If an implicit value of type u.
If an implicit value of type u.
Test two objects for inequality.
Test two objects for inequality.
true
if !(this == that), false otherwise.
Equivalent to x.hashCode
except for boxed numeric types and null
.
Equivalent to x.hashCode
except for boxed numeric types and null
.
For numerics, it returns a hash value which is consistent
with value equality: if two value type instances compare
as true, then ## will produce the same hash value for each
of them.
For null
returns a hashcode where null.hashCode
throws a
NullPointerException
.
a hash value consistent with ==
Test two objects for equality.
Test two objects for equality.
The expression x == that
is equivalent to if (x eq null) that eq null else x.equals(that)
.
true
if the receiver object is equivalent to the argument; false
otherwise.
Cast the receiver object to be of type T0
.
Cast the receiver object to be of type T0
.
Note that the success of a cast at runtime is modulo Scala's erasure semantics.
Therefore the expression 1.asInstanceOf[String]
will throw a ClassCastException
at
runtime, while the expression List(1).asInstanceOf[List[String]]
will not.
In the latter example, because the type argument is erased as part of compilation it is
not possible to check whether the contents of the list are of the requested type.
the receiver object.
if the receiver object is not an instance of the erasure of type T0
.
Create a copy of the receiver object.
Create a copy of the receiver object.
The default implementation of the clone
method is platform dependent.
a copy of the receiver object.
not specified by SLS as a member of AnyRef
Tests whether the argument (arg0
) is a reference to the receiver object (this
).
Tests whether the argument (arg0
) is a reference to the receiver object (this
).
The eq
method implements an equivalence relation on
non-null instances of AnyRef
, and has three additional properties:
x
and y
of type AnyRef
, multiple invocations of
x.eq(y)
consistently returns true
or consistently returns false
.x
of type AnyRef
, x.eq(null)
and null.eq(x)
returns false
.null.eq(null)
returns true
. When overriding the equals
or hashCode
methods, it is important to ensure that their behavior is
consistent with reference equality. Therefore, if two objects are references to each other (o1 eq o2
), they
should be equal to each other (o1 == o2
) and they should hash to the same value (o1.hashCode == o2.hashCode
).
true
if the argument is a reference to the receiver object; false
otherwise.
The equality method for reference types.
Called by the garbage collector on the receiver object when there are no more references to the object.
Called by the garbage collector on the receiver object when there are no more references to the object.
The details of when and if the finalize
method is invoked, as
well as the interaction between finalize
and non-local returns
and exceptions, are all platform dependent.
not specified by SLS as a member of AnyRef
Returns string formatted according to given format
string.
Returns string formatted according to given format
string.
Format strings are as for String.format
(@see java.lang.String.format).
A representation that corresponds to the dynamic class of the receiver object.
A representation that corresponds to the dynamic class of the receiver object.
The nature of the representation is platform dependent.
a representation that corresponds to the dynamic class of the receiver object.
not specified by SLS as a member of AnyRef
The hashCode method for reference types.
Test whether the dynamic type of the receiver object is T0
.
Test whether the dynamic type of the receiver object is T0
.
Note that the result of the test is modulo Scala's erasure semantics.
Therefore the expression 1.isInstanceOf[String]
will return false
, while the
expression List(1).isInstanceOf[List[String]]
will return true
.
In the latter example, because the type argument is erased as part of compilation it is
not possible to check whether the contents of the list are of the specified type.
true
if the receiver object is an instance of erasure of type T0
; false
otherwise.
Equivalent to !(this eq that)
.
Equivalent to !(this eq that)
.
true
if the argument is not a reference to the receiver object; false
otherwise.
Wakes up a single thread that is waiting on the receiver object's monitor.
Wakes up a single thread that is waiting on the receiver object's monitor.
not specified by SLS as a member of AnyRef
Wakes up all threads that are waiting on the receiver object's monitor.
Wakes up all threads that are waiting on the receiver object's monitor.
not specified by SLS as a member of AnyRef
Creates a String representation of this object.
Creates a String representation of this object. The default representation is platform dependent. On the java platform it is the concatenation of the class name, "@", and the object's hashcode in hexadecimal.
a String representation of the object.
(typeTags: StringAdd).self
(typeTags: StringFormat).self
(typeTags: ArrowAssoc[TypeTags]).x
(Since version 2.10.0) Use leftOfArrow
instead
(typeTags: Ensuring[TypeTags]).x
(Since version 2.10.0) Use resultOfEnsuring
instead
Type tags encapsulate a representation of type T. They are supposed to replace the pre-2.10 concept of a scala.reflect.Manifest. TypeTags are much better integrated with reflection than manifests are, and are consequently much simpler.
