Document that a derived archive must call init() itself#357
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Since Boost 1.73, the CRTP base archive classes no longer call `init()` from their constructors: doing so downcast this to the most derived class while it was still being constructed, which is undefined behavior and tripped the sanitizers. The derivation guide still showed the old pattern, in which a class derived from one of the `xxx_oarchive_impl` templates relied on the base to write the archive header. Following it now yields an archive that never writes its header. The guide now states that the most derived class is responsible for calling `init()` in its own constructor body, honoring `no_header`, and shows how, noting that the log_archive example is empty only because it suppresses the header. This also corrects the curiously recurring template argument in the further-derivation sketch, which named `xml_oarchive` instead of `log_archive`. Refs issue #182.
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Since Boost 1.73, the CRTP base archive classes no longer call
init()from their constructors: doing so downcast this to the most derived class while it was still being constructed, which is undefined behavior and tripped the sanitizers. The derivation guide still showed the old pattern, in which a class derived from one of thexxx_oarchive_impltemplates relied on the base to write the archive header. Following it now yields an archive that never writes its header.The guide now states that the most derived class is responsible for calling
init()in its own constructor body, honoringno_header, and shows how, noting that the log_archive example is empty only because it suppresses the header.This also corrects the curiously recurring template argument in the further-derivation sketch, which named
xml_oarchiveinstead oflog_archive.Refs issue #182.