Expressions (Delphi)

RAD Studio (Common)

Contents

Index

Previous

Next

Expressions (Delphi)

Description | Expressions | Operators | Function Calls | Set Constructors | Indexes | Typecasts | See Also

circuit evaluation is usually preferable because it guarantees minimum execution time and, in most cases, minimum code size. Complete evaluation is sometimes convenient when one operand is a function with side effects that alter the execution of the program.

circuit evaluation also allows the use of constructions that might otherwise result in illegal runtime operations. For example, the following code iterates through the string S, up to the first comma.

while (I <= Length(S)) and (S[I] <> ',') do
             begin
                  ...
                  Inc(I);
             end;

In the case where S has no commas, the last iteration increments I to a value which is greater than the length of S. When the while condition is next tested, complete evaluation results in an attempt to read S[I], which could cause a runtime error. Under short-circuit evaluation, in contrast, the second part of the while condition (S[I] <> ',') is not evaluated after the first part fails.

Use the $B compiler directive to control evaluation mode. The default state is {$B}, which enables short-circuit evaluation. To enable complete evaluation locally, add the {$B+} directive to your code. You can also switch to complete evaluation on a project-wide basis by selecting Complete Boolean Evaluation in the Compiler Options dialog (all source units will need to be recompiled).

Note: If either operand involves a Variant, the compiler always performs complete evaluation (even in the {$B} state).

Logical (Bitwise) Operators

The following logical operators perform bitwise manipulation on integer operands. For example, if the value stored in X (in binary) is 001101 and the value stored in Y is 100001, the statement:

Z := X or Y;

assigns the value 101101 to Z.

Logical (Bitwise) Operators

Operator	Operation	Operand Types	Result Type	Example
not	bitwise negation	integer	integer	not X
and	bitwise and	integer	integer	X and Y
or	bitwise or	integer	integer	X or Y
xor	bitwise xor	integer	integer	X xor Y
shl	bitwise shift left	integer	integer	X shl 2
shr	bitwise shift right	integer	integer	Y shr I

The following rules apply to bitwise operators.

The result of a not operation is of the same type as the operand.
If the operands of an and, or, or xor operation are both integers, the result is of the predefined integer type with the smallest range that includes all possible values of both types.
The operations x shl y and x shr y shift the value of x to the left or right by y bits, which (if x is an unsigned integer) is equivalent to multiplying or dividing x by 2^y; the result is of the same type as x. For example, if N stores the value 01101 (decimal 13), then N sh 1 returns 11010 (decimal 26). Note that the value of y is interpreted modulo the size of the type of x. Thus for example, if x is an integer, x shl 40 is interpreted as x shl 8 because an integer is 32 bits and 40 mod 32 is 8.

String Operators

The relational operators =, <>, <, >, <=, and >= all take string operands (see Relational operators). The + operator concatenates two strings.

String Operators

Operator	Operation	Operand Types	Result Type	Example
+	concatenation	string, packed string, character	string	S + '.'

The following rules apply to string concatenation.

The operands for + can be strings, packed strings (packed arrays of type Char), or characters. However, if one operand is of type WideChar, the other operand must be a long string (UnicodeString, AnsiString or WideString).
The result of a + operation is compatible with any string type. However, if the operands are both short strings or characters, and their combined length is greater than 255, the result is truncated to the first 255 characters.

Pointer Operators

The relational operators <, >, <=, and >= can take operands of type PAnsiChar and PWideChar (see Relational operators). The following operators also take pointers as operands. For more information about pointers, see Pointers and pointer types.

Character-pointer operators

Operator	Operation	Operand Types	Result Type	Example
+	pointer addition	character pointer, integer	character pointer	P + I
-	pointer subtraction	character pointer, integer	character pointer, integer	P - Q
^	pointer dereference	pointer	base type of pointer	P^
=	equality	pointer	Boolean	P = Q
<>	inequality	pointer	Boolean	P <>Q

The ^ operator dereferences a pointer. Its operand can be a pointer of any type except the generic Pointer, which must be typecast before dereferencing.

P = Q is True just in case P and Q point to the same address; otherwise, P <> Q is True.

You can use the + and - operators to increment and decrement the offset of a character pointer. You can also use - to calculate the difference between the offsets of two character pointers. The following rules apply.

If I is an integer and P is a character pointer, then P + I adds I to the address given by P; that is, it returns a pointer to the address I characters after P. (The expression I + P is equivalent to P + I.) P - I subtracts I from the address given by P; that is, it returns a pointer to the address I characters before P. This is true for PAnsiChar pointers; for PWideChar pointers P + I adds SizeOf(WideChar) to P.
If P and Q are both character pointers, then P - Q computes the difference between the address given by P (the higher address) and the address given by Q (the lower address); that is, it returns an integer denoting the number of characters between P and Q. P + Q is not defined.

