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Run-time type information
View on WikipediaIn computer programming, run-time type information or run-time type identification (RTTI)[1] is a feature of some programming languages (such as C++,[2] Object Pascal, and Ada[3]) that exposes information about an object's data type at runtime. Run-time type information may be available for all types or only to types that explicitly have it (as is the case with Ada). Run-time type information is a specialization of a more general concept called type introspection.
In the original C++ design, Bjarne Stroustrup did not include run-time type information, because he thought this mechanism was often misused.[4]
Overview
[edit]In C++, RTTI can be used to do safe typecasts using the dynamic_cast<> operator, and to manipulate type information at runtime using the typeid operator and std::type_info class. In Object Pascal, RTTI can be used to perform safe type casts with the as operator, test the class to which an object belongs with the is operator, and manipulate type information at run time with classes contained in the RTTI unit[5] (i.e. classes: TRttiContext, TRttiInstanceType, etc.). In Ada, objects of tagged types also store a type tag, which permits the identification of the type of these object at runtime. The in operator can be used to test, at runtime, if an object is of a specific type and may be safely converted to it.[6]
RTTI is available only for classes that are polymorphic, which means they have at least one virtual method. In practice, this is not a limitation because base classes must have a virtual destructor to allow objects of derived classes to perform proper cleanup if they are deleted from a base pointer.
Some compilers have flags to disable RTTI. Using these flags may reduce the overall size of the application, making them especially useful when targeting systems with a limited amount of memory.[7]
C++ typeid operator
[edit]The typeid reserved word (keyword) is used to determine the class of an object at runtime. It returns a reference to std::type_info object, which exists until the end of the program.[8] The use of typeid, in a non-polymorphic context, is often preferred over dynamic_cast<class_type> in situations where just the class information is needed, because typeid is always a constant-time procedure, whereas dynamic_cast may need to traverse the class derivation lattice of its argument at runtime.[citation needed] Some aspects of the returned object are implementation-defined, such as std::type_info::name(), and cannot be relied on across compilers to be consistent.
Objects of class std::bad_typeid are thrown when the expression for typeid is the result of applying the unary * operator on a null pointer. Whether an exception is thrown for other null reference arguments is implementation-dependent. In other words, for the exception to be guaranteed, the expression must take the form typeid(*p) where p is any expression resulting in a null pointer.
The typeid operator, if used in a context where std::type_info is not visible, is ill-informed, meaning <typeinfo> must be included any time it is used.
Example
[edit]import std;
using std::bad_typeid;
using std::type_info;
class Person {
public:
virtual ~Person() = default;
};
class Employee: public Person {
// ...
};
int main() {
Person person;
Employee employee;
Person* ptr = &employee;
Person& ref = employee;
type_info personType = typeid(person);
type_info employeeType = typeid(employee);
type_info ptrType = typeid(ptr);
type_info refType = typeid(ref);
// The string returned by std::type_info::name() is implementation-defined.
std::println("{}", personType.name());
// Person (statically known at compile-time).
std::println("{}", employeeType.name());
// Employee (statically known at compile-time).
std::println("{}", ptrType.name());
// Person* (statically known at compile-time).
std::println("{}", refType.name());
// Employee
// (looked up dynamically at run-time
// because it is the dereference of a
// pointer to a polymorphic class).
std::println("{}", typeid(ref).name());
// Employee (references can also be polymorphic)
Person* p = nullptr;
try {
typeid(*p); // Not undefined behavior; throws std::bad_typeid.
} catch (const bad_typeid& e) {
std::println(stderr, "Exception caught: {}", e.what());
}
Person& ref2 = *p; // Undefined behavior: dereferencing null
// Does not meet requirements to throw std::bad_typeid
// because the expression for typeid is not the result
// of applying the unary * operator.
type_info ref2Type = typeid(ref2);
}
Output (exact output varies by system and compiler):
Person Employee Person* Employee Employee
C++ dynamic_cast and Java cast
[edit]The dynamic_cast operator in C++ is used for downcasting a reference or pointer to a more specific type in the class hierarchy. Unlike the static_cast, the target of the dynamic_cast must be a pointer or reference to class. Unlike static_cast and C-style typecast (where type check occurs while compiling), a type safety check is performed at runtime. If the types are not compatible, an exception will be thrown (when dealing with references) or a null pointer will be returned (when dealing with pointers).
