Glossary V

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validation rules

A set of functions that dialog data verification (DDV) employs to validate user entries in a dialog control. For example, the function DDV_MaxChars could validate that the string entered in the text-box control is not greater than a specified length.

variable

In programming, a named storage location capable of containing a certain type of data that can be modified during program execution. See also data structure, data type.

variant

  1. In OLE Automation, an instance of the VARIANT datatype, which can represent values of many different types, such as integers, floats, booleans, strings, pointers, etc.

  2. In general, one of two or more data elements, functions, libraries, or whatever, exhibiting (usually slight) differences.

VBX

A custom control that can be used in Visual Basic as well as other Microsoft programming systems, including Visual C++. VBX controls are 16-bit dynamic-link libraries and are not supported in 32-bit applications. Visual Basic control files have a .VBX filename extension.

vector font

A scalable font in which the characters are drawn in arrangements of line segments rather than arrangements of curves or bits. Vector fonts are often used in applications that are optimized for output to plotters instead of printers. See also raster font, TrueType font.

Veronica

A database of the names of almost every menu item on thousands of Gopher servers. This database can be searched from most major gopher menus. See also Archie.

version information

An application's company and product identification, product release number, and copyright and trademark notification. Version information is held in a standard form in the executable (.EXE) file or dynamic-link library (DLL) file and is accessible by various tools and Windows functions.

version resource

A resource that contains either text or binary data about an application's version information.

Very Easy Rodent-Oriented Net-wide Index to Computerized Archives

A database of the names of almost every menu item on thousands of Gopher servers. This database can be searched from most major gopher menus. See also Archie.

view

  1. (noun) A window object through which a user interacts with a document.

  2. Or aspect. (noun) Generally, the manner in which data or a graphical image is displayed.

  3. (verb) To display information on a computer screen, as in "to view a file."

viewer

Program for opening image files and files in other special formats, such as audio and video.

viewport

A rectangle in device space that is used to specify a transformation between page and device space. The viewport extents (height and width) are always measured in pixels for a video display or in dots for printers.

viewport origin

The corner of the viewport from which the height and width of the viewport are measured.

VIRTKEY

In an accelerator table resource, a flag used to indicate that the keystroke value in the definition is a virtual key code.

virtual base class

A base class whose derived classes contain only one shared instance of its members even if the indirectly derived classes have done multiple inheritance. The class declaration for the derived class must have the keyword virtual before the base class specifier in order for it to be a virtual base class. In the following example, class MultiC has only one subobject of class Base even though it has inherited from more than one class derived from Base:

class Base {  };
class DerivedA: virtual public Base {  };
class DerivedB: virtual public Base {  };
class MultiC: public DerivedA, public DerivedB {  };

virtual destructor

A destructor implemented by declaring a base class's destructor with the keyword virtual. A virtual destructor ensures that, when delete is applied to a base class pointer or reference, it calls the destructor implemented in the derived class, if an implementation exists.

virtual device driver

A low-level software component that manages a single resource, such as a display screen or a serial port, on behalf of all running processes. A VxD is always 32-bit protected mode code and is frequently written in assembly language.

virtual function

A member function of a base class, where the function is declared with the keyword virtual. If a base class contains a virtual function and a derived class defines the same function, the function from the derived class is invoked for objects of the derived class, even if it is called using a pointer or reference to the base class.

virtual function table

A table of function pointers, such as an implementation of a class in C++. The pointers in the VTBL point to the members of the interfaces that an object supports. See also dual interface.

virtual inheritance

In C++, a method of deriving classes by preceding the name of the base class with the keyword virtual. With virtual inheritance, indirectly derived classes contain only one shared instance of the base class's members even if they have done multiple inheritance. See also virtual base class.

virtual key code

In Windows, a device-independent value translated from the scan code by the keyboard device driver. The code identifies the keyboard key. See also VIRTKEY.

virtual memory

A memory-management scheme that allows an application to "see" a  large, continuous block of primary memory (RAM) that, in reality, is a much smaller block of primary memory supplemented by secondary memory (such as a hard disk), with blocks of data being moved into and out of primary memory. See also swap file.

Virtual Reality Modeling Language

A vector-based language for modeling three-dimensional environments. It sends ASCII text files over the Internet, which are translated by the VRML viewing engine at the other end. VRML complements HTML.

visible region

In Windows, the portion of a window visible to the user.

Visual Basic control

A custom control that can be used in Visual Basic as well as other Microsoft programming systems, including Visual C++. VBX controls are 16-bit dynamic-link libraries and are not supported in 32-bit applications. Visual Basic control files have a .VBX filename extension.

void pointer

A pointer to an object of unknown type. See also typed pointer.

volatile

  1. In C/C++ and J++, a type qualifier that declares an item whose value can legitimately be changed by something beyond the control of the program in which it appears, such as a concurrently executing thread.

  2. More generally, describes information that is stored in memory and not preserved when a process or application terminates.

VRML

A vector-based language for modeling three-dimensional environments. It sends ASCII text files over the Internet, which are translated by the VRML viewing engine at the other end. VRML complements HTML.

VTBL

A table of function pointers, such as an implementation of a class in C++. The pointers in the VTBL point to the members of the interfaces that an object supports. See also dual interface.

VxD

A low-level software component that manages a single resource, such as a display screen or a serial port, on behalf of all running processes. A VxD is always 32-bit protected mode code and is frequently written in assembly language.