computer science Computer science is the study of computation, automation, and information. Computer science spans theoretical disciplines (such as algorithms, theory of computation, information theory, and automation) to practical disciplines (includin ...

, an opaque data type is a

data type In computer science and computer programming, a data type (or simply type) is a set of possible values and a set of allowed operations on it. A data type tells the compiler or interpreter how the programmer intends to use the data. Most progra ...

whose concrete

data structure In computer science, a data structure is a data organization, management, and storage format that is usually chosen for Efficiency, efficient Data access, access to data. More precisely, a data structure is a collection of data values, the rel ...

is not defined in an

interface Interface or interfacing may refer to: Academic journals * ''Interface'' (journal), by the Electrochemical Society * '' Interface, Journal of Applied Linguistics'', now merged with ''ITL International Journal of Applied Linguistics'' * '' Int ...

. This enforces

information hiding In computer science, information hiding is the principle of segregation of the ''design decisions'' in a computer program that are most likely to change, thus protecting other parts of the program from extensive modification if the design decisio ...

, since its

values In ethics and social sciences, value denotes the degree of importance of something or action, with the aim of determining which actions are best to do or what way is best to live (normative ethics in ethics), or to describe the significance of dif ...

can only be manipulated by calling

subroutines In computer programming, a function or subroutine is a sequence of program instructions that performs a specific task, packaged as a unit. This unit can then be used in programs wherever that particular task should be performed. Functions may ...

that have access to the missing information. The concrete representation of the type is hidden from its users, and the visible implementation is incomplete. A data type whose representation is visible is called transparent. Opaque data types are frequently used to implement

abstract data type In computer science, an abstract data type (ADT) is a mathematical model for data types. An abstract data type is defined by its behavior (semantics) from the point of view of a '' user'', of the data, specifically in terms of possible values, po ...

s. Typical examples of opaque data types include handles for

resources Resource refers to all the materials available in our environment which are technologically accessible, economically feasible and culturally sustainable and help us to satisfy our needs and wants. Resources can broadly be classified upon their ...

provided by an

operating system An operating system (OS) is system software that manages computer hardware, software resources, and provides common daemon (computing), services for computer programs. Time-sharing operating systems scheduler (computing), schedule tasks for ef ...

application software Application may refer to: Mathematics and computing * Application software, computer software designed to help the user to perform specific tasks ** Application layer, an abstraction layer that specifies protocols and interface methods used in a ...

. For example, the POSIX standard for threads defines an application programming interface based on a number of opaque types that represent

threads Thread may refer to: Objects * Thread (yarn), a kind of thin yarn used for sewing ** Thread (unit of measurement), a cotton yarn measure * Screw thread, a helical ridge on a cylindrical fastener Arts and entertainment * ''Thread'' (film), 2016 ...

synchronization primitive In computer science, synchronization refers to one of two distinct but related concepts: synchronization of processes, and synchronization of data. ''Process synchronization'' refers to the idea that multiple processes are to join up or handshak ...

s like mutexes or condition variables. An

opaque pointer In computer programming, an opaque pointer is a special case of an opaque data type, a data type declared to be a pointer to a record or data structure of some unspecified type. Opaque pointers are present in several programming languages includin ...

is a special case of an opaque data type, a datatype that is declared to be a

pointer Pointer may refer to: Places * Pointer, Kentucky * Pointers, New Jersey * Pointers Airport, Wasco County, Oregon, United States * The Pointers, a pair of rocks off Antarctica People with the name * Pointer (surname), a surname (including a list ...

to a

record A record, recording or records may refer to: An item or collection of data Computing * Record (computer science), a data structure ** Record, or row (database), a set of fields in a database related to one entity ** Boot sector or boot record, ...

of some unspecified data type. For example, the standard library that forms part of the specification of the

C programming language ''The C Programming Language'' (sometimes termed ''K&R'', after its authors' initials) is a computer programming book written by Brian Kernighan and Dennis Ritchie, the latter of whom originally designed and implemented the language, as well as ...

provides functions for file input and output that return or take values of type "pointer to FILE" that represent file streams (see C file input/output), but the concrete implementation of the type FILE is not specified.

Uses in various languages

Some languages, such as C, allow the declaration of opaque records (structs), whose size and fields are hidden from the client. The only thing that the client can do with an object of such a type is to take its

memory address In computing, a memory address is a reference to a specific memory location used at various levels by software and hardware. Memory addresses are fixed-length sequences of digits conventionally displayed and manipulated as unsigned integers. ...

, to produce an

. If the information provided by the interface is sufficient to determine the type's size, then clients can declare variables, fields, and arrays of that type, assign their values, and possibly compare them for equality. This is usually the case for opaque pointers. In some languages, such as

Java Java (; id, Jawa, ; jv, ꦗꦮ; su, ) is one of the Greater Sunda Islands in Indonesia. It is bordered by the Indian Ocean to the south and the Java Sea to the north. With a population of 151.6 million people, Java is the world's mo ...

, the only kind of opaque type provided is the opaque pointer. Indeed, in Java (and several other languages) records are always handled through pointers. Some languages allow partially opaque types, e.g. a record which has some ''public'' fields, known and accessible to all clients, and some ''hidden'' fields which are not revealed in the interface. Such types play a fundamental role in

object-oriented programming Object-oriented programming (OOP) is a programming paradigm based on the concept of " objects", which can contain data and code. The data is in the form of fields (often known as attributes or ''properties''), and the code is in the form of ...

. The information which is missing in the interface may be declared in its

implementation Implementation is the realization of an application, or execution of a plan, idea, model, design, specification, standard, algorithm, or policy. Industry-specific definitions Computer science In computer science, an implementation is a real ...

, or in another "friends-only" interface. This second option allows the hidden information to be shared by two or more modules.

References

Data types Object-oriented programming {{comp-sci-stub

Uses in various languages

See also

References