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const attribute

The const keyword modifies the type of a type declaration or the type of a function parameter, preventing the value from varying.


const const-type identifier = const-expression ;

[ typedef [ , type-attribute-list ] ] const const-type declarator-list;

[ typedef [ , type-attribute-list ] ] pointer-type const declarator-list;

[ [ function-attr-list ] ] type-specifier [ ptr-decl ] function-name(
    [ [ parameter-attribute-list ] ] ) const; 

const-type [declarator], [ [ parameter-attribute-list ] ] pointer-type const [declarator], ...);

Parameters

const-type

Specifies a valid MIDL integer, character, string, or Boolean type. Valid MIDL types include small, short, long, char, char *, wchar_t, wchar_t *, byte, byte *, and void *. The integer and character types can be signed or unsigned.

identifier

Specifies a valid MIDL identifier. Valid MIDL identifiers consist of up to 31 alphanumeric and/or underscore characters and must start with an alphabetic or underscore character.

const-expression

Specifies an expression, identifier, or numeric or character constant appropriate for the specified type: constant integer literals or constant integer expressions for integer constants; Boolean expressions that can be computed at compilation for Boolean types; single-character constants for char types; and string constants for [string] types. The void * type can be initialized only to NULL.

type-attribute-list

Specifies one or more attributes that apply to the type.

pointer-type

Specifies a valid MIDL pointer type.

declarator and declarator-list

Specifies standard C declarators, such as identifiers, pointer declarators, and array declarators. For more information, see Array and Sized-Pointer Attributes, arrays, and Arrays and Pointers. The declarator-list consists of one or more declarators, separated by commas. The parameter-name identifier in the function declarator is optional.

function-attr-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.

ptr-decl

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.

parameter-attribute-list

Specifies zero or more directional attributes, field attributes, usage attributes, and pointer attributes appropriate for the specified parameter type. Separate multiple attributes with commas.

Remarks

MIDL allows you to declare constant integer, character, string, and Boolean types in the interface body of the IDL file. Const type declarations are reproduced in the generated header file as #define directives.

DCE IDL compilers do not support constant expressions. Therefore, this feature is not available when you use the MIDL compiler /osf switch.

A previously defined constant can be used as the assigned value of a subsequent constant. The value of a constant integral expression is automatically converted to the respective integer type in accordance with C conversion rules.

The value of a character constant must be a single-quoted ASCII character. When the character constant is the single-quote character itself ('), the backslash character (\) must precede the single-quote character, as in \'.

The value of a character-string constant must be a double-quoted string. Within a string, the backslash (\) character must precede a literal double-quote character ( " ), as in \". Within a string, the backslash character (\) represents an escape character. String constants can consist of up to 255 characters.

The value NULL is the only valid value for constants of type void *. Any attributes associated with the const declaration are ignored.

The MIDL compiler does not check for range errors in const initialization. For example, when you specify "const short x = 0xFFFFFFFF;" the MIDL compiler does not report an error and the initializer is reproduced in the generated header file.

Examples

const void *  p1        = NULL; 
const char    my_char1  = 'a'; 
const char    my_char2  = my_char1; 
const wchar_t my_wchar3 = L'a'; 
const wchar_t * pszNote = L"Note"; 
const unsigned short int x = 123; 
 
typedef [string] const char *LPCSTR; 
 
HRESULT GetName([out] wchar_t * const pszName );

See also

arrays
MIDL Base Types
Boolean
byte
callback
char
context_handle
enum
Interface Definition (IDL) File
ignore
local
long
/osf
ptr
ref
short
signed
small
string
struct
union
unique
unsigned
void
wchar_t

 

 

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