# Application Development with Harbour/Harbour Overview

Unlike Java which is intended to be write once, run anywhere, Harbour aims to be write once, compile anywhere. As the same compiler is available for all of the above operating systems, there is no need for recoding to produce identical products for different platforms, except when operating system dependent features are used. Cross-compiling is supported with MinGW32. Under Microsoft Windows, Harbour is more stable but less well-documented than Clipper, but has multi-platform capability and is more transparent, customizable and can run from a USB flash drive.
Under Linux and Windows Mobile, Clipper source code can be compiled with Harbour with very little adaptation. Most software originally written to run on Xbase++, Flagship, FoxPro, xHarbour and others dialects can be compiled with Harbor with some adaptation. As 2010 many efforts have been made to turn the transition from other xBase dialects easier.
Harbour can use the following C compilers, among others: GCC, MinGW, Clang, ICC, Microsoft Visual C++ (6.0+), Borland C++, Watcom C, Pelles C and Sun Studio.
Harbour can make use of multiple Graphic Terminal emulations, including console drivers, and Hybrid Console/GUIs, such as GTWvt, and GTWvg.
Harbour supports external GUIs, free (e.g. HWGui, MiniGUI and Qt) and commercial (e.g. FiveWin, Xailer). HBQt is a library provinding bindings to Qt. HBIDE application included in official distribution and SVN repository is a sample of HBQt potencial.
Harbour is 100% Clipper-compatible and supports many language syntax extensions including greatly extended run-time libraries such as OLE, Blat, OpenSSL, FreeImage, GD, TIP, Tpathy, PCRE, HbZip (zlib and bzip2), cURL, Cairo, its own implementation of CA-Tools and NanFor libraries and many others. Harbour has an active development community and extensive third party support.
Any xBase language provides a very productive way to build business and data intensive applications. Harbour is not an exception.

### Macro Operator (runtime compiler)

One of the most powerful features of xBase languages is the Macro Operator '&'. Harbour's implementation of the Macro Operator allows for runtime compilation of any valid Harbour expression. Such a compiled expression may be used as a VALUE, i.e. the right side of an assignment (rvalue), but more interestingly, such a compiled expression may be used to resolve the left side (lvalue) of an assignment, i.e. PRIVATE, or PUBLIC variables, or a database FIELD.
Additionally, the Macro Operator may compile and execute function calls, complete assignments, or even list of arguments, and the result of the macro may be used to resolve any of the above contexts in the compiled application. In other words, any Harbour application may be extended and modified at runtime to compile and execute additional code on-demand.
Latest Macro compiler can compile any valid Harbour code including code to pre-process before compile.
Syntax:
The text value of the expression '...' will be compiled, and the value resulting from the execution of the compiled code is the result.
is the short form for &( SomeId ).
is the short form of &( SomeId + "postfix" ).

### Object Oriented Programming

Programming in an OOP style is a broader issue than a specific library or a specific interface, but OOP programming is something many Clipper programmers have come to expect. CA-Clipper 5.2 and especially 5.3 added a number of base classes, and a matching OOP syntax. Libraries such as CLASSy, Fivewin, Clip4Win, and TopClass provide additional OOP functionality.
Harbour has OOP extensions with full support for classes including inheritance, based on CLASSy syntax. OOP syntax in Harbour is very similar to that of earlier Clipper class libraries so it should be possible to maintain legacy Clipper code with minimal changes.

## Syntax and semantics

Harbour as every xBase language is case insensitive and can optionally accept keywords written just by first four characters.

### Built-in data types

Harbour has 6 scalar types : Nil, String, Date, Logical, Number, Pointer, and 4 complex types: Array, Object, CodeBlock, and Hash. A scalar holds a single value, such as a string, number, or reference to any other type. Arrays are ordered lists of scalars or complex types, indexed by number, starting at 1. Hashes, or associative arrays, are unordered collections of any type values indexed by their associated key, which may be of any scalar or complex type.
Literal (static) representation of scalar types:
* Nil: NIL
* String: "hello", 'hello', [hello]
* Date: 0d20100405
* Logical: .T., .F.
* Number: 1, 1.1, -1, 0xFF
Complex Types may also be represent as literal values:
* Array: { "String"", 1, { "Nested Array" }, .T., FunctionCall, @FunctionPointer }
* CodeBlock: { |Arg1, ArgN| Arg1 := ArgN + OuterVar + FunctionCall }
* Hash: { "Name" => "John", 1 => "Numeric key", { "Nested" => "Hash" } }
Hashes may use any type including other Hashes as the Key for any element. Hashes and Arrays may contain any type as the Value of any member, including nesting arrays, and Hashes.
Codeblocks may have references to Variables of the Procedure/Function>method in which it was defined. Such Codeblocks may be returned as a value, or by means of an argument passed BY REFERENCE, in such case the Codeblock will "outlive" the routine in which it was defined, and any variables it references, will be a DETACHED variable.
Detached variables will maintain their value for as long as a Codeblock referencing them still exists. Such values will be shared with any other Codeblock which may have access to those same variables. If the Codeblock did not outlive its containing routine, and will be evaluated within the life time of the routine in which it is defined, changes to its Detached Variables(s) by means of its evaluation, will be reflected back at its parent routine.
Codeblocks can be evaluated any number of times, by means of the Eval( BlockExp ) function.

### Variables

All types can be assigned to named variables. Named variable identifiers are 1 to 63 characters long, start with [A-Z|_] and further consist of the characters [A-Z|0-9|_] up to a maximum of 63 characters. Named variables are not case sensitive.
Variables have one of the following scopes:
* LOCAL: Visible only within the routine which declared it. Value is lost upon exit of the routine.
* STATIC: Visible only within the routine which declared it. Value is preserved for subsequent invocations of the routine. If a STATIC variable is declared before any Procedure/Function/Method is defined, it has a MODULE scope, and is visible within any routine defined within that same source file, it will maintain its life for the duration of the application life time.
* PRIVATE: Visible within the routine which declared it, and all routines called by that routine.
* PUBLIC: Visible by all routines in the same application.
LOCAL and STATIC are resolved at compile time, and thus are much faster than PRIVATE and PUBLIC variables which are dynamic entities accessed by means of a runtime Symbol table. For this same reason, LOCAL and STATIC variables are not exposed to the Macro compiler, and any macro code which attempts to reference them will generate a runtime error.
Due to the dynamic nature of PRIVATE and PUBLIC variables, they can be created and destroyed at runtime, can be accessed and modified by means of runtime macros, and can be accessed and modified by Codeblocks created on the fly.

## References

* ↑ Official Harbour page