What is an opaque value in C++?

from Wikipedia

In computer science, an opaque data type is a data type that is incompletely defined in an interface, so that its values can only be manipulated by calling subroutines that have access to the missing information. The concrete representation of the type is hidden from its users

Typical examples of opaque data types include handles for resources provided by an operating system to application software.

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, 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.