This piece of C++ code implements a simple [Tcl] extension. The API as seen from [Tcl] is:

+++
   example::create   Creates a new example object and returns the object handle
   ''handle'' ping       Ping the object
   ''handle'' destroy    Destroy the object
+++

======
#include "tcl.h"

#include <iostream>

class MyObjectData {
    int id;
public:
    MyObjectData(int iid) : id(iid) {}
    void ping() { std::cout << "pong from " << id << std::endl; }
};

extern "C" int example_obj_command(ClientData     clientData, 
				   Tcl_Interp*    interp,
				   int	    objc,
				   Tcl_Obj* const objv[]) 
{
    if (objc < 2) {
	Tcl_WrongNumArgs(interp, 1, objv, "method ?argument ...?");
	return TCL_ERROR;
    }
  
    static const char* methods[] = {"ping", "destroy", NULL};
    enum ExObjMethods {EXOBJ_PING, EXOBJ_DESTROY};

    int index = -1;

    if (Tcl_GetIndexFromObj(interp, objv[1], methods, "method", 0,
			    &index) != TCL_OK)
	return TCL_ERROR;

    MyObjectData* p = (MyObjectData*)clientData;

    switch((ExObjMethods)index) {
    case EXOBJ_PING:
	p->ping();
	break;
    case EXOBJ_DESTROY:
	Tcl_DeleteCommand(interp, Tcl_GetString(objv[0]));
	delete p;
	break;
    }
    
    return TCL_OK;
}

extern "C" int example_create(ClientData     clientData, 
			      Tcl_Interp*    interp,
			      int	    objc,
			      Tcl_Obj* const objv[]) 
{
    static int obj_count = 0;

    MyObjectData* p = new MyObjectData(obj_count);
    
    char obj_name[13 + TCL_INTEGER_SPACE];
    sprintf(obj_name, "::exampleobj%d", obj_count++);
    
    Tcl_CreateObjCommand(interp, obj_name,
			 (Tcl_ObjCmdProc*)example_obj_command,
			 (ClientData) p, (Tcl_CmdDeleteProc *) NULL);

    Tcl_SetObjResult(interp, Tcl_NewStringObj(obj_name, strlen(obj_name)));
   
    return TCL_OK;
}

extern "C" DLLEXPORT int Example_Init(Tcl_Interp *interp)
{
    if (Tcl_InitStubs(interp, TCL_VERSION, 0) == NULL) {
	return TCL_ERROR;
    }
    if (Tcl_PkgRequire(interp, "Tcl", TCL_VERSION, 0) == NULL) {
	return TCL_ERROR;
    }
    if (Tcl_PkgProvide(interp, "example", "0.1") != TCL_OK) {
	return TCL_ERROR;
    }

    Tcl_CreateObjCommand(interp, "example::create",
			 (Tcl_ObjCmdProc*)example_create,
			 (ClientData)NULL, (Tcl_CmdDeleteProc*)NULL);

    return TCL_OK;
}
 
======

Compile it with this `Makefile` (Linux only) after adjusting the `TCLDIR`:

======
TCLDIR = /activetcl/8.5.0.0
TCLINCDIR = $(TCLDIR)/include
TCLLIBDIR = $(TCLDIR)/lib
TCLLIB = $(TCLLIBDIR)/libtcl8.5.so

all: example.so

example.so : example.o
	g++ -shared -o example.so example.o $(TCLLIB)

example.o : example.c
	g++ -fPIC -I$(TCLINCDIR) example.c -c -o example.o
======

You can use a `pkgIndex.tcl` file with this line in it 

======
package ifneeded example 0.1 [list load [file join $dir example.so]]
======

and 

======
package require example
======

or simply

======
load example.so
======

to load the extension in an interpreter.

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See also: [Writing Extensions], [Invoking Tcl commands from Cplusplus]

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%| [Arts and Crafts of Tcl-Tk Programming] | [Category Foreign Interfaces] | [Category Example] |%
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