Selasa, 23 Oktober 2012

VISUAL BASIC











Icons for the main controls in Visual Basic 6.
Like the BASIC programming language, Visual Basic was designed to be easily learned and used by beginner programmers. The language not only allows programmers to create simple GUI applications, but to also develop complex applications. Programming in VB is a combination of visually arranging components or controls on a form, specifying attributes and actions of those components, and writing additional lines of code for more functionality. Since default attributes and actions are defined for the components, a simple program can be created without the programmer having to write many lines of code. Performance problems were experienced by earlier versions, but with faster computers and native code compilation this has become less of an issue.
Although VB programs can be compiled into native code executables from version 5 onwards, they still require the presence of runtime libraries of approximately 1 MB in size. Runtime libraries are included by default in Windows 2000 and later, however for earlier versions of Windows, i.e. 95/98/NT, runtime libraries must be distributed together 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 will automatically display its list and allow the user to select any element. An event handler is called when an item is selected, which can then execute additional code created by the programmer to perform some action based on which element was selected, such as populating a related list.
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", resulting in the very common problem of memory leaks. 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, although it will transform keywords into a standard case configuration and force the case of variable names to conform to the case of the entry within the symbol table. String comparisons are case sensitive by default.
The Visual Basic compiler is shared with other Visual Studio languages (C, C++), but restrictions in the IDE do not allow the creation of some targets (Windows model DLLs) and threading models.

Characteristics

The code windows in Visual Basic, showing a Function using the If, Then, Else and Dim statements.
Visual Basic has the following traits which differ from C-derived languages:
  • Statements tend to be terminated 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.[5] 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 will return 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.[6] 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 BASE was 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 Round function.[7] ? Round(2.5, 0) gives 2, ? Round(3.5, 0) gives 4.
  • Integers are automatically promoted to reals in expressions involving the normal division operator (/) so that division of one integer by another produces the intuitively correct result. There is a specific integer divide operator (\) which 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.

History

Alan Cooper created the drag and drop design for the user interface of Visual Basic.
VB 1.0 was introduced in 1991. The drag and drop design for creating the user interface is derived from a prototype form generator developed by Alan Cooper and his company called Tripod. Microsoft contracted with Cooper and his associates to develop Tripod into a programmable form system for Windows 3.0, under the code name Ruby (no relation to the Ruby programming language).
Tripod did not include a programming language at all. Microsoft decided to combine Ruby with the Basic language to create Visual Basic.
The Ruby interface generator provided the "visual" part of Visual Basic and this was combined with the "EB" Embedded BASIC engine designed for Microsoft's abandoned "Omega" database system. Ruby also provided the ability to load dynamic link libraries containing additional controls (then called "gizmos"), which later became the VBX interface.[8]

Timeline

  • Project 'Thunder' was initiated in 1990.[citation needed]
  • Visual Basic 1.0 (May 1991) was released for Windows at the Comdex/Windows World trade show in Atlanta, Georgia.
  • Visual Basic 1.0 for DOS was released in September 1992. The language itself was not quite compatible with Visual Basic for Windows, as it was actually the next version of Microsoft's DOS-based BASIC compilers, QuickBASIC and BASIC Professional Development System. The interface used a Text user interface, using extended ASCII characters to simulate the appearance of a GUI.
Visual Basic for MS-DOS
  • Visual Basic 2.0 was released in November 1992. The programming environment was easier to use, and its speed was improved. Notably, forms became instantiable objects, thus laying the foundational concepts of class modules as were later offered in VB4.
  • Visual Basic 3.0 was released in the summer of 1993 and came in Standard and Professional versions. VB3 included version 1.1 of the Microsoft Jet Database Engine that could read and write Jet (or Access) 1.x databases.
  • Visual Basic 4.0 (August 1995) was the first version that could create 32-bit as well as 16-bit Windows programs. It has three editions; Standard, Professional, and Enterprise. It also introduced the ability to write non-GUI classes in Visual Basic. Incompatibilities between different releases of VB4 caused installation and operation problems. While previous versions of Visual Basic had used VBX controls, Visual Basic now used OLE controls (with files names ending in .OCX) instead. These were later to be named ActiveX controls.
  • With version 5.0 (February 1997), Microsoft released Visual Basic exclusively for 32-bit versions of Windows. Programmers who preferred to write 16-bit programs were able to import programs written in Visual Basic 4.0 to Visual Basic 5.0, and Visual Basic 5.0 programs can easily be converted with Visual Basic 4.0. Visual Basic 5.0 also introduced the ability to create custom user controls, as well as the ability to compile to native Windows executable code, speeding up calculation-intensive code execution. A free, downloadable Control Creation Edition was also released for creation of ActiveX controls. It was also used as an introductory form of Visual Basic: a regular .exe project could be created and run in the IDE, but not compiled.
  • Visual Basic 6.0 (Mid 1998) improved in a number of areas[9] including the ability to create web-based applications. VB6 has entered Microsoft's "non-supported phase" as of March 2008. Although the Visual Basic 6.0 development environment is no longer supported, the runtime is supported on Windows Vista, Windows Server 2008 and Windows 7.[10]
  • Mainstream Support for Microsoft Visual Basic 6.0 ended on March 31, 2005. Extended support ended in March 2008.[11] In response, the Visual Basic user community expressed its grave concern and lobbied users to sign a petition to keep the product alive.[12] Microsoft has so far refused to change their position on the matter. (but see [13]) Ironically, around this time (2005), it was exposed that Microsoft's new anti-spyware offering, Microsoft AntiSpyware (part of the GIANT Company Software purchase), was coded in Visual Basic 6.0.[14] Its replacement, Windows Defender, was rewritten as C++ code.[15]

