Supertrait for typeclasses that enable certain contain matcher syntax for containers.

A Containing[C] provides access to the "containing nature" of type C in such a way that relevant contain matcher syntax can be used with type C. A C can be any type of "container," a type that in some way can contains one or more other objects. ScalaTest provides implicit implementations for several types. You can enable the contain matcher syntax on your own type U by defining an Containing[U] for the type and making it available implicitly.

ScalaTest provides implicit Containing instances for scala.collection.GenTraversable, java.util.Collection, java.util.Map, String, Array, and scala.Option in the Containing companion object.

Containing versus Aggregating

The difference between Containing and Aggregating is that Containing enables contain matcher syntax that makes sense for "box" types that can contain at most one value (for example, scala.Option), whereas Aggregating enables contain matcher syntax for full-blown collections and other aggregations of potentially more than one object. For example, it makes sense to make assertions like these, which are enabled by Containing, for scala.Option:

val option: Option[Int] = Some(7)
option should contain (7)
option should contain oneOf (6, 7, 8)
option should contain noneOf (3, 4, 5)

However, given a scala.Option can only ever contain at most one object, it doesn't make sense to make assertions like the following, which are enabled via Aggregation:

// Could never succeed, so does not compile
option should contain allOf (6, 7, 8)

The above assertion could never succceed, because an option cannot contain more than one value. By default the above statement does not compile, because contain allOf is enabled by Aggregating, and ScalaTest provides no implicit Aggregating instance for type scala.Option.