for floating point operands are not associative.

## Unary Expressions

    UnaryExpression:
    	PostfixExpression
    	& UnaryExpression
    	++ UnaryExpression
    	-- UnaryExpression
    	* UnaryExpression
    	- UnaryExpression
    	+ UnaryExpression
    	! UnaryExpression
    	~ UnaryExpression
    	( Type ) . Identifier
    	NewExpression
    	DeleteExpression
    	CastExpression
    	NewAnonClassExpression

### New Expressions

    NewExpression:
    	NewArguments Type [ AssignExpression ]
    	NewArguments Type ( ArgumentList )
    	NewArguments Type
    	NewArguments ClassArguments BaseClasslistopt { DeclDefs }

    NewArguments:
    	new ( ArgumentList )
    	new ( )
    	new

    ClassArguments:
    	class ( ArgumentList )
    	class ( )
    	class

    ArgumentList:
    	AssignExpression
    	AssignExpression , ArgumentList

NewExpressions are used to allocate memory on the garbage collected heap (default) or using a class or struct specific allocator.

To allocate multidimensional arrays, the declaration reads in the same order as the prefix array declaration order.

    char[][] foo;	// dynamic array of strings
    ...
    foo = new char[][30];	// allocate array of 30 strings

The above allocation can also be written as:

    foo = new char[][](30);	// allocate array of 30 strings

To allocate the nested arrays, multiple arguments can be used:

    int[][][] bar;
    ...
    bar = new int[][][](5,20,30);

Which is equivalent to:

    bar = new int[][][5];
    foreach (ref a; bar)
    {
        a = new int[][20];
        foreach (ref b; a)
        {
    	b = new int[30];
        }
    }

If there is a new ( ArgumentList ), then those arguments are passed to the class or struct specific allocator function after the size argument.

If a NewExpression is used as an initializer for a function local variable with scope storage class, and the ArgumentList to new is empty, then the instance is allocated on the stack rather than the heap or using the class specific allocator.

### Delete Expressions

    DeleteExpression:
    	delete UnaryExpression

If the UnaryExpression is a class object reference, and there is a destructor for that class, the destructor is called for that object instance.

Next, if the UnaryExpression is a class object reference, or a pointer to a struct instance, and the class or struct has overloaded operator delete, then that operator delete is called for that class object instance or struct instance.

Otherwise, the garbage collector is called to immediately free the memory allocated for the class instance or struct instance. If the garbage collector was not used to allocate the memory for the instance, undefined behavior will result.

If the UnaryExpression is a pointer or a dynamic array, the garbage collector is called to immediately release the memory. If the garbage collector was not used to allocate the memory for the instance, undefined behavior will result.

The pointer, dynamic array, or reference is set to null after the delete is performed.

If UnaryExpression is a variable allocated on the stack, the class destructor (if any) is called for that instance. Neither the garbage collector nor any class deallocator is called.

### Cast Expressions

    CastExpression:
    	cast ( Type ) UnaryExpression

A CastExpression converts the UnaryExpression to Type.

    cast(foo) -p;	// cast (-p) to type foo
    (foo) - p;	// subtract p from foo

Any casting of a class reference to a derived class reference is done with a runtime check to make sure it really is a downcast. null is the result if it isn't. Note: This 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
    }

Casting a floating point literal from one type to another changes its type, but internally it is retained at full precision for the purposes of constant folding.

    void test()
    {
        real a = 3.40483L;
        real b;
        b = 3.40483;         // literal is not truncated to double precision
        assert(a == b);
        assert(a == 3.40483);
        assert(a == 3.40483L);
        assert(a == 3.40483F);
        double d = 3.40483;	// truncate literal when assigned to variable
        assert(d != a);     // so it is no longer the same
        const double x = 3.40483; // assignment to const is not
        assert(x == a);           // truncated if the initializer is visible
    }

Casting a value v to a struct S, when value is not a struct of the same type, is equivalent to:

    S(v)

## Postfix Expressions

    PostfixExpression:
    	PrimaryExpression
    	PostfixExpression . Identifier
    	PostfixExpression . NewExpression
    	PostfixExpression ++
    	PostfixExpression --
    	PostfixExpression ( )
    	PostfixExpression ( ArgumentList )
    	IndexExpression
    	SliceExpression

## Index Expressions

    IndexExpression:
    	PostfixExpression [ ArgumentList ]

PostfixExpression is evaluated. If PostfixExpression is an expression of type static array or dynamic array, the symbol $ is set to be the the number of elements in the array. If PostfixExpression is an ExpressionTuple, the symbol $ is set to be the the number of elements in the tuple. A new declaration scope is created for the evaluation of the ArgumentList and $ appears in that scope only.

