What is the difference between aggregation and composition?

This question has bothered me for some time and I always forget. So I wrote it down here and added an answer.

Composition : An object contains another object. When the container object dies, so does the composited objects.

Aggregation : An object pseudo-contains another object (contains a pointer to it). When the container object dies, the containees do not.

Sometimes aggregation is called composition when the difference doesn’t matter.
Note: In UML, aggregation is an unfilled diamond, whereas composition is a filled diamond.


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