A Java typecast behaves similarly; if the object being cast is not actually an instance of the target type, and cannot be converted to one by a language-defined method, an instance of java.lang.ClassCastException will be thrown.[9]
Example
[edit]Suppose some function takes an object of type Base as its argument, and wishes to perform some additional operation if the object passed is an instance of Derived, a subclass of Base. This can be done using dynamic_cast as follows.
import std;
using std::array;
using std::bad_cast;
using std::unique_ptr;
class Base {
private:
void specificToBase() const {
std::println("Method specific for Base was invoked");
}
public:
// Since RTTI is included in the virtual method table there should be at
// least one virtual function.
virtual ~Base() = default;
};
class Derived: public Base {
public:
void specificToDerived() const {
std::println("Method specific for B was invoked");
}
};
void myFunction(Base& base) {
try {
// Cast will be successful only for B type objects.
Derived& derived = dynamic_cast<Derived&>(base);
derived.specificToDerived();
} catch (const bad_cast& e) {
std::println(stderr, "Exception {} thrown.", e.what());
std::println("Object is not of type Derived");
}
}
int main(int argc, char* argv[]) {
// Array of pointers to base class A.
array<unique_ptr<Base>, 3> arrayOfBase = {
std::make_unique<Derived>(); // Pointer to Derived object.
std::make_unique<Derived>(); // Pointer to Derived object.
std::make_unique<Base>(); // Pointer to Base object.
}
for (Base b: arrayOfBase) {
myFunction(*b);
}
return 0;
}
Console output:
Method specific for Derived was invoked Method specific for Derived was invoked Exception std::bad_cast thrown. Object is not of type Derived
A similar version of myFunction can be written with pointers instead of references:
void myFunction(Base* base) {
Derived* derived = dynamic_cast<Derived*>(base);
if (derived) {
derived->specificToDerived();
} else {
std::println(stderr, "Object is not Derived type");
}
}
Object Pascal, Delphi
[edit]In Object Pascal and Delphi, the operator is is used to check the type of a class at runtime. It tests the belonging of an object to a given class, including classes of individual ancestors present in the inheritance hierarchy tree (e.g. Button1 is a TButton class that has ancestors: TWinControl → TControl → TComponent → TPersistent → TObject, where the latter is the ancestor of all classes). The operator as is used when an object needs to be treated at run time as if it belonged to an ancestor class.
The RTTI unit is used to manipulate object type information at run time. This unit contains a set of classes that allow you to: get information about an object's class and its ancestors, properties, methods and events, change property values and call methods. The following example shows the use of the RTTI module to obtain information about the class to which an object belongs, creating it, and to call its method. The example assumes that the TSubject class has been declared in a unit named SubjectUnit.
uses
RTTI, SubjectUnit;
procedure WithoutReflection;
var
MySubject: TSubject;
begin
MySubject := TSubject.Create;
try
Subject.Hello;
finally
Subject.Free;
end;
end;
procedure WithReflection;
var
RttiContext: TRttiContext;
RttiType: TRttiInstanceType;
Subject: TObject;
begin
RttiType := RttiContext.FindType('SubjectUnit.TSubject') as TRttiInstanceType;
Subject := RttiType.GetMethod('Create').Invoke(RttiType.MetaclassType, []).AsObject;
try
RttiType.GetMethod('Hello').Invoke(Subject, []);
finally
Subject.Free;
end;
end;
See also
[edit]References
[edit]- ^ Sun Microsystems (2000). "Runtime Type Identification". C++ Programming Guide. Oracle. Retrieved 16 April 2015.
- ^ "Language support library [support.rtti]". eel.is. Retrieved 2021-07-13.
- ^ "Object-oriented programming". learn.adacore.com. Retrieved 2021-07-13.
- ^ Bjarne Stroustrup (March 1993). "A History of C++: 1979—1991" (PDF). Bjarne Stroustrup. p. 50. Retrieved 2009-05-18.
- ^ "Working with RTTI - RAD Studio". docwiki.embarcadero.com. Retrieved 2021-06-06.
- ^ English, John (2002-02-22). "Chapter 15". Ada 95: The Craft of Object-Oriented Programming. Retrieved 2021-07-13.
- ^ "Avoiding RTTI, and support for -fno-rtti in Arm Compiler 6". Arm Developer. Retrieved 2021-07-13.
