trait TypeTags extends AnyRef
A TypeTag[T]
encapsulates the runtime type representation of some type T
.
Like scala.reflect.Manifest, the prime use case of TypeTag
s is to give access
to erased types. However, TypeTag
s should be considered to be a richer
replacement of the pre-2.10 notion of a Manifest, that
are, in addition, fully integrated with Scala reflection.
There exist three different types of TypeTags
:
- scala.reflect.api.TypeTags#TypeTag.
A full type descriptor of a Scala type. For example, aTypeTag[List[String]]
contains all type information, in this case, of typescala.List[String]
. - scala.reflect.ClassTag.
A partial type descriptor of a Scala type. For example, aClassTag[List[String]]
contains only the erased class type information, in this case, of typescala.collection.immutable.List
.ClassTag
s provide access only to the runtime class of a type. Analogous to scala.reflect.ClassManifest - scala.reflect.api.TypeTags#WeakTypeTag.
A type descriptor for abstract types (see description below).
Like Manifests, TypeTag
s are always generated by the
compiler, and can be obtained in three ways:
#1 Via the methods typeTag, classTag, or weakTypeTag
For example:
import scala.reflect.runtime.universe._ val tt = typeTag[Int] import scala.reflect._ val ct = classTag[String]
Each of these methods constructs a TypeTag[T]
or ClassTag[T]
for the given
type argument T
.
#2 Using an implicit parameter of type TypeTag[T]
, ClassTag[T]
, or WeakTypeTag[T]
For example:
import scala.reflect.runtime.universe._ def paramInfo[T](x: T)(implicit tag: TypeTag[T]): Unit = { val targs = tag.tpe match { case TypeRef(_, _, args) => args } println(s"type of $x has type arguments $targs") } scala> paramInfo(42) type of 42 has type arguments List() scala> paramInfo(List(1, 2)) type of List(1, 2) has type arguments List(Int)
#3 Context bound of a type parameter
...on methods or classes. The above example can be implemented as follows:
import scala.reflect.runtime.universe._ def paramInfo[T: TypeTag](x: T): Unit = { val targs = typeOf[T] match { case TypeRef(_, _, args) => args } println(s"type of $x has type arguments $targs") } scala> paramInfo(42) type of 42 has type arguments List() scala> paramInfo(List(1, 2)) type of List(1, 2) has type arguments List(Int)
WeakTypeTag
s
WeakTypeTag[T]
generalizes TypeTag[T]
. Unlike a regular TypeTag
, components of
its type representation can be references to type parameters or abstract types.
However, WeakTypeTag[T]
tries to be as concrete as possible, i.e. if type tags
are available for the referenced type arguments or abstract types, they are used to
embed the concrete types into the WeakTypeTag[T]
.
Continuing the example above:
def weakParamInfo[T](x: T)(implicit tag: WeakTypeTag[T]): Unit = { val targs = tag.tpe match { case TypeRef(_, _, args) => args } println(s"type of $x has type arguments $targs") } scala> def foo[T] = weakParamInfo(List[T]()) foo: [T]=> Unit scala> foo[Int] type of List() has type arguments List(T)
TypeTags and Manifests
TypeTag
s correspond loosely to the pre-2.10 notion of
scala.reflect.Manifests. While scala.reflect.ClassTag corresponds to
scala.reflect.ClassManifest and scala.reflect.api.TypeTags#TypeTag mostly
corresponds to scala.reflect.Manifest, other pre-2.10 Manifest
types do not
have a direct correspondence with a 2.10 "Tag
" type.
- scala.reflect.OptManifest is not supported.
This is becauseTag
s can reify arbitrary types, so they are always available. - - There is no equivalent for scala.reflect.AnyValManifest.
Instead, one can compare theirTag
with one of the baseTag
s (defined in the corresponding companion objects) in order to find out whether or not it represents a primitive value class. Additionally, it's possible to simply use<tag>.tpe.typeSymbol.isPrimitiveValueClass
. - There are no replacement for factory methods defined in the
Manifest
companion objects.
Instead, one could generate corresponding types using the reflection APIs provided by Java (for classes) and Scala (for types). - Certain manifest operations(i.e., <:<, >:> and typeArguments) are not
supported.
Instead, one could use the reflection APIs provided by Java (for classes) and Scala (for types).
