If you create them before, they will be part of the design phase.

If you create them after, you can just use them for documentation. But documenting is better and faster if you do that during design and coding.

By the way, you don't lose time in doing design! The coding part will be faster. And better.

After all, you think that doing design of a skyscraper or of a bridge is a lose of time instead of just starting building it?

A compromise is to starting creating some dummy code during the design phase (just prototypes of classes/functions) and use this code skeleton to build the class diagrams.

If you create them before, they will be part of the design phase.

If you create them after, you can just use them for documentation. But documenting is better and faster if you do that during design and coding.

By the way, you don't lose time in doing design! The coding part will be faster. And better.

After all, you think that doing design of a skyscraper or of a bridge is a loss of time instead of just starting building it?

A compromise is to starting creating some dummy code during the design phase (just prototypes of classes/functions) and use this code skeleton to build the class diagrams.