Overview
Type tags are organized in a hierarchy of three classes: scala.reflect.ClassTag, scala.reflect.base.Universe#TypeTag and scala.reflect.base.Universe#AbsTypeTag.
A scala.reflect.ClassTag carries a runtime class that corresponds to the source type T. As of such, it possesses the knowledge about how to build single- and multi-dimensional arrays of elements of that type. It guarantees that the source type T did not to contain any references to type parameters or abstract types. scala.reflect.ClassTag corresponds to a previous notion of scala.reflect.ClassManifest.
A scala.reflect.base.Universe#AbsTypeTag value wraps a full Scala type in its tpe field. A scala.reflect.base.Universe#TypeTag value is an scala.reflect.base.Universe#AbsTypeTag that is guaranteed not to contain any references to type parameters or abstract types.
[Eugene++] also mention sensitivity to prefixes, i.e. that rb.TypeTag is different from ru.TypeTag [Eugene++] migratability between mirrors and universes is also worth mentioning
Splicing
Tags can be spliced, i.e. if compiler generates a tag for a type that contains references to tagged type parameters or abstract type members, it will retrieve the corresponding tag and embed it into the result. An example that illustrates the TypeTag embedding, consider the following function:
import reflect.mirror._ def f[T: TypeTag, U] = { type L = T => U implicitly[AbsTypeTag[L]] }
Then a call of f[String, Int] will yield a result of the form
AbsTypeTag(<[ String => U ]>).
Note that T has been replaced by String, because it comes with a TypeTag in f, whereas U was left as a type parameter.
AbsTypeTag vs TypeTag
Be careful with AbsTypeTag, because it will reify types even if these types are abstract. This makes it easy to forget to tag one of the methods in the call chain and discover it much later in the runtime by getting cryptic errors far away from their source. For example, consider the following snippet:
def bind[T: AbsTypeTag](name: String, value: T): IR.Result = bind((name, value)) def bind(p: NamedParam): IR.Result = bind(p.name, p.tpe, p.value) object NamedParam { implicit def namedValue[T: AbsTypeTag](name: String, x: T): NamedParam = apply(name, x) def apply[T: AbsTypeTag](name: String, x: T): NamedParam = new Typed[T](name, x) }
This fragment of Scala REPL implementation defines a
bind
function that carries a named value along with its type into the heart of the REPL. Using a scala.reflect.base.Universe#AbsTypeTag here is reasonable, because it is desirable to work with all types, even if they are type parameters or abstract type members.However if any of the three
AbsTypeTag
context bounds is omitted, the resulting code will be incorrect, because the missingAbsTypeTag
will be transparently generated by the compiler, carrying meaningless information. Most likely, this problem will manifest itself elsewhere, making debugging complicated. IfAbsTypeTag
context bounds were replaced withTypeTag
, then such errors would be reported statically. But in that case we wouldn't be able to usebind
in arbitrary contexts.Backward compatibility
Type tags correspond loosely to manifests.
More precisely: The previous notion of a scala.reflect.ClassManifest corresponds to a scala.reflect.ClassTag, The previous notion of a scala.reflect.Manifest corresponds to scala.reflect.runtime.universe.TypeTag,
In Scala 2.10, manifests are deprecated, so it's adviseable to migrate them to tags, because manifests might be removed in the next major release.
In most cases it will be enough to replace ClassManifests with ClassTags and Manifests with TypeTags, however there are a few caveats:
1) The notion of OptManifest is no longer supported. Tags can reify arbitrary types, so they are always available. // [Eugene++] it might be useful, though, to guard against abstractness of the incoming type.
2) There's no equivalent for AnyValManifest. Consider comparing your tag with one of the base tags (defined in the corresponding companion objects) to find out whether it represents a primitive value class. You can also use
<tag>.tpe.typeSymbol.isPrimitiveValueClass
for that purpose (requires scala-reflect.jar).3) There's no replacement for factory methods defined in
ClassManifest
andManifest
companion objects. Consider assembling corresponding types using reflection API provided by Java (for classes) and Scala (for types).4) Certain manifest functions (such as
<:<
,>:>
andtypeArguments
) weren't included in the tag API. Consider using reflection API provided by Java (for classes) and Scala (for types) instead.