Тизер.

...
int main()
{  
  char const* str = "myvalue=2*(yourvalue=1);myvalue=myvalue+1;";

  typedef AstParseInfo<char const*> parse_info_t;
  parse_info_t tpi =
    ast_parse(str, str+strlen(str), MyGrammar(), *ch_p(' '));

  typedef std::map<std::string, int> context_t;
  context_t context;
  int v = evaluate(tpi.nodes[0], context);

  std::cout << "CONTEXT: " << std::endl;
  typedef context_t::const_iterator context_iter_t;
  for(context_iter_t it = context.begin(); it != context.end(); ++it)
  {
    std::cout
    << " '"
    << (*it).first
    << "' is "
    << (*it).second
    << std::endl;
  }
}
...

debug: set 'yourvalue' to 1
debug: set 'myvalue' to 2
debug: set 'myvalue' to 3
CONTEXT: 
 'myvalue' is 3
 'yourvalue' is 1
Press [Enter] to close the terminal ...

P.S. Наконец-то отпуск.