Suppose we want to read a parameter form a config file which represents some integral value, let's call it "MaxValue". It is possible that this parameter is not specified; such situation is no error. It is valid to not specify the parameter and in that case the program is supposed to behave slightly different. Also suppose that any possible value of type int is a valid value for "MaxValue", so we cannot jut use -1 to represent the absence of the parameter in the config file.