In Scala 2.10, scala.reflect.ClassManifests are deprecated, and it is planned
to deprecate scala.reflect.Manifest in favor of TypeTag
s and ClassTag
s in
an upcoming point release. Thus, it is advisable to migrate any Manifest
-based
APIs to use Tag
s.
For more information about TypeTag
s, see the
Reflection Guide: TypeTags
- Self Type
- Universe
- Source
- TypeTags.scala
- See also
scala.reflect.ClassTag, scala.reflect.api.TypeTags#TypeTag, scala.reflect.api.TypeTags#WeakTypeTag
- Grouped
- Alphabetic
- By Inheritance
- TypeTags
- AnyRef
- Any
- by any2stringadd
- by StringFormat
- by Ensuring
- by ArrowAssoc
- Hide All
- Show All
- Public
- All
Type Members
-
trait
TypeTag
[T] extends Universe.WeakTypeTag[T] with Equals with Serializable
A
TypeTag
is a scala.reflect.api.TypeTags#WeakTypeTag with the additional static guarantee that all type references are concrete, i.e.A
TypeTag
is a scala.reflect.api.TypeTags#WeakTypeTag with the additional static guarantee that all type references are concrete, i.e. it does not contain any references to unresolved type parameters or abstract types.- Annotations
- @implicitNotFound( "No TypeTag available for ${T}" )
- See also
-
trait
WeakTypeTag
[T] extends Equals with Serializable
If an implicit value of type
WeakTypeTag[T]
is required, the compiler will create one, and the reflective representation ofT
can be accessed via thetpe
field.If an implicit value of type
WeakTypeTag[T]
is required, the compiler will create one, and the reflective representation ofT
can be accessed via thetpe
field. Components ofT
can be references to type parameters or abstract types. Note thatWeakTypeTag
makes an effort to be as concrete as possible, i.e. ifTypeTag
s are available for the referenced type arguments or abstract types, they are used to embed the concrete types into the WeakTypeTag. Otherwise the WeakTypeTag will contain a reference to an abstract type. This behavior can be useful, when one expectsT
to be perhaps be partially abstract, but requires special care to handle this case. However, ifT
is expected to be fully known, use scala.reflect.api.TypeTags#TypeTag instead, which statically guarantees this property.For more information about
TypeTag
s, see the Reflection Guide: TypeTags- Annotations
- @implicitNotFound( "No WeakTypeTag available for ${T}" )
- See also
Abstract Value Members
-
abstract
def
symbolOf[T](implicit arg0: Universe.WeakTypeTag[T]): Universe.TypeSymbol
Type symbol of
x
as derived from a type tag.
Concrete Value Members
-
final
def
!=(arg0: Any): Boolean
Test two objects for inequality.
Test two objects for inequality.
- returns
true
if !(this == that), false otherwise.
- Definition Classes
- AnyRef → Any
-
final
def
##(): Int
Equivalent to
x.hashCode
except for boxed numeric types andnull
.Equivalent to
x.hashCode
except for boxed numeric types andnull
. 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. Fornull
returns a hashcode wherenull.hashCode
throws aNullPointerException
.- returns
a hash value consistent with ==
- Definition Classes
- AnyRef → Any
- def +(other: String): String
- def ->[B](y: B): (TypeTags, B)
-
final
def
==(arg0: Any): Boolean
The expression
x == that
is equivalent toif (x eq null) that eq null else x.equals(that)
.The expression
x == that
is equivalent toif (x eq null) that eq null else x.equals(that)
.- returns
true
if the receiver object is equivalent to the argument;false
otherwise.
- Definition Classes
- AnyRef → Any
-
final
def
asInstanceOf[T0]: T0
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 aClassCastException
at runtime, while the expressionList(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.- returns
the receiver object.
- Definition Classes
- Any
- Exceptions thrown
ClassCastException
if the receiver object is not an instance of the erasure of typeT0
.
-
def
clone(): AnyRef
Create a copy of the receiver object.
Create a copy of the receiver object.
The default implementation of the
clone
method is platform dependent.- returns
a copy of the receiver object.
