Talk again to interface and implementation
Users can choose any C compiler, but eventually they must use a compiler of C (not other languages).
COM provides a language, which only uses the syntax of the C language that everyone is familiar with, and adds some capabilities used to eliminate unity in the C language, namely the interface definition language
Idl (Interface Definition Language)
COM IDL adds to COM-related extensions based on OSF DCE RPC IDL (inheritance, polymorphism)
MIDL.EXE is an IDL compiler,
Generate C / C compatible header files
Generate a binary, called type library file
Method and effect
Each COM method returns a HRESULT value,
Divided into three parts: severity, opcode and information code
If HRESULT indicates an abnormal result, the Java virtual opportunity throws an exception.
And C must manually check HRESULT.
Interface and IDL
IDL uses interface keyword definition interface
Definition: Interface name, base interface name, interface body, interface attribute
There are at least two properties per COM interface: one is [object] indicates that COM instead of the DCE-style interface; one is the actual name of the interface [UUID]
GUID is a 128-bit large number, which guarantees unique in time and spatial, standard text format for [UUID]
Iunknown
Method: queryinterface (), addref (), release ();
IUNKNOWN is the root of all interfaces
CoM prohibits multiple interface inheritance, but implementing multiple interfaces can be exposed
Resource Management and IUNKNOWN
CF reference count rules:
1. When a non-empty interface pointer is copied from a memory location to another memory location, addRef ();
2. For the memory location already containing a non-empty interface pointer, Release () is called before replying the memory;
3. If possible, excess addRef () and release () can be optimized;
Call rules:
AddRef ()
A1 When a non-empty interface pointer is written to a local variable;
A2 When the called party writes a non-empty interface pointer to the method or function [OUT] or [IN, OUT] parameters
A3 When the caller returns a non-empty interface pointer as the actual result of the function
A4 When writing a non-empty interface pointer to a data member of an object
RELEASE ()
R1 rewrites a non-air local variable or data member
R2 before the scope of the non-air local variable
R3 is called the [IN, OUT] parameters of the method or function, and the initial value of the parameter is not empty.
R4 rewrites before non-empty data member
R5 before leaving the object paratory function
Special rule
When the S1 caller passes a non-empty interface pointer to the function through the [IN] parameter, do not need to call a or R
Type forced conversion and iUnknown
HRESULT QueryInterface ([in] refiid riid, [out] void ** ppv);
If the object does not support the requested interface type, the PPV is set to NULL, return E_NOIInterface
If support, then rewrite the pointer, return S_OK, call addRef ();
AddRef and Release are not actions for the entire object, but an operation for interface pointers.
Void ** caused by unsafe vulnerability, available macro IID_PPV_ARG to solve
Implement IUNKNOWN
AddRef () and Release ()
The destructor in the class definition is protected, ensuring that the object is always created on the pile, and Delete this is always correct.
But sometimes we want the object not allocating in the heap, then the reference count can be optimized, return a reference value
Queryinterface ()
When a request is supported by multiple interfaces, the type conversion operation must clearly select a more accurate base class.
Use the COM interface pointer
Because the C language mapping of COM does not provide Runtime Layer, the C programmer must display iUnknown method, but this code efficiency is higher.
For Java and VB, IUNKNOWN details are hidden after virtual machines.
The smart pointer can simplify the operation of using the COM interface pointer in C .
But it also brings a lot of problems.
Optimize QueryInterface
Each COM-compatible class provides a table that maps the IID supported by this class to a place supported to the object by a fixed offset or other technology. (Similar to message mapping macro in the MFC)
type of data
Conversion of OleChar to TCHAR
BSTR: String Type
IDL and COM also support United (UNION), to ensure that there is no secondary meaning, providing full discriminator
COM provides a general purpose, useful for VB, namely Variant
The interface of the COM can also be passed as a parameter: static or dynamic
IDL attribute and COM properties
Indicates an object to have some disclosed properties and can access or modify these attributes through the COM interface. Some are useful.
The COM IDL allows the interface to add some comments to reflect the method to read and write an object.
abnormal
Abnormal API: Throw SterrorInfo () Capture Exception GeterrorInfo ();
COM exception object must support IrrorInfo interface
Throw an abnormal object must implement the ISUPPORTERRORINFO interface to determine which interface supports exception
COM requires that "pure C abnormal" spreads outside the border of the method
