Two routes for implicit type conversion

Given C++ classes A and B, what are the two mechanisms that allow an A to be converted to a B implicitly?

A converting constructor in B or a conversion operator in A. Either B has a non-explicit constructor callable with one A argument, such as B(const A&), or A defines operator B() const. Mark them explicit when silent conversions would be surprising, and avoid defining both, which makes conversions ambiguous.