In Section 3.4, we have seen how to expose member variables of a C++ object into OTcl space. This, however, does not apply to static member variables of a C++ class. Of course, one may create an OTcl variable for the static member variable of every C++ object; obviously this defeats the whole meaning of static members.
We cannot solve this binding problem using a similar solution as binding in TclObject, which is based on InstVar, because InstVars in TclCL require the presence of a TclObject. However, we can create a method of the corresponding TclClass and access static members of a C++ class through the methods of its corresponding TclClass. The procedure is as follows:
As an example, we show a simplified version of PacketHeaderClass in ~ns/packet.cc. Suppose we have the following class Packet which has a static variable hdrlen_ that we want to access from OTcl:
class Packet { ...... static int hdrlen_; };Then we do the following to construct an accessor for this variable:
class PacketHeaderClass : public TclClass { protected: PacketHeaderClass(const char* classname, int hdrsize); TclObject* create(int argc, const char*const* argv); /* These two implements OTcl class access methods / virtual void bind(); virtual int method(int argc, const char*const* argv); }; void PacketHeaderClass::bind() { /* Call to base class bind() must precede add_method() / TclClass::bind(); add_method("hdrlen"); } int PacketHeaderClass::method(int ac, const char*const* av) { Tcl& tcl = Tcl::instance(); \* Notice this argument translation; we can then handle them \ as if in TclObject::command() */ int argc = ac - 2; const char*const* argv = av + 2; if (argc == 2) { if (strcmp(argv[1], "hdrlen") == 0) { tcl.resultf("%d", Packet::hdrlen_); return (TCL_OK); } } else if (argc == 3) { if (strcmp(argv[1], "hdrlen") == 0) { Packet::hdrlen_ = atoi(argv[2]); return (TCL_OK); } } return TclClass::method(ac, av); }After this, we can then use the following OTcl command to access and change values of Packet::hdrlen_:
PacketHeader hdrlen 120 set i [PacketHeader hdrlen]
Tom Henderson 2011-11-05