= External Classes
:table-number: 0
:toc:

There is currently no {cpp} skeleton generator for external classes, so any external classes have to be implemented manually.

External class implementation must abide by the following rules:

* The file name, where the external class is declared, must be the name of the external class followed by `.hh`. Please note that the name mapping rules described in <<6-mapping_ttcn3_data_types_to_c++_constructs.adoc#mapping-of-names-and-identifiers, Mapping of Names and Identifiers>> also apply to the class and file name. The implementations of class methods can be in any {cpp} source file that is linked to the executable test suite.
* The new `.hh` file must include the generated header file of the TTCN-3 module containing the external class declaration.
* The include to the new `.hh` file is already in the desired namespace in the generated {cpp} code, so no further namespaces are needed around the external class declaration.
* The external class must inherit the built-in {cpp} class `OBJECT` (`object` in TTCN-3).
* The C++ equivalents of all functions in the TTCN-3 external function declaration must be virtual.
* The class must also have a virtual destructor (even if it is empty).
* For more information about function parameters and return values of class types see <<6-mapping_ttcn3_data_types_to_c++_constructs.adoc#object-references, Object References>>.

== Example

Example.ttcn:
[source]
----
type external class ExternalClass {
  public external function f_ext(in integer x) return charstring;
  external function f_ext2(inout OtherClass p);
}
----

ExternalClass.hh:
[source]
----
#include "Example.hh"

#ifndef EXTERNALCLASS_HH
#define EXTERNALCLASS_HH

// NOTE: no namespace specification needed, since the 'include' command is
// already in the desired namespace!

class ExternalClass : public OBJECT {
public:
  virtual CHARSTRING f__ext(const INTEGER& x);
  
protected:
  virtual void f__ext2(OBJECT_REF<OtherClass>& p);
  
public:
  virtual ~ExternalClass() { }

// additional members and methods

};

#endif
----

ExternalClass.cc:
[source]
----
#include "ExternalClass.hh"

namespace Example {

CHARSTRING ExternalClass::f__ext(const INTEGER& x)
{
  // method implementation
}

void ExternalClass::f__ext2(OBJECT_REF<OtherClass>& p)
{
  // method implementation
}

}
----

NOTE: If external class methods are implemented in a different {cpp} file, than the mentioned new `.hh` file (such as `ExternalClass.cc` in this case), then they need to be placed in the TTCN-3 module's namespace.