- ^ C++ standard (ISO/IEC14882) section 5.2.8 [expr.typeid], 18.5.1 [lib.type.info] – http://cs.nyu.edu/courses/fall11/CSCI-GA.2110-003/documents/c++2003std.pdf
- ^ "ClassCastException (Java Platform SE 8)".
External links
[edit]Run-time type information
View on Grokipediastd::bad_cast (for references) if the cast fails; and typeid, which returns a std::type_info object describing the dynamic type of an expression, enabling comparisons and name retrieval.[4] The std::type_info class, defined in the <typeinfo> header, provides methods such as name() for an implementation-defined string representation of the type and hash_code() (since C++11) for generating a unique hash value, though it is neither copy-constructible nor copy-assignable to prevent unintended modifications.[5] RTTI requires compiler support and can be disabled in some implementations to reduce overhead, as it incurs runtime costs for type metadata storage and checks, typically implemented via virtual tables extended with type descriptors.[1] While RTTI offers limited reflection compared to languages like Java, it remains essential for scenarios involving dynamic polymorphism, such as in framework design or legacy code integration.[4]
Fundamentals
Definition and Core Principles
Run-time type information (RTTI) is a mechanism in object-oriented programming languages that enables the determination and inspection of an object's exact dynamic type during program execution, particularly in contexts involving polymorphism where the compile-time type may differ from the runtime type.[1] This feature allows programs to access metadata about types beyond what is available at compile time, facilitating operations that require awareness of an object's most derived class in inheritance hierarchies.[4] In contrast to static typing systems, where type resolution and checking occur entirely at compile time to ensure type safety before execution, RTTI defers certain type-related decisions to runtime, enabling greater flexibility in handling polymorphic objects whose types cannot be fully known statically.[1] Core principles of RTTI center on the persistent storage of type metadata in runtime-accessible structures, such as descriptors that encode class relationships, member details, and type identities, ensuring this information survives across program phases without recomputation.[6] Fundamental components of RTTI implementations include type descriptors, which provide opaque but comparable representations of types, often including names and equality checks, and their seamless integration with virtual function tables (vtables) in polymorphic classes.[5] Vtables, used primarily for dynamic dispatch of virtual functions, incorporate pointers to these type descriptors, allowing RTTI to leverage the existing infrastructure for polymorphism to support safe navigation and verification along inheritance chains, such as confirming compatibility before downcasting from a base to a derived type.[6] RTTI was first formally standardized in the ISO/IEC 14882:1998 specification for the C++ programming language, establishing a consistent model for runtime type handling across implementations.[7] Its foundational ideas, however, originate from earlier reflective systems like those in Smalltalk-80, where built-in introspection mechanisms permitted objects and code to query and alter their own types and behaviors dynamically during execution.[8]Purposes and Common Use Cases
Run-time type information (RTTI) primarily enables the resolution of polymorphic behavior by providing mechanisms for safe downcasting from base class pointers or references to derived types at runtime, ensuring type safety in inheritance hierarchies.[1] It supports type-safe exception handling, where catch clauses can match exceptions based on their dynamic types, allowing polymorphic exceptions to be caught at appropriate levels without slicing.[9] Beyond standard virtual functions, RTTI facilitates advanced dynamic dispatching, such as multimethods or type-based method selection, extending runtime flexibility in object-oriented designs.[10] Common use cases for RTTI include debugging tools that leverage type identification to log or inspect object types during execution, simplifying the diagnosis of issues in complex, polymorphic codebases.[11] In serialization and deserialization processes, such as converting objects to and from formats like JSON or binary streams, RTTI determines class types to automate mapping and reconstruction without manual type specification.[12] Plugin architectures employ RTTI to verify type compatibility when dynamically loading external modules, ensuring seamless integration while preventing mismatches in class hierarchies.[13] GUI frameworks utilize RTTI for event routing, dispatching user interactions to specific handlers based on the runtime type of UI components, which enhances modularity in interactive applications.[14] RTTI finds application in game engines within entity-component systems, where it aids in querying and attaching components of particular types to entities for dynamic behavior composition. In integrated development environments (IDEs), it supports runtime code introspection, enabling features like dynamic type visualization and automated debugging probes during program execution.[15] The evolution of RTTI's uses traces back to the early 1990s, when it was introduced in C++ primarily for safe downcasting in polymorphic scenarios to address inconsistencies in vendor implementations. In modern reflection-heavy frameworks, such as those extending C++ with enhanced introspection libraries, RTTI serves as a foundational mechanism for automating tasks like binding generation and property exposure, adapting to demands for greater runtime adaptability in large-scale software systems.[15]Key Mechanisms
