For the latest documentation on Visual Studio 2017, see Specifying When and Where an Annotation Applies on docs.microsoft.com.
When an annotation is conditional, it may require other annotations to specify that to the analyzer. For example, if a function has a variable that can be either synchronous or asynchronous, the function behaves as follows: In the synchronous case it always eventually succeeds, but in the asynchronous case it reports an error if it can’t succeed immediately. When the function is called synchronously, checking the result value provides no value to the code analyzer because it would not have returned. However, when the function is called asynchronously and the function result is not checked, a serious error could occur. This example illustrates a situation in which you could use the _When_ annotation—described later in this article—to enable checking.
To control when and where annotations apply, use the following structural annotations.
expr is an expression that yields an lvalue. The annotations in anno-list are applied to the object that is named by expr. For each annotation in anno-list, expr is interpreted in pre-condition if the annotation is interpreted in pre-condition, and in post-condition if the annotation is interpreted in post-condition.
_At_buffer_(expr, iter, elem-count, anno-list)
expr is an expression that yields an lvalue. The annotations in anno-list are applied to the object that is named by expr. For each annotation in anno-list, expr is interpreted in pre-condition if the annotation is interpreted in precondition, and in post-condition if the annotation is interpreted in post-condition.
iter is the name of a variable that is scoped to the annotation (inclusive of anno-list). iter has an implicit type long. Identically named variables in any enclosing scope are hidden from evaluation.
elem-count is an expression that evaluates to an integer.
The annotations in anno-list are all considered to have any qualifier that applies to the group annotation that is applied to each annotation.
expr is an expression that can be converted to bool. When it is non-zero (true), the annotations that are specified in anno-list are considered applicable.
By default, for each annotation in anno-list, expr is interpreted as using the input values if the annotation is a precondition, and as using the output values if the annotation is a post-condition. To override the default, you can use the _Old_ intrinsic when you evaluate a post-condition to indicate that input values should be used. Note: Different annotations might be enabled as a consequence of using _When_ if a mutable value—for example, *pLength—is involved because the evaluated result of expr in precondition may differ from its evaluated result in post-condition.