switch_is attribute

The [switch_is] attribute specifies the expression or identifier acting as the union discriminant that selects the union member.

typedef struct [[ struct-tag ]] 
{
    [ switch_is(limited-expr) [[ , field-attr-list ]] ] union-type-specifier declarator;
    ...
}

[[ [function-attribute-list] ]] type-specifier [[pointer-declarator]] function-name(
    [ switch_is(limited-expr) [[ , param-attr-list ]] ] union-type [[declarator]]
    , ...);

Parameters

struct-tag

Specifies an optional tag for a structure.

limited-expr

Specifies a C-language expression supported by MIDL. Almost all C-language expressions are supported. The MIDL compiler supports conditional expressions, logical expressions, relational expressions, and arithmetic expressions. MIDL does not allow function invocations in expressions and does not allow pre- and post-increment and pre- and post-decrement operators.

field-attr-list

Specifies zero or more field attributes that apply to a union member. Valid field attributes include [first_is], [last_is], [length_is], [max_is], [size_is]; the usage attributes [string], [ignore], and [context_handle]; the pointer attribute [ref], [unique], or [ptr]; and for members that are themselves unions, the union attribute [switch_type]. Separate multiple field attributes with commas.

union-type-specifier

Specifies the union type identifier. An optional storage specification can precede type-specifier.

declarator and declarator-list

Specifies a standard C declarator, such as an identifier, pointer declarator, and array declarator. (Function declarators and bit-field declarations are not allowed in unions that are transmitted in remote procedure calls. These declarators are allowed in unions that are not transmitted.) Separate multiple declarators with commas.

function-attribute-list

Specifies zero or more attributes that apply to the function. Valid function attributes are [callback], [local]; the pointer attribute [ref], [unique], or [ptr]; and the usage attributes [string], [ignore], and [context_handle].

type-specifier

Specifies a base type, struct, union, enum type, or type identifier. An optional storage specification can precede type-specifier.

pointer-declarator

Specifies zero or more pointer declarators. A pointer declarator is the same as the pointer declarator used in C; it is constructed from the * designator, modifiers such as far, and the qualifier const.

function-name

Specifies the name of the remote procedure.

param-attr-list

Specifies zero or more attributes appropriate for the specified parameter type. Parameter attributes can take the directional attributes [in] and [out], the field attributes [first_is], [last_is], [length_is], [max_is], [size_is], and [switch_type]; the pointer attribute [ref], [unique], or [ptr]; and the usage attributes [context_handle] and [string]. The usage attribute [ignore] cannot be used as a parameter attribute. Separate multiple attributes with commas.

union-type

Identifies the union type specifier.

Remarks

The discriminant associated with the [switch_is] attribute must be defined at the same logical level as the union:

• When the union is a parameter, the union discriminant must be another parameter.
• When the union is a field of a structure, the discriminant must be another field of the same structure.

The sequence in a structure or a function parameter list is not significant. The union can either precede or follow the discriminant.

The [switch_is] attribute can appear as a field attribute or as a parameter attribute.

Examples

typedef [switch_type(short)] union _WILLIE_UNION_TYPE 
{ 
    [case(24)] 
        float fMays; 
    [case(25)] 
        double dMcCovey; 
    [default] 
        ; 
} WILLIE_UNION_TYPE; 
 
typedef struct _WINNER_TYPE 
{ 
    [switch_is(sUniformNumber)] WILLIE_UNION_TYPE w; 
    short sUniformNumber; 
} WINNER_TYPE;

See also

MIDL Base Types

callback

const

context_handle

Encapsulated Unions

enum

first_is

ignore

last_is

length_is

local

max_is

Nonencapsulated Unions

ptr

ref

size_is

string

struct

switch_type

union

unique