Set Operators

The following operators take sets as operands.

Set Operators

Operator	Operation	Operand Types	Result Type	Example
+	union	set	set	Set1 + Set2
-	difference	set	set	S - T
*	intersection	set	set	S * T
<=	subset	set	Boolean	Q <= MySet
>=	superset	set	Boolean	S1 >= S2
=	equality	set	Boolean	S2 = MySet
<>	inequality	set	Boolean	MySet <> S1
in	membership	ordinal, set	Boolean	A in Set1

The following rules apply to +, -, and *.

An ordinal O is in X + Y if and only if O is in X or Y (or both). O is in X - Y if and only if O is in X but not in Y. O is in X * Y if and only if O is in both X and Y.
The result of a +, -, or * operation is of the type set of A..B, where A is the smallest ordinal value in the result set and B is the largest.

The following rules apply to <=, >=, =, <>, and in.

X <= Y is True just in case every member of X is a member of Y; Z >= W is equivalent to W <= Z. U = V is True just in case U and V contain exactly the same members; otherwise, U <> V is True.
For an ordinal O and a set S, O in S is True just in case O is a member of S.

Relational Operators

Relational operators are used to compare two operands. The operators =, <>, <=, and >= also apply to sets.

Relational Operators

Operator	Operation	Operand Types	Result Type	Example
=	equality	simple, class, class reference, interface, string, packed string	Boolean	I = Max
<>	inequality	simple, class, class reference, interface, string, packed string	Boolean	X <> Y
<	less-than	simple, string, packed string, PChar	Boolean	X < Y
>	greater-than	simple, string, packed string, PChar	Boolean	Len > 0
<=	less-than-or-equal-to	simple, string, packed string, PChar	Boolean	Cnt <= I
>=	greater-than-or-equal-to	simple, string, packed string, PChar	Boolean	I >= 1

For most simple types, comparison is straightforward. For example, I = J is True just in case I and J have the same value, and I <> J is True otherwise. The following rules apply to relational operators.

Operands must be of compatible types, except that a real and an integer can be compared.
Strings are compared according to the ordinal values that make up the characters that make up the string. Character types are treated as strings of length 1.
Two packed strings must have the same number of components to be compared. When a packed string with n components is compared to a string, the packed string is treated as a string of length n.
Use the operators <, >, <=, and >= to compare PAnsiChar (and PWideChar) operands only if the two pointers point within the same character array.
The operators = and <> can take operands of class and class-reference types. With operands of a class type, = and <> are evaluated according the rules that apply to pointers: C = D is True just in case C and D point to the same instance object, and C <> D is True otherwise. With operands of a class-reference type, C = D is True just in case C and D denote the same class, and C <> D is True otherwise. This does not compare the data stored in the classes. For more information about classes, see Classes and objects.

Class Operators

The operators as and is take classes and instance objects as operands; as operates on interfaces as well. For more information, see Classes and objects and Object interfaces.

The relational operators = and <> also operate on classes.

The @ Operator

The @ operator returns the address of a variable, or of a function, procedure, or method; that is, @ constructs a pointer to its operand. For more information about pointers, see Pointers and pointer types. The following rules apply to @.

If X is a variable, @X returns the address of X. (Special rules apply when X is a procedural variable; see Procedural types in statements and expressions.) The type of @X is Pointer if the default {$T} compiler directive is in effect. In the {$T+} state, @X is of type ^T, where T is the type of X (this distinction is important for assignment compatibility, see Assignment-compatibility).
If F is a routine (a function or procedure), @F returns F's entry point. The type of @F is always Pointer.
When @ is applied to a method defined in a class, the method identifier must be qualified with the class name. For example,

@TMyClass.DoSomething

points to the DoSomething method of TMyClass. For more information about classes and methods, see Classes and objects.

Note: When using the @ operator, it is not possible to take the address of an interface method as the address is not known at compile time and cannot be extracted at runtime.

Operator Precedence

In complex expressions, rules of precedence determine the order in which operations are performed.

Precedence of operators

Operators	Precedence
@, not	first (highest)
, /, div, mod, and, shl, shr*, as	second
+, -, or, xor	third
=, <>, <, >, <=, >=, in, is	fourth (lowest)

An operator with higher precedence is evaluated before an operator with lower precedence, while operators of equal precedence associate to the left. Hence the expression

X + Y * Z

multiplies Y times Z, then adds X to the result; * is performed first, because is has a higher precedence than +. But

X - Y + Z

first subtracts Y from X, then adds Z to the result; - and + have the same precedence, so the operation on the left is performed first.

You can use parentheses to override these precedence rules. An expression within parentheses is evaluated first, then treated as a single operand. For example,

(X + Y) * Z

multiplies Z times the sum of X and Y.