If PostfixExpression is an ExpressionTuple, then the ArgumentList must consist of only one argument, and that must be statically evaluatable to an integral constant. That integral constant n then selects the nth expression in the ExpressionTuple, which is the result of the IndexExpression. It is an error if n is out of bounds of the ExpressionTuple.

## Slice Expressions

    SliceExpression:
    	PostfixExpression [ ]
    	PostfixExpression [ AssignExpression .. AssignExpression ]

PostfixExpression is evaluated. if PostfixExpression is an expression of type static array or dynamic array, the variable length (and the special variable $) is declared and set to be the length of the array. A new declaration scope is created for the evaluation of the AssignExpression..AssignExpression and length (and $) appears in that scope only.

The first AssignExpression is taken to be the inclusive lower bound of the slice, and the second AssignExpression is the exclusive upper bound. The result of the expression is a slice of the PostfixExpression array.

If the [ ] form is used, the slice is of the entire array.

The type of the slice is a dynamic array of the element type of the PostfixExpression.

If PostfixExpression is an ExpressionTuple, then the result of the slice is a new ExpressionTuple formed from the upper and lower bounds, which must statically evaluate to integral constants. It is an error if those bounds are out of range.

## Primary Expressions

    PrimaryExpression:
    	Identifier
    	.Identifier
    	this
    	super
    	null
    	true
    	false
    	$
    	__FILE__
    	__LINE__
    	NumericLiteral
    	CharacterLiteral
    	StringLiterals
    	ArrayLiteral
    	AssocArrayLiteral
    	FunctionLiteral
    	AssertExpression
    	MixinExpression
    	ImportExpression
    	BasicType . Identifier
    	typeid ( Type )
    	IsExpression
    	( Expression )
    	TraitsExpression

### .Identifier

Identifier is looked up at module scope, rather than the current lexically nested scope.

### this

Within a non-static member function, this resolves to a reference to the object for which the function was called. If the object is an instance of a struct, this will be a pointer to that instance. If a member function is called with an explicit reference to typeof(this), a non-virtual call is made:

    class A
    {
        char get() { return 'A'; }

        char foo() { return typeof(this).get(); }
        char bar() { return this.get(); }
    }

    class B : A
    {
        char get() { return 'B'; }
    }

    void main()
    {
        B b = new B();

        b.foo();		// returns 'A'
        b.bar();		// returns 'B'
    }

### super

super is identical to this, except that it is cast to this's base class. It is an error if there is no base class. It is an error to use super within a struct member function. (Only class `Object` has no base class.) super is not allowed in struct member functions. If a member function is called with an explicit reference to super, a non-virtual call is made.

### null

null represents the null value for pointers, pointers to functions, delegates, dynamic arrays, associative arrays, and class objects. If it has not already been cast to a type, it is given the type (void *) and it is an exact conversion to convert it to the null value for pointers, pointers to functions, delegates, etc. After it is cast to a type, such conversions are implicit, but no longer exact.

### true, false

These are of type bool and when cast to another integral type become the values 1 and 0, respectively.

### Character Literals

Character literals are single characters and resolve to one of type char, wchar, or dchar. If the literal is a \u escape sequence, it resolves to type wchar. If the literal is a \U escape sequence, it resolves to type dchar. Otherwise, it resolves to the type with the smallest size it will fit into.

### String Literals

    StringLiterals:
    	StringLiteral
    	StringLiterals StringLiteral

### Array Literals

    ArrayLiteral:
    	[ ArgumentList ]

Array literals are a comma-separated list of AssignExpressions between square brackets [ and ]. The AssignExpressions form the elements of a static array, the length of the array is the number of elements. The type of the first element is taken to be the type of all the elements, and all elements are implicitly converted to that type. If that type is a static array, it is converted to a dynamic array.

    [1,2,3];	// type is int[3], with elements 1, 2 and 3
    [1u,2,3];	// type is uint[3], with elements 1u, 2u, and 3u

If any of the arguments in the ArgumentList are an ExpressionTuple, then the elements of the ExpressionTuple are inserted as arguments in place of the tuple.

Array literals are allocated on the memory managed heap. Thus, they can be returned safely from functions:

    int[] foo()
    {
        return [1, 2, 3];
    }

### Associative Array Literals

    AssocArrayLiteral:
    	[ KeyValuePairs ]

    KeyValuePairs:
    	KeyValuePair
    	KeyValuePair , KeyValuePairs

    KeyValuePair:
    	KeyExpression : ValueExpression

    KeyExpression:
    	ConditionalExpression

    ValueExpression:
    	ConditionalExpression

Associative array literals are a comma-separated list of key:value pairs between square brackets [ and ]. The list cannot be empty. The type of the first key is taken to be the type of all the keys, and all subsequent keys are implicitly converted to that type. The type of the first value is taken to be the type of all the values, and all subsequent values are implicitly converted to that type. An AssocArrayLiteral cannot be used to statically initialize anything.

