Type Conversion

In computer science, type conversion, typecasting, and coercion are different ways of, implicitly or explicitly, changing an entity of one data type into another. This is done to take advantage of certain features of type hierarchies or type representations. One example would be small integers, which can be stored in a compact format and converted to a larger representation when used in arithmetic computations. In object-oriented programming, type conversion allows programs to treat objects of one type as one of their ancestor types to simplify interacting with them.

Each programming language has its own rules on how types can be converted. In general, both objects and fundamental data types can be converted. In most languages, the word coercion is used to denote an implicit conversion, either during compilation or during run time. A typical example would be an expression mixing integer and floating point numbers (like 5 + 0.1), where the integers are normally converted into the latter. Explicit type conversions can either be performed via built-in routines (or a special syntax) or via separately defined conversion routines such as an overloaded object constructor.

In most Algol-based languages with nested function definitions, such as Ada, Delphi, Modula 2 and Pascal, conversion and casting are distinctly different concepts. In these languages, conversion refers to either implicitly or explicitly changing a value from one data type to another, e.g. a 16-bit integer to a 32-bit integer. The storage requirements may change as a result of the conversion. A loss of precision or truncation may also occur. The word cast, on the other hand, refers to explicitly changing the interpretation of the bit pattern representing a value from one type to another. For example 32 contiguous bits may be treated as an array of 32 booleans, a two character Unicode string, an unsigned 32-bit integer or an IEEE single precision floating point value. While the storage requirements are never changed, it still requires knowledge of low level details such as representation format, byte order, and alignment requirements in order to be meaningful.

In the C family of languages and Algol 68, the word cast typically refers to an explicit type conversion (as opposed to an implicit conversion), regardless of whether this is a re-interpretation of a bit-pattern or a real conversion.