Parentheses are sometimes needed in situations where, at first glance, they seem not to be. For example, consider the expression

X = Y or X = Z

The intended interpretation of this is obviously

(X = Y) or (X = Z)

Without parentheses, however, the compiler follows operator precedence rules and reads it as

(X = (Y or X)) = Z

which results in a compilation error unless Z is Boolean.

Parentheses often make code easier to write and to read, even when they are, strictly speaking, superfluous. Thus the first example could be written as

X + (Y * Z)

Here the parentheses are unnecessary (to the compiler), but they spare both programmer and reader from having to think about operator precedence.

This topic describes syntax rules of forming Delphi expressions.

The following items are covered in this topic:

Valid Delphi Expressions
Operators
Function calls
Set constructors
Indexes
Typecasts

An expression is a construction that returns a value. The following table shows examples of Delphi expressions:

X	variable
@X	address of the variable X
15	integer constant
InterestRate	variable
Calc(X, Y)	function call
X * Y	product of X and Y
Z / (1 - Z)	quotient of Z and (1 - Z)
X = 1.5	Boolean
C in Range1	Boolean
not Done	negation of a Boolean
['a', 'b', 'c']	set
Char(48)	value typecast

The simplest expressions are variables and constants (described in Data types). More complex expressions are built from simpler ones using operators, function calls, set constructors, indexes, and typecasts.

Operators behave like predefined functions that are part of the Delphi language. For example, the expression (X + Y) is built from the variables X and Y, called operands, with the + operator; when X and Y represent integers or reals, (X + Y) returns their sum. Operators include @, not, ^, *, /, div, mod, and, shl, shr, as, +, -, or, xor, =, >, <, <>, <=, >=, in, and is.

The operators @, not, and ^ are unary (taking one operand). All other operators are binary (taking two operands), except that + and - can function as either a unary or binary operator. A unary operator always precedes its operand (for example, -B), except for ^, which follows its operand (for example, P^). A binary operator is placed between its operands (for example, A = 7).

Some operators behave differently depending on the type of data passed to them. For example, not performs bitwise negation on an integer operand and logical negation on a Boolean operand. Such operators appear below under multiple categories.

Except for ^, is, and in, all operators can take operands of type Variant.

The sections that follow assume some familiarity with Delphi data types.

For information about operator precedence in complex expressions, see Operator Precedence Rules, later in this topic.

Arithmetic Operators

Arithmetic operators, which take real or integer operands, include +, -, *, /, div, and mod.

Binary Arithmetic Operators

Operator	Operation	Operand Types	Result Type	Example
+	addition	integer, real	integer, real	X + Y
-	subtraction	integer, real	integer, real	Result -1
*	multiplication	integer, real	integer, real	P * InterestRate
/	real division	integer, real	real	X / 2
div	integer division	integer	integer	Total div UnitSize
mod	remainder	integer	integer	Y mod 6

Unary arithmetic operators

Operator	Operation	Operand Type	Result Type	Example
+	sign identity	integer, real	integer, real	+7
-	sign negation	integer, real	integer, real	-X

The following rules apply to arithmetic operators.

The value of x / y is of type Extended, regardless of the types of x and y. For other arithmetic operators, the result is of type Extended whenever at least one operand is a real; otherwise, the result is of type Int64 when at least one operand is of type Int64; otherwise, the result is of type Integer. If an operand's type is a subrange of an integer type, it is treated as if it were of the integer type.
The value of x div y is the value of x / y rounded in the direction of zero to the nearest integer.
The mod operator returns the remainder obtained by dividing its operands. In other words, x mod y = x —(x div y) * y.
A runtime error occurs when y is zero in an expression of the form x / y, x div y, or x mod y.

Boolean Operators

The Boolean operators not, and, or, and xor take operands of any Boolean type and return a value of type Boolean.

Boolean Operators

Operator	Operation	Operand Types	Result Type	Example
not	negation	Boolean	Boolean	not (C in MySet)
and	conjunction	Boolean	Boolean	Done and (Total >0)
or	disjunction	Boolean	Boolean	A or B
xor	exclusive disjunction	Boolean	Boolean	A xor B

These operations are governed by standard rules of Boolean logic. For example, an expression of the form x and y is True if and only if both x and y are True.

Complete Versus Short-Circuit Boolean Evaluation

The compiler supports two modes of evaluation for the and and or operators: complete evaluation and short-circuit (partial) evaluation. Complete evaluation means that each conjunct or disjunct is evaluated, even when the result of the entire expression is already determined. Short-circuit evaluation means strict left-to-right evaluation that stops as soon as the result of the entire expression is determined. For example, if the expression A and B is evaluated under short-circuit mode when A is False, the compiler won't evaluate B; it knows that the entire expression is False as soon as it evaluates A.

