# Exported Symbols

This section will be of interest only to writers of low-level libraries, such as replacements for the basic system library.

In order to provide maximum flexibility, the T3 Virtual Machine is designed to let the running program define as much of its own behavior as practical. As a result, the VM sometimes relies on the program to perform an operation on behalf of the VM, so that the operation doesn't have to be hard-coded into the VM. The VM accesses the program-defined code to perform these operations using "exports." An export is simply a symbol that the program defines and which the VM wishes to access. The program exports a symbol to tell the VM that a particular object or property performs an operation that is special to the VM.

For example, when a run-time error occurs, the VM throws an exception, which means the VM must create an exception object to represent the error condition. The VM does not have its own built-in exception class, because there's no practical need for such a class to be implemented in native code. Instead, the VM depends upon the program to define the object class that represents run-time errors, and uses an exported symbol to determine which class this is.

An exported symbol has two parts: an externally visible name, and the associated meaning. The meaning is an object or property value, and the external name is the name by which the VM knows the symbol. Since the purpose of this mechanism is to allow the VM to ask the program to provide values for certain things, the external names are defined by the VM.

## The export Statement

To export a symbol, you use the export statement.

    export symbol [ 'externalName' ]  ;

The symbol is the object or property name you wish to export. The externalName, if present, gives the name by which the VM knows the entity. If the external_name is not present, the symbol name is used as the external name. For example:

    export RuntimeError;

This statement exports the RuntimeError object defined in the program, using 'RuntimeError' as the external name as well.

The statement allows you to specify an external name so that you can use a different name for the entity within the program. If you provide an external name in the statement, the VM doesn't care that your program uses a different name internally. Some libraries have their own naming conventions, so they might want not want to use the VM-defined symbol names internally.

An export statement can appear anywhere a top-level statement (such as an object or function definition) can. It doesn't matter if an export comes before or after the actual definition of the object or property it exports; all of the exports are resolved during linking, so where they appear within the source code is unimportant.

A given internal symbol can be exported multiple times with different external names; if you do this, the VM will use the same entity for each different purpose it associates with an external name. A given external name can only be associated with a single internal entity, though - it is an error to export multiple objects or properties with the same external name.

## VM Symbols

The VM looks for the external names listed below to be exported by the program. (The VM also looks for the symbols supplied automatically by the compiler, as explained below, but library code only provides the ones listed here.)

RuntimeError - the exception class for run-time error exceptions. The basic system library provides an exported definition for this object.

exceptionMessage - the property to which to assign an explanatory message string in a run-time error exception object. The basic system library provides an exported definition for this property.

propNotDefined - a method which the VM invokes when a call is made to an undefined property. This method is invoked with the original property ID as the first argument, and the original argument list as the remaining arguments. The adv3 library provides an exported definition for this proeprty. See the section on capturing undefined method calls.

## Compiler-Supplied Exports

The compiler automatically provides a number of exports. Libraries do not need to provide exports for these; they are listed here only for completeness.

Constructor - the property invoked to construct new object. The compiler exports the construct property for this symbol.

Destructor - the property invoked to finalize an object during garbage collection. The compiler exports the finalize property for this symbol.

LastProp - the highest property ID value allocated by the compiler.

ObjectCallProp - a property assigned for invocation of anonymous function pointers.

TADS 3 System Manual
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