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The #import Directive

C++ Specific

The #import directive is used to incorporate information from a type library. The content of the type library is converted into C++ classes, mostly describing the COM interfaces.

Syntax

#import "filename" [attributes]

#import <filename> [attributes]

attributes:

attribute1, attribute2, ...

attribute1 attribute2 ...

filename is the name of the file containing the type library information. A file can be one of the following types:

a type library (.TLB or .ODL) file
an executable (.EXE) file
a library (.DLL) file containing a type library resource (such as .OCX)
a compound document holding a type library
any other file format that can be understood by the LoadTypeLib API

The filename is optionally preceded by a directory specification. The filename must name an existing file. The difference between the two forms is the order in which the preprocessor searches for the type library files when the path is incompletely specified.

Syntax Form	Action
Quoted form	This form instructs the preprocessor to first look for type library files in the same directory of the file that contains the #import statement, and then in the directories of whatever files that include (#include) that file. The preprocessor then searches along the paths shown below.
Angle-bracket form	This form instructs the preprocessor to search for type library files along the paths shown below.

The compiler will search in the following directories for the named file:

the PATH environment variable path list
the LIB environment variable path list
the path specified by the /I (additional include directories) compiler option

#import can optionally include one or more attributes. These attributes tell the compiler to modify the contents of the type-library headers. A backslash (\) symbol can be used to include additional lines in a single #import statement. For example:

#import “test.lib” no_namespace \
   rename(“OldName”, “NewName”)

The #import attributes are listed below:

exclude	high_method_prefix
high_property_prefixes	implementation_only
include(…)	inject_statement
named_guids	no_auto_exclude
no_implementation	no_namespace
raw_dispinterfaces	raw_interfaces_only
raw_method_prefix	raw_native_types
raw_property_prefixes	rename
rename_namespace

#import creates two header files that reconstruct the type library contents in C++ source code. The primary header file is similar to that produced by the Microsoft Interface Definition Language (MIDL) compiler, but with additional compiler-generated code and data. The primary header file has the same base name as the type library, plus a .TLH extension. The secondary header file has the same base name as the type library, with a .TLI extension. It contains the implementations for compiler-generated member functions, and is included (#include) in the primary header file.

Both header files are placed in the output directory specified by the /Fo (name object file) option. They are then read and compiled by the compiler as if the primary header file was named by a #include directive.

The following compiler optimizations come with the #import directive:

The header file, when created, is given the same timestamp as the type library.
When #import is processed, the compiler first checks if the header exists and is up to date. If yes, then it does not need to be recreated.
The compiler delays initializing the OLE subsystem until the first #import command is encountered.

The #import directive also participates in minimal rebuild and can be placed in a precompiled header file.