Because functions return a value, function calls are expressions. For example, if you've defined a function called Calc that takes two integer arguments and returns an integer, then the function call Calc(24,47) is an integer expression. If I and J are integer variables, then I + Calc(J,8) is also an integer expression. Examples of function calls include

Sum(A, 63)
Maximum(147, J)
Sin(X + Y)
Eof(F)
Volume(Radius, Height)
GetValue
TSomeObject.SomeMethod(I,J);

For more information about functions, see Procedures and functions.

Set Constructors

A set constructor denotes a set-type value. For example,

[5, 6, 7, 8]

denotes the set whose members are 5, 6, 7, and 8. The set constructor

[ 5..8 ]

could also denote the same set.

The syntax for a set constructor is

[ item1, ..., itemn ]

where each item is either an expression denoting an ordinal of the set's base type or a pair of such expressions with two dots (..) in between. When an item has the form x..y, it is shorthand for all the ordinals in the range from x to y, including y; but if x is greater than y, then x..y, the set [x..y], denotes nothing and is the empty set. The set constructor [ ] denotes the empty set, while [x] denotes the set whose only member is the value of x.

Examples of set constructors:

[red, green, MyColor]
[1, 5, 10..K mod 12, 23]
['A'..'Z', 'a'..'z', Chr(Digit + 48)]

For more information about sets, see Sets.

Strings, arrays, array properties, and pointers to strings or arrays can be indexed. For example, if FileName is a string variable, the expression FileName[3] returns the third character in the string denoted by FileName, while FileName[I + 1] returns the character immediately after the one indexed by I. For information about strings, see String types. For information about arrays and array properties, see Arrays and Array properties.

It is sometimes useful to treat an expression as if it belonged to different type. A typecast allows you to do this by, in effect, temporarily changing an expression's type. For example, Integer('A') casts the character A as an integer.

The syntax for a typecast is

typeIdentifier(expression)

If the expression is a variable, the result is called a variable typecast; otherwise, the result is a value typecast. While their syntax is the same, different rules apply to the two kinds of typecast.

Value Typecasts

In a value typecast, the type identifier and the cast expression must both be ordinal or pointer types. Examples of value typecasts include

Integer('A')
Char(48) 
Boolean(0)
Color(2)
Longint(@Buffer)

The resulting value is obtained by converting the expression in parentheses. This may involve truncation or extension if the size of the specified type differs from that of the expression. The expression's sign is always preserved.

The statement

I := Integer('A');

assigns the value of Integer('A'), which is 65, to the variable I.

A value typecast cannot be followed by qualifiers and cannot appear on the left side of an assignment statement.

Variable Typecasts

You can cast any variable to any type, provided their sizes are the same and you do not mix integers with reals. (To convert numeric types, rely on standard functions like Int and Trunc.) Examples of variable typecasts include

Char(I)
Boolean(Count)
TSomeDefinedType(MyVariable)

Variable typecasts can appear on either side of an assignment statement. Thus

var MyChar: char;
  ...
  Shortint(MyChar) := 122;

assigns the character z (ASCII 122) to MyChar.

You can cast variables to a procedural type. For example, given the declarations

type Func = function(X: Integer): Integer;
var
  F: Func;
  P: Pointer;
  N: Integer;

you can make the following assignments.

F := Func(P);     { Assign procedural value in P to F }
Func(P) := F;     { Assign procedural value in F to P }
@F := P;          { Assign pointer value in P to F }
P := @F;          { Assign pointer value in F to P }
N := F(N);        { Call function via F }
N := Func(P)(N);  { Call function via P }

Variable typecasts can also be followed by qualifiers, as illustrated in the following example.

type
  TByteRec = record
     Lo, Hi: Byte;
  end;

  TWordRec = record
     Low, High: Word;
  end;

var
  B: Byte;
  W: Word;
  L: Longint;
  P: Pointer;

begin
  W := $1234;
  B := TByteRec(W).Lo;
  TByteRec(W).Hi := 0;
  L := $1234567;
  W := TWordRec(L).Low;
  B := TByteRec(TWordRec(L).Low).Hi;
  B := PByte(L)^;
end;

In this example, TByteRec is used to access the low- and high-order bytes of a word, and TWordRec to access the low- and high-order words of a long integer. You could call the predefined functions Lo and Hi for the same purpose, but a variable typecast has the advantage that it can be used on the left side of an assignment statement.

For information about typecasting pointers, see Pointers and pointer types. For information about casting class and interface types, see The as operator and Interface typecasts.

Fundamental Syntactic Elements

Declarations and Statements

Copyright(C) 2009 Embarcadero Technologies, Inc. All Rights Reserved.

What do you think about this topic? Send feedback!