current value. If it returns false for the
default value, ParseCommandLineFlags will die.

<p>Here is an example use of this functionality:</p>
<pre>
static bool ValidatePort(const char* flagname, int32 value) {
   if (value > 0 && value < 32768)   // value is ok
     return true;
   printf("Invalid value for --%s: %d\n", flagname, (int)value);
   return false;
}
DEFINE_int32(port, 0, "What port to listen on");
DEFINE_validator(port, &ValidatePort);
</pre>

<p>By doing the registration at global initializat