Enum type is the abstract base class of all enumeration types. It provides a number of methods to get information about an enumeration type and its values. Beginning with C 7. Enum in a base class constraint that is known as the enum constraint to specify that a type parameter is an enumeration type. Any enumeration type also satisfies the struct constraint, which is used to specify that a type parameter is a non-nullable value type. For any enumeration type, there exist explicit conversions between the enumeration type and its underlying integral type.
If you cast an enum value to its underlying type, the result is the associated integral value of an enum member. Use the Enum. IsDefined method to determine whether an enumeration type contains an enum member with the certain associated value. For any enumeration type, there exist boxing and unboxing conversions to and from the System.
Enum type, respectively. For more information, see the following sections of the C language specification :. Feedback will be sent to Microsoft: By pressing the submit button, your feedback will be used to improve Microsoft products and services.
Privacy policy. Skip to main content. This browser is no longer supported. Download Microsoft Edge More info. If the numeric value is quoted, it is still interpreted as an index if there is no matching string in the list of enumeration values. For these reasons, it is not advisable to define an ENUM column with enumeration values that look like numbers, because this can easily become confusing.
For example, the following column has enumeration members with string values of '0' , '1' , and '2' , but numeric index values of 1 , 2 , and 3 :. If you store 2 , it is interpreted as an index value, and becomes '1' the value with index 2. If you store '2' , it matches an enumeration value, so it is stored as '2'.
If you store '3' , it does not match any enumeration value, so it is treated as an index and becomes '2' the value with index 3. An enumeration value can also be the empty string '' or NULL under certain circumstances:. If you insert an invalid value into an ENUM that is, a string not present in the list of permitted values , the empty string is inserted instead as a special error value.
See Index Values for Enumeration Literals for details about the numeric indexes for the enumeration values. ENUM values are sorted based on their index numbers, which depend on the order in which the enumeration members were listed in the column specification.
For example, 'b' sorts before 'a' for ENUM 'b', 'a'. The empty string sorts before nonempty strings, and NULL values sort before all other enumeration values. Specify the ENUM list in alphabetic order. An enumeration value cannot be an expression, even one that evaluates to a string value.
You also cannot employ a user variable as an enumeration value. This pair of statements do not work:. If you do use a number as an enumeration value, always enclose it in quotation marks. If the quotation marks are omitted, the number is regarded as an index.
See Handling of Enumeration Literals to see how even a quoted number could be mistakenly used as a numeric index value. Duplicate values in the definition cause a warning, or an error if strict SQL mode is enabled. Numeric Type Attributes. Out-of-Range and Overflow Handling.
Date and Time Data Type Syntax. ROOK ,. QUEEN ,. KING ;. Company cName ;. ATT - my carrier provider.. EBAY ;. YAHOO ;. ATT ;. ATT - my carrier provider. Process finished with exit code 0. Point-6 You can not create instance of enums by using new operator in Java because constructor of Enum in Java can only be private and Enums constants can only be created inside Enums itself. Point-7 Instance of Enum in Java is created when any Enum constants are first called or referenced in code.
Point-8 An enum specifies a list of constant values assigned to a type. Point-9 An enum can be declared outside or inside a class, but NOT in a method. Point An enum declared outside a class must NOT be marked static, final , abstract , protected , or private Point Enums can contain constructors , methods, variables, and constant class bodies. Point enum constants can send arguments to the enum constructor, using the syntax BIG 8 , where the int literal 8 is passed to the enum constructor.
Point enum constructors can have arguments, and can be overloaded.
0コメント