Composition, moisture-capturing cured product, and electronic devices

This composition comprises both a compound (A) represented by the following general formula (1): (R 1 )nM [wherein R 1 s may be the same or different, at least one of the R 1 moieties being a group having one or more unsaturated bonds; n is 2 or 3, and equals the valence of M; and M is one element selected from among aluminum, boron, magnesium and calcium] and a radical generator (C).