Type Identification Techniques
Run-time type identification techniques provide mechanisms for querying the dynamic type of an object at execution time, enabling retrieval of type names, verification of inheritance relationships, and inspection of associated metadata without modifying the object's reference. These methods support non-destructive operations essential for applications such as debugging, where logging the precise type aids in error diagnosis, and serialization, where type information ensures correct data reconstruction. Unlike type casting, which attempts to reinterpret an object as another type, identification focuses solely on informational queries to maintain type safety and avoid runtime errors from invalid conversions. A fundamental implementation involves type descriptors embedded in or accessible from the object's representation, often via a type metadata structure. These descriptors store the type's name as a string for human-readable identification and include structural details like field offsets and method signatures, allowing programs to access metadata dynamically. For instance, in systems requiring precise layout information for memory management, such descriptors are derived from compiler-generated data to describe in-memory object structures beyond basic language types. This approach extends standard RTTI by incorporating compiler-specific details, ensuring portability across environments like persistent stores and garbage collectors. Type equality checks typically rely on unique identifiers within the descriptor, such as pointers to static instances or hashed values, enabling fast comparisons without string operations. Subtype queries, which verify is-a relationships, examine the inheritance hierarchy encoded in the descriptor—often as a linked list of base types or a precomputed subgraph—to confirm if the object's most derived type inherits from a specified class. Handling abstract types occurs through resolution to the concrete derived type at runtime, as identification operates on the actual object instance rather than the static reference type, preventing ambiguity in polymorphic scenarios. For efficiency, hash-based methods predominate to minimize overhead in performance-critical code. These techniques collectively distinguish identification from casting by emphasizing query-only access, preserving the object's integrity while providing comprehensive type introspection.Runtime Type Casting
Runtime type casting enables the conversion of object references from one type to another compatible type during program execution, leveraging runtime type information (RTTI) to ensure type safety in object-oriented systems. Upcasting, which converts a reference from a derived class to its base class, is inherently safe and often performed implicitly, as every derived object is a valid instance of its base class. In contrast, downcasting—from a base class to a derived class—carries risks, as the actual object may not belong to the target derived type, potentially leading to undefined behavior if unchecked. Safety mechanisms in runtime type casting mitigate these risks through dynamic verification using RTTI. These checks inspect the object's actual type against the target type within the class hierarchy, preventing invalid conversions that could cause memory corruption or type confusion attacks. Upon failure, safe casting operations typically return a null pointer for pointer-based conversions or throw an exception for reference-based ones, allowing programs to handle errors gracefully without invoking undefined behavior. Such runtime validations, often implemented via polymorphic hierarchies with virtual functions, ensure that only compatible casts succeed, enhancing overall type security in object-oriented designs. The general process of runtime type casting begins with querying the runtime type information of the source object, typically through mechanisms that expose the complete type hierarchy. Compatibility is then verified by traversing the inheritance structure to confirm that the target type is a valid subtype or related base in the graph. If verified, the conversion adjusts the reference to point to the appropriate offset or entry in the type metadata, enabling access to derived-specific members; otherwise, the operation aborts safely. This process relies on RTTI as a precursor for accurate type queries before attempting the cast. Edge cases in runtime type casting arise with complex language features like templates and generics, where type parameterization may limit available RTTI, complicating verification and sometimes necessitating additional runtime checks to avoid type erasure pitfalls.Implementations in C++
typeid Operator
Thetypeid operator in C++ provides a mechanism for querying type information at runtime, primarily for polymorphic objects, by returning a reference to a std::type_info object that encapsulates details about the type. This operator is essential for runtime type identification (RTTI) and is declared in the <typeinfo> header.[16]
The syntax of typeid is straightforward: typeid(type-id) yields the std::type_info for a specified type at compile time, while typeid(expression) determines the type based on the expression, which can resolve statically or dynamically depending on the context. For instance, typeid(int).name() retrieves a string representation of the type name for int, whereas applying it to an expression involving a polymorphic object pointer or reference performs a runtime evaluation via the virtual table.[16] The returned std::type_info object supports comparison operators such as == and != for type equality checks, as well as a name() member function that returns an implementation-defined string describing the type.
