__in，__ out，__ in_opt的含义

Question

在函数参数中的变量之前使用这些关键字的含义是什么？

• __in
• __out
• __in_opt

Answer 1

Those are some of the older decoration macro's from Microsoft's SAL Annotations ( the newer ones now follow different casing, starting with a capital). These have no real affect on compilation (under VS 2010 they aren't even expanded), they are there for inline API documentation.

1. __in : this parameter is an input to the function (read-only, caller initialized).
2. __out : this parameter contains output from the function when it returns (write-only, caller initialized).
3. __in_opt : a compound annotation formed from _in and _opt , _opt indications that the parameter is optional and can be set to a default value (generally NULL ).

You can get the full explanation here of the older decorations here .

Answer 2

As answered by Nercolis, these are SAL annotation attributes. However, these aren't just internal or just for API documentation . The real purpose is for Code Analysis . When you build the project with /analyze compiler option (Project Properties, Code Analysis -> General), these play important role in finding the coding issues.

For example, if a particular pointer argument says __in then it must not be passed a null pointer. The function will not check for null and may cause SEH. __in_opt argument may be null. When compiler finds some issue, it reports it as a warning. See this article .