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In computer science, an opaque data type is a data type that is incompletely defined in an interface, so that ordinary client programs can only manipulate data of that type by calling procedures that have access to the missing information.
Some languages, such as C, allow the declaration of opaque records (struct's), 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 address, to produce an opaque pointer.
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, 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.
The information which is missing in the interface may be declared in its implementation, or in another "friends-only" interface. This second option allows the hidden information to be shared by two or more modules.
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