Behaviorally, typeid evaluates to the static type for non-polymorphic expressions, meaning it does not perform dynamic dispatch in such cases. However, for polymorphic classes—those with at least one virtual function—it resolves to the dynamic type of the object at runtime, enabling identification of the actual derived type even when accessed through a base pointer or reference. However, if the expression is a glvalue that refers to a null object of a polymorphic type (such as dereferencing a null pointer), typeid throws a std::bad_typeid exception during dynamic type evaluation. The type_info objects are guaranteed to have static storage duration but may not refer to the same instance across multiple typeid invocations, even for identical types.[16]
Limitations include the requirement for complete types; incomplete types result in undefined behavior. For non-polymorphic types, only static type information is available, limiting its utility for dynamic queries.[16] Additionally, the output of name() is implementation-defined and may be abbreviated or mangled in optimized builds, potentially providing incomplete or non-human-readable type information. Runtime type information must be enabled via compiler flags like /GR in Microsoft Visual C++.[16]
The following example demonstrates typeid usage with a polymorphic base and derived class:
#include <iostream>
#include <typeinfo>
class Base {
public:
virtual ~Base() {} // Ensures polymorphic behavior
};
class Derived : public Base {};
int main() {
Base* ptr = new Derived();
std::cout << typeid(*ptr).name() << std::endl; // Outputs something like "class Derived" (implementation-defined)
std::cout << typeid(Base).name() << std::endl; // Outputs something like "class Base"
delete ptr;
return 0;
}
#include <iostream>
#include <typeinfo>
class Base {
public:
virtual ~Base() {} // Ensures polymorphic behavior
};
class Derived : public Base {};
int main() {
Base* ptr = new Derived();
std::cout << typeid(*ptr).name() << std::endl; // Outputs something like "class Derived" (implementation-defined)
std::cout << typeid(Base).name() << std::endl; // Outputs something like "class Base"
delete ptr;
return 0;
}
typeid on a dereferenced base pointer to a derived object reveals the dynamic type, contrasting with the static type for the base class itself.[16]
The typeid operator was introduced in the C++98 standard as part of the RTTI facilities, with subsequent refinements in later standards to address edge cases like null pointer handling.
dynamic_cast Operator
Thedynamic_cast operator in C++ provides a safe mechanism for performing runtime type conversions, particularly for downcasting pointers or references from a base class to a derived class within an inheritance hierarchy. It performs these conversions at runtime, leveraging run-time type information (RTTI) to verify the actual type of the object before completing the cast, thereby preventing undefined behavior that could arise from incorrect type assumptions. This operator is essential in polymorphic scenarios where the exact derived type is not known at compile time, such as in generic processing of base class pointers.[17]
The syntax for dynamic_cast is dynamic_cast<target-type>(expression), where target-type is typically a pointer or reference to a class type, and expression is a pointer or reference (glvalue since C++11) to a polymorphic class object. For pointers, it is commonly used as dynamic_cast<Derived*>(base_ptr) to attempt a downcast from a base class pointer to a derived class pointer; a similar form applies to references as dynamic_cast<Derived&>(base_ref). The operator supports upcasting (derived to base), downcasting (base to derived), and cross-casting (between sibling derived classes), but it cannot convert to unrelated types or remove constness—though it can add it.[17]
To use dynamic_cast, the source and target classes must be polymorphic, meaning the class containing the pointer or reference must have at least one virtual function (such as a virtual destructor), enabling the compiler to generate the necessary RTTI metadata. The operator traverses the inheritance graph at runtime, querying the object's dynamic type to determine if the cast is valid, which requires public and unambiguous base class relationships for successful downcasts or crosscasts. If the classes lack polymorphism, the program is ill-formed, and compilation fails.[17]
On failure—such as when downcasting to an incompatible derived type—dynamic_cast for pointers returns a null pointer value (nullptr), allowing the caller to check and handle the failure gracefully without dereferencing an invalid pointer. For references, an unsuccessful cast throws a std::bad_cast exception, which must be caught to avoid program termination, making reference casts suitable only when failure is not expected or is explicitly handled via try-catch blocks. This dual behavior ensures type safety while providing flexibility in error management.[17][18]
