#include <map>
#include <string>

...

struct ltstr {
  bool operator()(const char *s1, const char *s2) const {
    return strcmp(s1, s2) < 0;
  }
};

...

using namespace std;

map<const char *, char *, ltstr> value;	// variables
				// the symbol table implements a
				// function (map) from char * to char *
...

enum Bool {NO=0, YES=1};
static inline Bool
is_defined(char *name) {	// YES if name is in the symbol table
  return ((value.find(name) == value.end()) ? NO : YES);
}

...

char	*v = value["X"];	// set v to point to the value of X
value["X"] = "";		// set the value of the var X to ""


<< Comments >>

o If you map from char * to char *, the pointers should not point to
  memory that will disappear (e.g., variables on the stack).
    BAD:
  	void foo (...) {
	  char	buf[100];
	  ... some code that puts a string into buf[] ...
	  value["Some_Name"] = buf;	// buf[] will disappear upon
	}				//   return from foo
    BETTER:
  	void foo (...) {
	  char	buf[100];
	  ... some code that puts a string into buf[] ...
	  value["Some_Name"] = strdup(buf);
	}	// strdup() allocates from heap; need to free space later

o Mapping from char * to char * is OK, but C++ string to C++ string might
  be better for real C++ programmers.