    [21u:"he",38:"ho",2:"hi"]; // type is char[2][uint], with keys 21u, 38u and 2u
                               // and values "he", "ho", and "hi"

If any of the keys or values in the KeyValuePairs are an ExpressionTuple, then the elements of the ExpressionTuple are inserted as arguments in place of the tuple.

### Function Literals

    FunctionLiteral:
    	function Typeopt ParameterAttributes opt FunctionBody
    	delegate Typeopt ParameterAttributes opt FunctionBody
    	ParameterAttributes FunctionBody
    	FunctionBody

    ParameterAttributes:
    	Parameters
    	Parameters FunctionAttributes

FunctionLiterals enable embedding anonymous functions and anonymous delegates directly into expressions. Type is the return type of the function or delegate, if omitted it is inferred from any ReturnStatements in the FunctionBody. ( ArgumentList ) forms the arguments to the function. If omitted it defaults to the empty argument list (). The type of a function literal is pointer to function or pointer to delegate. If the keywords function or delegate are omitted, it defaults to being a 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; } );
    }

and the following where the return type int is inferred:

    int abc(int delegate(long i));

    void test()
    {   int b = 3;

        abc( (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, { d += 1; } );
        loop(3, 10,  { f += 3; } );

        return d + f;
    }

When comparing with nested functions, the function form is analogous to static or non-nested functions, and the delegate form is analogous to non-static nested functions. In other words, a delegate literal can access stack variables in its enclosing function, a function literal cannot.

### Assert Expressions

    AssertExpression:
    	assert ( AssignExpression )
    	assert ( AssignExpression , AssignExpression )

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.

The expression `assert(0)` is a special case; it signifies that it is unreachable code. Either AssertError is thrown at runtime if it is reachable, or the execution is halted (on the x86 processor, a HLT instruction can be used to halt execution). The optimization and code generation phases of compilation may assume that it is unreachable code.

The second Expression, if present, must be implicitly convertible to type `char[]`. It is evaluated if the result is false, and the string result is appended to the AssertError's message.

    void main()
    {
        assert(0, "an" ~ " error message");
    }

When compiled and run, it will produce the message:

    Error: AssertError Failure test.d(3) an error message

### Mixin Expressions

    MixinExpression:
    	mixin ( AssignExpression )

The AssignExpression must evaluate at compile time to a constant string. The text contents of the string must be compilable as a valid AssignExpression, and is compiled as such.

    int foo(int x)
    {
        return mixin("x + 1") * 7;  // same as ((x + 1) * 7)
    }

### Import Expressions

    ImportExpression:
    	import ( AssignExpression )

The AssignExpression must evaluate at compile time to a constant string. The text contents of the string are interpreted as a file name. The file is read, and the exact contents of the file become a string literal.

    void foo()
    {
        // Prints contents of file foo.txt
        writefln( import("foo.txt") );
    }

### Typeid Expressions

    TypeidExpression:
        typeid ( Type )

Returns an instance of class TypeInfo corresponding to Type.

### IsExpression

    IsExpression:
    	is ( Type )
    	is ( Type : TypeSpecialization )
    	is ( Type == TypeSpecialization )
    	is ( Type Identifier )
    	is ( Type Identifier : TypeSpecialization )
    	is ( Type Identifier == TypeSpecialization )
    	is ( Type Identifier : TypeSpecialization , TemplateParameterList )
    	is ( Type Identifier == TypeSpecialization , TemplateParameterList )

    TypeSpecialization:
    	Type
    	typedef
    	struct
    	union
    	class
    	interface
    	enum
    	function
    	delegate
    	super
    	const
    	interface

IsExpressions are evaluated at compile time and are used for checking for valid types, comparing types for equivalence, determining if one type can be implicitly converted to another, and deducing the subtypes of a type. The result of an IsExpression is an int of type 0 if the condition is not satisified, 1 if it is.

Type is the type being tested. It must be syntactically correct, but it need not be semantically correct. If it is not semantically correct, the condition is not satisfied.

Identifier is declared to be an alias of the resulting type if the condition is satisfied. The Identifier forms can only be used if the IsExpression appears in a StaticIfCondition.

TypeSpecialization is the type that Type is being compared against.

