Delphi Const Array

Array of const Delphi supports a language construct called an array of const. This argument type is the same as taking an open array of TVarRec by value. The following is an Delphi code segment declared to accept an array of const.

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The constkeyword has three uses in a Pascalprogram:

• to start a constant declaration section
• to declare const parameter for a function or procedure
• to declare a function with a variable number of parameters passed via a variable-sized array of different element types

Const Section

The declaration const in a Pascal program is used to inform the compiler that certain identifiers which are being declared are constants, that is, they are initialized with a specific value at compile time as opposed to a variable which is initialized at run time.

However, the default setting in Free Pascal is to allow const identifiers to be re-assigned to. In order to make them unchangeable, the {$J} (short form) or {$WriteableConst} (long form) compiler directives must be used to turn off the ability to assign to constant identifiers. That is {$J-} or {$WriteableConst OFF}.

In some Pascal compilers, the Const declaration is used to define variables which are initialized at compile time to a certain specific value, and that the variables so defined can change as the program executes. This can be used for initializing arrays at compile time as opposed to setting values when the program is executed.

Const Parameter

A function or procedure parameter may be declared const. Any assignment to a const parameter within a procedure or function and the compiler will flag it as an error: 'Can't assign values to const variable'. Declaring a parameter as const allows the compiler the possibility to do optimizations it couldn't do otherwise, such as passing by reference while retaining the semantics of passing by value. A const parameter cannot be passed to another function or procedure that requires a variable parameter.

Array of Const

A function can declare a parameter as an array of const. This allows a a routine to effectively take a variable amount of different types of parameters, via a single variable-length array parameter. At run-time when the function/procedure is called, the actual elements of the array are turned into variant records of type TVarRec. The called routine can use High() to determine the number of elements in the array and look at the VType field of each TVarRec element, to determine the type it contains. Record types cannot be passed as part of an array of const, but simple types, classes and interfaces can be. You create an array of const by placing the components within []. For example:

This feature is only available in compiler modeObjFPC or Delphi.

Note: Due to Delphi compatibility, arrays of const can't have an unsigned 32-bit variable type. Trying to pass a Dword with the highest bit set will cause a range check error, or gets interpreted as a Longint.

See Also

Several different language constructions are referred to as 'constants'. There are numeric constants (also called numerals) like 17, and string constants (also called character strings or string literals) like 'Hello world!' Every enumerated type defines constants that represent the values of that type. There are predefined constants like True, False, and nil.

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Finally, there are constants that, like variables, are created individually by declaration.Declared constants are either true constants or typed constants. These two kinds of constant are superficially similar, but they are governed by different rules and used for different purposes.

Const MyNumber = Int64(17);declares a constant called MyNumber, of type Int64, that returns the integer 17. Otherwise, the type of the declared constant is the type of the constantExpression. If constantExpression is a character string, the declared constant is compatible with any string type. If the character string is of length 1, it is also compatible with any character type. If constantExpression is a real, its type is Extended.

If it is an integer, its type is given by the table below.for integer constants. Constant ExpressionsA constant expression is an expression that the compiler can evaluate without executing the program in which it occurs.

Constant expressions include numerals; character strings; true constants; values of enumerated types; the special constants True, False, and nil; and expressions built exclusively from these elements with operators, typecasts, and set constructors. Constant expressions cannot include variables, pointers, or function calls, except calls to the following predefined functions.

Record ConstantsTo declare a record constant, specify the value of each field - as fieldName: value, with the field assignments separated by semicolons - in parentheses at the end of the declaration. The values must be represented by constant expressions. The fields must be listed in the order in which they appear in the record type declaration, and the tag field, if there is one, must have a value specified; if the record has a variant part, only the variant selected by the tag field can be assigned values.Examples.