The following example demonstrates dynamic_cast with a simple inheritance hierarchy:
#include <iostream>
#include <typeinfo>
class Base {
public:
virtual ~Base() = default; // Ensures polymorphism
};
class Derived : public Base {};
class Unrelated : public Base {};
int main() {
Base* base_ptr = new Derived(); // Points to Derived object
// Successful downcast
Derived* derived_ptr = dynamic_cast<Derived*>(base_ptr);
if (derived_ptr) {
std::cout << "Cast successful." << std::endl;
}
// Change to point to Unrelated
delete base_ptr;
base_ptr = new Unrelated();
derived_ptr = dynamic_cast<Derived*>(base_ptr);
if (!derived_ptr) {
std::cout << "Cast failed: null pointer returned." << std::endl;
}
// Reference example (throws on failure)
try {
Base& base_ref = *base_ptr;
Derived& derived_ref = dynamic_cast<Derived&>(base_ref); // Would throw std::bad_cast
} catch (const std::bad_cast& e) {
std::cout << "Reference cast failed: " << e.what() << std::endl;
}
delete base_ptr;
return 0;
}
#include <iostream>
#include <typeinfo>
class Base {
public:
virtual ~Base() = default; // Ensures polymorphism
};
class Derived : public Base {};
class Unrelated : public Base {};
int main() {
Base* base_ptr = new Derived(); // Points to Derived object
// Successful downcast
Derived* derived_ptr = dynamic_cast<Derived*>(base_ptr);
if (derived_ptr) {
std::cout << "Cast successful." << std::endl;
}
// Change to point to Unrelated
delete base_ptr;
base_ptr = new Unrelated();
derived_ptr = dynamic_cast<Derived*>(base_ptr);
if (!derived_ptr) {
std::cout << "Cast failed: null pointer returned." << std::endl;
}
// Reference example (throws on failure)
try {
Base& base_ref = *base_ptr;
Derived& derived_ref = dynamic_cast<Derived&>(base_ref); // Would throw std::bad_cast
} catch (const std::bad_cast& e) {
std::cout << "Reference cast failed: " << e.what() << std::endl;
}
delete base_ptr;
return 0;
}
nullptr for safe checking, and a reference cast throwing std::bad_cast on mismatch.[17][18]
Implementation-wise, dynamic_cast relies on virtual table (vtable) lookups and RTTI data structures to resolve types at runtime, incurring a modest overhead compared to compile-time casts like static_cast, though this cost is negligible in most applications involving polymorphism. Compilers may optimize these lookups, but the feature can be disabled entirely via the -fno-rtti flag in GCC or equivalent, rendering dynamic_cast unavailable.[17][18]
Implementations in Java and C#
Type Checking Operators
In Java, theinstanceof operator performs runtime type checking to determine if an object is an instance of a specified class, interface, or subclass thereof, returning a boolean value.[19][20] For example, given a polymorphic object referenced as Object obj = new Derived();, the expression obj instanceof Derived evaluates to true because Derived extends the base class.[19] This operator supports hierarchy checks, including interfaces and arrays, and integrates with the Java Virtual Machine (JVM) for efficient verification without explicit casting.[20] Complementing instanceof, the getClass() method, inherited from Object, returns a Class object representing the exact runtime type of the instance, providing metadata for further reflection such as method introspection or field access.[21][22]
In C#, the is operator similarly checks for type compatibility at runtime, evaluating whether an expression's type is assignable to a given type, including derived classes or implemented interfaces, and returns a boolean.[23][24] For instance, with object obj = new Derived();, obj is Derived yields true due to inheritance.[24] This operator leverages the Common Language Runtime (CLR) for optimized checks and supports pattern matching extensions.[23] The GetType() method on Object retrieves a Type object encapsulating the precise runtime type, enabling access to type details like properties, methods, and attributes via the System.Type class.[25][26]
Both Java and C# type checking operators exhibit shared behaviors, including seamless integration with generics for type-safe collections and operations, where runtime checks respect parameterized types within the JVM and CLR environments for performance. They return either booleans for compatibility tests or type objects for metadata retrieval, facilitating polymorphic designs without runtime errors.[19][24]
