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Implicit type conversion, also known as coercion or type juggling, is an automatic type conversion by the compiler. Some programming languages allow compilers to provide coercion; others require it. In a mixed-type expression, data of one or more subtypes can be converted to a supertype as needed at runtime so that the program will run correctly.
Autoboxing is the term for getting a reference type out of a value type just through type conversion (either implicit or explicit). The compiler automatically supplies the extra source code that creates the object. For example, in versions of Java prior to J2SE 5.0, the following code did not compile:
While we could also convert myObject to a compile-time String using the universal java.lang.Object.toString(), this would risk calling the default implementation of toString() where it was unhelpful or insecure, and exception handling could not prevent this. In C++, run-time type checking is implemented through dynamic_cast.
There are many examples of languages that allow implicit type conversions, but in a type-safe manner. For example, both C++ and C# allow programs to define operators to convert a value from one type to another with well-defined semantics. When a C++ compiler encounters such a conversion, it treats the operation just like a function call.
To obtain the information required to infer the type of an expression, the compiler either gathers this information as an aggregate and subsequent reduction of the type annotations given for its subexpressions, or through an implicit understanding of the type of various atomic values (e.g. true : Bool; 42 : Integer; 3.14159 : Real; etc.).
In coercive subtyping systems, subtypes are defined by implicit type conversion functions from subtype to supertype. For each subtyping relationship (S <: T), a coercion function coerce: S → T is provided, and any object s of type S is regarded as the object coerce S → T (s) of type T.
The process of verifying and enforcing the constraints of types—type checking—may occur at compile time (a static check) or at run-time (a dynamic check). If a language specification requires its typing rules strongly, more or less allowing only those automatic type conversions that do not lose information, one can refer to the process as strongly typed; if not, as weakly typed.
In the Java example below, ... Implicit type conversion has also been defined as a form of polymorphism, referred to as "coercion polymorphism". [1] [6]