const

const const-type identifier = const-expression ;

/* IDL file typedef syntax */
[ 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]
    , ...
);

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 character 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 pointersH20NZW and arrays1EV5ZM5. 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.

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 );

Remarks

MIDL allows you to declare constant integer, character, string, and boolean types in the interface body of the IDL file. You can use the const keyword to modify the type of a type declaration or the type of a function parameter. 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 (char *) 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.

See Also