The forms of the IsExpression are:

1. is ( Type )
   The condition is satisfied if Type is semantically correct (it must be syntactically correct regardless).

       alias int func(int);	// func is a alias to a function type
       void foo()
       {
           if ( is(func[]) )	// not satisfied because arrays of
       			// functions are not allowed
       	writefln("satisfied");
           else
       	writefln("not satisfied");

           if (is([][]))	// error, [][] is not a syntactically valid type
       	...
       }

2. is ( Type : TypeSpecialization )
   The condition is satisfied if Type is semantically correct and it is the same as or can be implicitly converted to TypeSpecialization. TypeSpecialization is only allowed to be a Type.

       alias short bar;
       void foo(bar x)
       {
           if ( is(bar : int) )	// satisfied because short can be
       				// implicitly converted to int
       	writefln("satisfied");
           else
       	writefln("not satisfied");
       }

3. is ( Type == TypeSpecialization )
   The condition is satisfied if Type is semantically correct and is the same type as TypeSpecialization.

   If TypeSpecialization is one of typedef struct union class interface enum function delegate const invariant then the condition is satisifed if Type is one of those.

       alias short bar;
       typedef char foo;
       void test(bar x)
       {
           if ( is(bar == int) )	// not satisfied because short is not
       				// the same type as int
       	writefln("satisfied");
           else
       	writefln("not satisfied");

           if ( is(foo == typedef) ) // satisfied because foo is a typedef
       	writefln("satisfied");
           else
       	writefln("not satisfied");
       }

4. is ( Type Identifier )
   The condition is satisfied if Type is semantically correct. If so, Identifier is declared to be an alias of Type.

       alias short bar;
       void foo(bar x)
       {
           static if ( is(bar T) )
       	alias T S;
           else
       	alias long S;
           writefln(typeid(S));   // prints "short"

           if ( is(bar T) )  // error, Identifier T form can
       		      // only be in StaticIfConditions
       	...
       }

5. is ( Type Identifier : TypeSpecialization )

   The condition is satisfied if Type is the same as TypeSpecialization, or if Type is a class and TypeSpecialization is a base class or base interface of it. The Identifier is declared to be either an alias of the TypeSpecialization or, if TypeSpecialization is dependent on Identifier, the deduced type.

       alias int bar;
       alias long* abc;
       void foo(bar x, abc a)
       {
           static if ( is(bar T : int) )
       	alias T S;
           else
       	alias long S;

           writefln(typeid(S));	// prints "int"

           static if ( is(abc U : U*) )
       	U u;

           writefln(typeid(typeof(u)));	// prints "long"
       }

   The way the type of Identifier is determined is analogous to the way template parameter types are determined by TemplateTypeParameterSpecialization.

6. is ( Type Identifier == TypeSpecialization )

   The condition is satisfied if Type is semantically correct and is the same as TypeSpecialization. The Identifier is declared to be either an alias of the TypeSpecialization or, if TypeSpecialization is dependent on Identifier, the deduced type.

   If TypeSpecialization is one of typedef struct union class interface enum function delegate const invariant then the condition is satisifed if Type is one of those. Furthermore, Identifier is set to be an alias of the type:

   keyword | alias type for Identifier
   --- | ---
   `typedef` | the type that Type is a typedef of
   `struct` | Type
   `union` | Type
   `class` | Type
   `interface` | Type
   `super` | TypeTuple of base classes and interfaces
   `enum` | the base type of the enum
   `function` | TypeTuple of the function parameter types
   `delegate` | the function type of the delegate
   `return` | the return type of the function, delegate, or function pointer
   `const` | Type
   `invariant` | Type

       alias short bar;
       enum E : byte { Emember }
       void foo(bar x)
       {
           static if ( is(bar T == int) ) // not satisfied, short is not int
       	alias T S;
           alias T U;			   // error, T is not defined

           static if ( is(E V == enum) )  // satisified, E is an enum
       	V v;			   // v is declared to be a byte
       }

7. is ( Type Identifier : TypeSpecialization , TemplateParameterList )
   is ( Type Identifier == TypeSpecialization , TemplateParameterList )

   More complex types can be pattern matched; the TemplateParameterList declares symbols based on the parts of the pattern that are matched, analogously to the way implied template parameters are matched.

       import std.stdio;

       void main()
       {
         alias long[char[]] AA;

         static if (is(AA T : T[U], U : const char[]))
         {
           writefln(typeid(T));	// long
           writefln(typeid(U));	// const char[]
         }

         static if (is(AA A : A[B], B : int))
         {
           assert(0);  // should not match, as B is not an int
         }

         static if (is(int[10] W : W[V], int V))
         {
           writefln(typeid(W));	// int
           writefln(V);		// 10
         }

         static if (is(int[10] X : X[Y], int Y : 5))
         {
           assert(0);	// should not match, Y should be 10
         }
       }

## Associativity and Commutativity

An implementation may rearrange the evaluation of expressions according to arithmetic associativity and commutativity rules as long as, within that thread of execution, no observable difference is possible.

This rule precludes any associative or commutative reordering of floating point expressions.

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