A programmer can create an application using the components provided by the Visual Basic program itself. Over time the community of programmers have developed new third party components, keeping this programming language to modern standards.[3][4][5][6][7] Programs written in Visual Basic can also use the Windows API, which requires external function declarations. Furthermore, new third party functions (which are open source) using part VB6 source code and part embedded machine code, make the Visual Basic 6.0 applications faster than those designed in C++.[3][8]
The final release was version 6 in 1998 (now known simply as Visual Basic). Though Visual Basic 6.0 IDE is unsupported as of April 8, 2008, the Visual Basic team is committed to “It Just Works” compatibility for Visual Basic 6.0 applications on Windows Vista, Windows Server 2008 including R2, Windows 7, and Windows 8.[9] In 2014 there are hundreds of thousands of developers who still prefer Visual Basic 6.0 over Visual Basic .NET.[3][10] Moreover, in recent years some developers lobbied aggressively for a new version of Visual Basic 6.0.[11][12][13] [14] A dialect of Visual Basic, Visual Basic for Applications (VBA), is used as a macro or scripting language within several Microsoft applications, including Microsoft Office.[15]
Language features
Like the BASIC programming language, Visual Basic was designed to accommodate a steep learning curve. Programmers can create both simple and complex GUI applications. Programming in VB is a combination of visually arranging components or controls on a form, specifying attributes and actions for those components, and writing additional lines of code for more functionality. Since VB defines default attributes and actions for the components, a programmer can develop a simple program without writing much code. Programs built with earlier versions suffered performance problems, but faster computers and native code compilation has made this less of an issue.
Though VB programs can be compiled into native code executables from version 5 on, they still require the presence of around 1 MB of runtime libraries. Runtime libraries are included by default in Windows 2000 and later. Earlier versions of Windows (95/98/NT), require that the runtime libraries be distributed with the executable.
An empty form in Visual Basic 6.
Forms are created using drag-and-drop techniques. A tool is used to place controls (e.g., text boxes, buttons, etc.) on the form (window). Controls have attributes and event handlers associated with them. Default values are provided when the control is created, but may be changed by the programmer. Many attribute values can be modified during run time based on user actions or changes in the environment, providing a dynamic application. For example, code can be inserted into the form resize event handler to reposition a control so that it remains centered on the form, expands to fill up the form, etc. By inserting code into the event handler for a keypress in a text box, the program can automatically translate the case of the text being entered, or even prevent certain characters from being inserted.
Visual Basic can create executables (EXE files), ActiveX controls, or DLL files, but is primarily used to develop Windows applications and to interface database systems. Dialog boxes with less functionality can be used to provide pop-up capabilities. Controls provide the basic functionality of the application, while programmers can insert additional logic within the appropriate event handlers.
For example, a drop-down combination box automatically displays a list. When the user selects an element, an event handler is called that executes code that the programmer created to perform the action for that list item.
Alternatively, a Visual Basic component can have no user interface, and instead provide ActiveX objects to other programs via Component Object Model (COM). This allows for server-side processing or an add-in module.
The runtime recovers unused memory using reference counting, which depends on variables passing out of scope or being set to Nothing, avoiding the problem of memory leaks common to other languages. There is a large library of utility objects, and the language provides basic object oriented support. Unlike many other programming languages, Visual Basic is generally not case sensitive—though it transforms keywords into a standard case configuration and forces the case of variable names to conform to the case of the entry in the symbol table. String comparisons are case sensitive by default.
The Visual Basic compiler is shared with other Visual Studio languages (C, C++). Nevertheless, by default the restrictions in the IDE do not allow creation of some targets (Windows model DLLs) and threading models, but over the years, developers have bypassed these restrictions.
Characteristics
The following Visual Basic traits differ from C-derived languages:
- Statements tend to terminate with keywords, such as "End If", instead of using "{}"s to group statements.
- Multiple variable assignment is not possible. A = B = C does not imply that the values of A, B and C are equal. The boolean result of "Is B = C?" is stored in A. The result stored in A would therefore be either false or true.
- Boolean constant True has numeric value −1.[16] This is because the Boolean data type is stored as a 16-bit signed integer. In this construct −1 evaluates to 16 binary 1s (the Boolean value True), and 0 as 16 0s (the Boolean value False). This is apparent when performing a Not operation on a 16 bit signed integer value 0, which returns the integer value −1, in other words True = Not False. This inherent functionality becomes especially useful when performing logical operations on the individual bits of an integer such as And, Or, Xor and Not.[17] This definition of True is also consistent with BASIC since the early 1970s Microsoft BASIC implementation and is also related to the characteristics of CPU instructions at the time.
- Logical and bitwise operators are unified. This is unlike some C-derived languages (such as Perl), which have separate logical and bitwise operators. This again is a traditional feature of BASIC.
- Variable array base. Arrays are declared by specifying the upper and lower bounds in a way similar to Pascal and Fortran. It is also possible to use the Option Base statement to set the default lower bound. Use of the Option Base statement can lead to confusion when reading Visual Basic code and is best avoided by always explicitly specifying the lower bound of the array. This lower bound is not limited to 0 or 1, because it can also be set by declaration. In this way, both the lower and upper bounds are programmable. In more subscript-limited languages, the lower bound of the array is not variable. This uncommon trait does exist in Visual Basic .NET but not in VBScript.
OPTION BASEwas introduced by ANSI, with the standard for ANSI Minimal BASIC in the late 1970s.
- Relatively strong integration with the Windows operating system and the Component Object Model. The native types for strings and arrays are the dedicated COM types, BSTR and SAFEARRAY.
- Banker's rounding as the default behavior when converting real numbers to integers with the
Roundfunction.[18]? Round(2.5, 0)gives 2,? Round(3.5, 0)gives 4. - Integers are automatically promoted to reals in expressions that involve the normal division operator (/) so that division of one integer by another produces the intuitively correct result. VB provides a specific integer divide operator (\) that does truncate.
- By default, if a variable has not been declared or if no type declaration character is specified, the variable is of type Variant. However this can be changed with Deftype statements such as DefInt, DefBool, DefVar, DefObj, DefStr. There are 12 Deftype statements in total offered by Visual Basic 6.0. The default type may be overridden for a specific declaration by using a special suffix character on the variable name (# for Double, ! for Single, & for Long, % for Integer, $ for String, and @ for Currency) or using the key phrase As (type). VB can also be set in a mode that only explicitly declared variables can be used with the command Option Explicit.
Source http://en.wikipedia.org
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