Derivative languages

Microsoft has developed derivatives of Visual Basic for use in scripting. Visual Basic itself is derived heavily from BASIC, and subsequently has been replaced with a .NET platform version.
Some of the derived languages are:
  • Visual Basic for Applications (VBA) is included in many Microsoft applications (Microsoft Office), and also in many third-party products like SolidWorks, AutoCAD, WordPerfect Office 2002, ArcGIS, Sage 300 ERP, and Business Objects Desktop Intelligence. There are small inconsistencies in the way VBA is implemented in different applications, but it is largely the same language as VB6 and uses the same runtime library. Although Visual Basic development ended with 6.0, in 2010 Microsoft introduced VBA 7 to provide extended features and 64-bit support for VBA. [16]
  • VBScript is the default language for Active Server Pages. It can be used in Windows scripting and client-side web page scripting. Although it resembles VB in syntax, it is a separate language and it is executed by vbscript.dll as opposed to the VB runtime. ASP and VBScript should not be confused with ASP.NET which uses the .NET Framework for compiled web pages.
  • Visual Basic .NET is Microsoft's designated successor to Visual Basic 6.0, and is part of Microsoft's .NET platform. Visual Basic.Net compiles and runs using the .NET Framework. It is not backwards compatible with VB6. An automated conversion tool exists, but fully automated conversion for most projects is impossible.[17]
  • StarOffice Basic is a Visual Basic compatible interpreter included in StarOffice suite, developed by Sun Microsystems.
  • Gambas is a Visual Basic inspired free software programming language. It is not a clone of Visual Basic, but it does have the ability to convert Visual Basic programs to Gambas.

Performance and other issues

Earlier versions of Visual Basic (prior to version 5) compiled the code to P-Code only. The P-Code is interpreted by the language runtime. The benefits of P-Code include portability and smaller binary file sizes, but it usually slows down the execution, since having a runtime adds an additional layer of interpretation. However, small amounts of code and algorithms can be constructed to run faster than compiled native code.
Visual Basic applications require Microsoft Visual Basic runtime MSVBVMxx.DLL, where xx is the relevant version number, either 50 or 60. MSVBVM60.dll comes as standard with Windows in all editions after Windows 98 while MSVBVM50.dll comes with all editions after Windows 95. A Windows 95 machine would however require inclusion with the installer of whichever dll was needed by the program.
Visual Basic 5 and 6 can compile code to either native or P-Code but in either case the runtime is still required for built in functions and forms management.
Criticisms levelled at Visual Basic editions prior to VB.NET include:[18]

Legacy development and support

All versions of the Visual Basic development environment from 1.0 to 6.0 have been retired and are now unsupported by Microsoft. The associated runtime environments are unsupported too, with the exception of the Visual Basic 6 core runtime environment, which will be officially supported by Microsoft for the lifetime of Windows 8.[21] Third party components that shipped with Visual Studio 6.0 are not included in this support statement. Some legacy Visual Basic components may still work on newer platforms, despite being unsupported by Microsoft and other vendors.
Development and maintenance development for Visual Basic 6 is possible on legacy Windows XP, Windows Vista and Windows 2003 using Visual Studio 6.0 platforms, but is unsupported. Documentation for Visual Basic 6.0, its application programming interface and tools is best covered in the last MSDN release before Visual Studio.NET 2002. Later releases of MSDN focused on .NET development and had significant parts of the Visual Basic 6.0 programming documentation removed. The Visual Basic IDE can be installed and used on Windows Vista, where it exhibits some minor incompatibilities which do not hinder normal software development and maintenance. As of August 2008, both Visual Studio 6.0 and the MSDN documentation mentioned above are available for download by MSDN subscribers.

Example code

The following code snippet displays a message box saying "Hello, World!" as the window loads:
Private Sub Form_Load()
    ' Execute a simple message box that will say "Hello, World!"
    MsgBox "Hello, World!"End Sub
This snippet makes a counter that moves up 1 every second (a label and a timer control need to be added to the form for this to work):
Option Explicit
Dim Count As Integer
Private Sub Form_Load()
    Count = 0
    Timer1.Interval = 1000 ' units of milliseconds
End Sub
Private Sub Timer1_Timer()
    Count = Count + 1
    Label1.Caption = Count
End Sub
Diposting oleh di

1 komentar:

Langganan: Posting Komentar (Atom)