Future perfect: Something completed before a future point

The future perfect uses will have + past participle to show that an action or change will be complete before a stated or understood point in the future.

Examples

By Friday, we will have finished the report.

The report is expected to be complete before Friday ends.
She will have left by the time you arrive.

Her departure will occur before the later future arrival.
By next year, the company will have opened three new offices.

The completed result is counted at a future reference point.

Future perfect vs. future continuous

Future continuous

At noon, I will be writing the final section.

The activity will be in progress at noon.

Future perfect

By noon, I will have written the final section.

The section will be complete before noon.

At noon often places the viewpoint inside an ongoing activity. By noon sets a deadline before which the result will be complete.

The deadline can be understood rather than stated

A sentence does not always need an explicit by phrase. The surrounding context can supply the future point from which completion is viewed.

You can use the room after lunch; the cleaners will have finished.

The understood completion point is before the room becomes available.
Call again at six. I will have spoken to the manager.

The conversation is expected to be complete before the later call.