Your thermos does not have a switch which you must set to "hot" or "cold" before inserting a liquid, handles solids as well as liquids, and doesn't require you to even think where the theshold might lie between the "hot" and "cold" settings. Instead it just does its best to prevent an energy exchange in either direction without having to even know the variables involved. And the "same subroutine" is basically used for different sized thermoses.
In code I see people not understand this all the time. Here's an example: let's say you present the user with their previous orders, and give them the option to filter those orders by some criterion.
The shitty way to do this is to have two variables:
Filter* orderFilter = NULL;
bool filterSpecified = false;
void setOrderFilter (filter* newFilter) {
orderFilter = newFilter;
filterSpecified = true;
}
So instead of just checking if a filter has been assigned, you have a separate boolean. What happens if the boolean is true but the filter is null? What would the vice versa case even mean?
The global codebase is riddled with dumb errors like this.