A key difference lies in the scope of exact type matching: Java's getClass() performs a strict equality check against the runtime class, ignoring inheritance, whereas C#'s is operator inherently accounts for inheritance hierarchies.[21][24] For array types, this distinction is evident; in Java, obj instanceof String[] succeeds if obj is a String[] or a compatible array subtype, but obj.getClass() == String[].class requires an exact match.[20] In C#, obj is string[] handles inheritance and covariance similarly, while obj.GetType() == typeof(string[]) demands precision.[24]
The following parallel code snippets illustrate instanceof/is on a polymorphic object in Java and C#, respectively:
Java:
class Base {}
class Derived extends Base {}
Object obj = new Derived();
boolean isDerived = obj instanceof Derived; // true
Class<?> type = obj.getClass(); // Derived.class
class Base {}
class Derived extends Base {}
Object obj = new Derived();
boolean isDerived = obj instanceof Derived; // true
Class<?> type = obj.getClass(); // Derived.class
class Base {}
class Derived : Base {}
object obj = new Derived();
bool isDerived = obj is Derived; // true
Type type = obj.GetType(); // typeof(Derived)
class Base {}
class Derived : Base {}
object obj = new Derived();
bool isDerived = obj is Derived; // true
Type type = obj.GetType(); // typeof(Derived)
getClass() and GetType() provide exact type objects for deeper inspection.[19][23][21][25]
Casting Mechanisms
In Java, runtime type casting to a reference type is performed using the syntax(TargetType) expression, which attempts to convert the runtime value to the specified type. If the conversion is invalid—such as when the actual runtime type is not a subclass, superclass, or implementor of the target type—a ClassCastException is thrown at runtime.[27] This mechanism relies on the Java Virtual Machine (JVM) verifying type compatibility using metadata from class files, ensuring safe narrowing conversions while handling interfaces and generics through isAssignableFrom checks on class objects.[28]
To mitigate the risks of explicit casts, Java introduced pattern matching for the instanceof operator starting in Java SE 14, allowing safer type tests and automatic casting in a single expression. For example, if (obj instanceof String s) { ... } checks the type and binds the cast variable s only if the match succeeds, avoiding separate casts and potential exceptions.[29] This enhancement reduces boilerplate and errors in polymorphic code involving inheritance hierarchies or interfaces.
Consider a simple inheritance example in Java:
class Animal { }
class Dog extends Animal { public void bark() { } }
Animal animal = new Dog();
try {
Dog dog = (Dog) animal; // Successful cast
dog.bark();
} catch (ClassCastException e) {
// Handle failure, e.g., if animal was a Cat instance
}
class Animal { }
class Dog extends Animal { public void bark() { } }
Animal animal = new Dog();
try {
Dog dog = (Dog) animal; // Successful cast
dog.bark();
} catch (ClassCastException e) {
// Handle failure, e.g., if animal was a Cat instance
}
Dog is a subclass of Animal, but failure would occur and trigger the exception if the runtime type mismatched. For generics, due to type erasure, runtime casts verify compatibility on the raw types (e.g., casting ListShape permitting only Circle and Square would disallow casting to an unrelated UtahTeapot, as the hierarchies have no overlap, thus reducing runtime errors.[30]
In C#, explicit casting uses the syntax (TargetType) expression, which converts the runtime object if possible but throws an InvalidCastException if the types are incompatible.[24] The safer as operator, expression as TargetType, attempts the cast and returns null on failure without throwing an exception, making it preferable for reference types and nullable value types. Both operators leverage the Common Language Runtime (CLR) metadata in assemblies to perform runtime verification, supporting interfaces (via implementor checks) and generics (through constraint validation).[24]
Starting with C# 7.0, pattern matching integrates type checking and casting via the is operator, combining the functionality of is and as into expressions like if (obj is string s) { ... }, which tests the type and binds the cast variable only on success.[31] This allows concise handling of inheritance and interfaces without explicit null checks post-cast.
An example in C# with inheritance:
class Animal { }
class Dog : Animal { public void Bark() { } }
Animal animal = new Dog();
Dog dog = animal as Dog; // Safe cast, null if not Dog
if (dog != null) {
dog.Bark();
}
class Animal { }
class Dog : Animal { public void Bark() { } }
Animal animal = new Dog();
Dog dog = animal as Dog; // Safe cast, null if not Dog
if (dog != null) {
dog.Bark();
}
if (animal is Dog d) { d.Bark(); }, which avoids exceptions and null checks. For generics, casting a List