C++ iterator in for loop pitfalls?

Question

I see somewhere it mentions:

for ( itr = files.begin(); itr < files.end(); ++itr )  // WRONG
for ( itr = files.begin(); itr != files.end(); ++itr ) // ok

Why is the first expression wrong? I always used the first expression, and didn't have any problems.

Answer 1

Ordering comparisons such as <, >, <=, >= will work for random-access iterators, but many other iterators (such as bidirectional iterators on linked lists) only support equality testing (== and !=). By using != you can later replace the container without needing to change as much code, and this is especially important for template code which needs to work with many different container types.