Enum type

An enumeration is a custom data type in Pascal. It is a discrete value that can be referred to by a unique identifier.

Definition

Basic

An enumeration is defined by surrounding a non-empty comma-separated list of identifiers that were not yet defined in the current scope with parentheses:

program enumerationDemo(input, output, stdErr);
type
    fruit = (apple, banana, citrus);

Thus, a variable of the type fruit may assume the values apple, banana, or citrus.

var
    snack: fruit;
begin
    snack := apple;
end.

Indices

Every member of an enumerated type has an ordinal value which can be obtained using the standard function ord. The first member of any enumeration has the ordinal value zero, and every following member is enumerated with consecutive numbers.

Override

In some cases, however, it would be quite useful if the ordinal values were different. The FPC allows Delphi-style explicit definition of indices by following member identifiers with an [[=|equal sign]] and an integer value:

type
    sign = (negative = -1, none = 0, positive = 1);

It is not necessary to define all members with an explicit ordinal value, this example just happens to define all of them. If the ordinal value is unspecified, the automatic enumeration mechanism applies again.

Properties

• An enumeration data type definition is ascending if the difference in ordinal value to every right-hand member is ≥ 1. Currently, non-ascending definitions merely emit a note. A set literal using two enumeration values having the same ordinal value, albeit represented by different symbols, will not be accepted.
• An enumeration data type is contiguous if the ordinal value of every (named) member has an absolute difference of one to its neighbors. Using a set constructor on a non-contiguous enumeration data type may produce unexpected results.
• An enumeration data type is normal if it is ascending and contiguous and the first member has an ordinal value of (strictly speaking) zero, or, in the broad sense (as implemented by the FPC), ≤ 0. The compiler intrinsic typeInfo is only available for those enumeration data types.

Prefixing

FPC prefixes all members of enumeration type definitions in the type section with their corresponding type name. By default, the short name without a prefix is inserted into the symbol table as an alias in order to meet Pascal standards.

Since version 2.6.0 FPC supports disabling member identifiers from being aliased by using the {$scopedEnums on} local compiler directive, thus requiring users to always specify the type name as a prefix:

program scopedEnumerationDemo(input, output, stdErr);
type
    {$scopedEnums on}
    logLevel = (error, warning, info, debug);
var
    verbosity: logLevel;
begin
    verbosity := logLevel.debug;
end.

This setting only affects definition of new enumeration data types. If the compiler switch {$scopedEnums …} was on during definition of a enumeration type, you have to precede any values of that enumeration type with the data type’s name and a dot (in the example above this means logLevel.).

This is useful if names are likely to clash with other identifiers, for example sysUtils.tUseBoolStrs is defined in that way (with {$scopedEnums on}), or if isolate identifiers would be meaningless or confusing like in objPas.tEndian. Usage introduces some incompatibility to other compilers though.

Size

The size an enumeration type occupies depends on

1. the minimum enumeration size compiler configuration, and
2. the range, that is ord(high(enumeration)) - ord(low(enumeration)).

The local compiler directive {$minEnumSize n} determines the minimum size in Bytes of an enumeration data type. {$minEnumSize n} is (for Delphi compatibility) an alias for {$packEnum n}/{$Z}.

• {$minEnumSize 1}, {$packEnum 1}, {$Z1} or {$Z‑} instructs the compiler to use just one Byte (at least) for the enumeration type in question.
• {$minEnumSize 2}, {$packEnum 2} and {$Z2}: 2 Bytes minimum.
• {$minEnumSize 4}, {$packEnum 4}, {$Z4} or {$Z+}: use at least 4 Bytes.

Furthermore, {$minEnumSize normal}, {$minEnumSize default} and {$minEnumSize 0} restores the default value. In {$mode TP} the default value is 1 whereas in all other modes it is 4. Such a “large” minimum size aids quick read/write access.

It stands to reason that an enumeration data type has to occupy at least as much space as all discrete values require to be stored. When there are 2⁸ (256) or fewer elements to be stored, the compiler may store a variable of this kind as a single Byte. A range of 2¹⁶ elements will be stored in two Bytes if a single Byte is insufficient. More than 2¹⁶ members will be stored in four Bytes. These rules may, of course, be superseded by the previously mentioned {$minEnumSize} setting.

Application

Out-of-the-box functionality

• The pre-defined data type Boolean is an enumeration data type.
• The functions low and high will give the lowest or highest available named item of an enumeration data type.
• The standard functions pred and succ return the predecessor and successor of a value. In non-Delphi modes this functionality is disabled if the enumeration data type is non-contiguous. Note that range checks may trigger run-time errors.

Masks

Enumeration data types are particularly useful for implementing “masks”: By defining a set of … you have a neat data type fulfilling certain nice properties.

Comparative remarks

In some programming languages enumeration type definitions become “synonyms” to certain integer values. In Pascal, however, this is not the case. You may not mix a symbol referring to a member of an enumeration with other integer values. You will have to utilize ord or an explicit typecast to the enumeration data type.

See also

• Enumerated types in the Basic Pascal Tutorial series

From the Free Pascal wiki, page "Enum type", as it stood on 2023-12-22 after a change by Alextpp, out of the wiki's dump of 2024-05-26. Copyright stays with the author who wrote it, according to the old wiki's Licensing page. If you have concerns about this material, message the admin.