- Attributes
- protected[java.lang]
- Definition Classes
- AnyRef
- Annotations
- @throws( ... )
- Note
not specified by SLS as a member of AnyRef
- def ensuring(cond: (TypeTags) ⇒ Boolean, msg: ⇒ Any): TypeTags
- def ensuring(cond: (TypeTags) ⇒ Boolean): TypeTags
- def ensuring(cond: Boolean, msg: ⇒ Any): TypeTags
- def ensuring(cond: Boolean): TypeTags
-
final
def
eq(arg0: AnyRef): Boolean
Tests whether the argument (
that
) is a reference to the receiver object (this
).Tests whether the argument (
that
) is a reference to the receiver object (this
).The
eq
method implements an equivalence relation on non-null instances ofAnyRef
, and has three additional properties:- It is consistent: for any non-null instances
x
andy
of typeAnyRef
, multiple invocations ofx.eq(y)
consistently returnstrue
or consistently returnsfalse
. - For any non-null instance
x
of typeAnyRef
,x.eq(null)
andnull.eq(x)
returnsfalse
. null.eq(null)
returnstrue
.
When overriding the
equals
orhashCode
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
).- returns
true
if the argument is a reference to the receiver object;false
otherwise.
- Definition Classes
- AnyRef
- It is consistent: for any non-null instances
-
def
equals(arg0: Any): Boolean
The equality method for reference types.
-
def
finalize(): Unit
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 betweenfinalize
and non-local returns and exceptions, are all platform dependent.- Attributes
- protected[java.lang]
- Definition Classes
- AnyRef
- Annotations
- @throws( classOf[java.lang.Throwable] )
- Note
not specified by SLS as a member of AnyRef
- def formatted(fmtstr: String): String
-
final
def
getClass(): Class[_]
Returns the runtime class representation of the object.
Returns the runtime class representation of the object.
- returns
a class object corresponding to the runtime type of the receiver.
- Definition Classes
- AnyRef → Any
-
def
hashCode(): Int
The hashCode method for reference types.
-
final
def
isInstanceOf[T0]: Boolean
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 returnfalse
, while the expressionList(1).isInstanceOf[List[String]]
will returntrue
. 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.- returns
true
if the receiver object is an instance of erasure of typeT0
;false
otherwise.
- Definition Classes
- Any
-
final
def
ne(arg0: AnyRef): Boolean
Equivalent to
!(this eq that)
.Equivalent to
!(this eq that)
.- returns
true
if the argument is not a reference to the receiver object;false
otherwise.
- Definition Classes
- AnyRef
-
final
def
notify(): Unit
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.
- Definition Classes
- AnyRef
- Note
not specified by SLS as a member of AnyRef
-
final
def
notifyAll(): Unit
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.
- Definition Classes
- AnyRef
- Note
not specified by SLS as a member of AnyRef
-
final
def
synchronized[T0](arg0: ⇒ T0): T0
- Definition Classes
- AnyRef
-
def
toString(): String
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.
- returns
a String representation of the object.
- Definition Classes
- AnyRef → Any
-
def
typeOf[T](implicit ttag: Universe.TypeTag[T]): Universe.Type
Shortcut for
implicitly[TypeTag[T]].tpe
-
def
typeTag[T](implicit ttag: Universe.TypeTag[T]): Universe.TypeTag[T]
Shortcut for
implicitly[TypeTag[T]]
-
final
def
wait(): Unit
- Definition Classes
- AnyRef
- Annotations
- @throws( ... )
-
final
def
wait(arg0: Long, arg1: Int): Unit
- Definition Classes
- AnyRef
- Annotations
- @throws( ... )
-
final
def
wait(arg0: Long): Unit
- Definition Classes
- AnyRef
- Annotations
- @throws( ... )
-
def
weakTypeOf[T](implicit attag: Universe.WeakTypeTag[T]): Universe.Type
Shortcut for
implicitly[WeakTypeTag[T]].tpe
-
def
weakTypeTag[T](implicit attag: Universe.WeakTypeTag[T]): Universe.WeakTypeTag[T]
Shortcut for
implicitly[WeakTypeTag[T]]
- def →[B](y: B): (TypeTags, B)
-
object
TypeTag
extends Serializable
Type tags corresponding to primitive types and constructor/extractor for WeakTypeTags.
-
object
WeakTypeTag
extends Serializable
Type tags corresponding to primitive types and constructor/extractor for WeakTypeTags.