Serialize optional<T> for T's that have no default constructor#352
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gennaroprota merged 1 commit intoJul 22, 2026
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The value of an `optional<T>` is now handled like a single element of a standard library container: when `T` is default constructible, it is serialized in place, exactly as before; otherwise, construction is routed through `save_construct_data`/`load_construct_data` so that `T` can be reconstructed on load. This lets `optional<T>` be serialized for types that cannot or should not provide a default constructor (const-qualified members, third party classes, and so on). The default-constructible path is left byte for byte unchanged, so archives written by earlier versions keep the same layout. The construct-data path writes an extra `item_version` plus whatever `save_construct_data` emits, which is new wire content only for types that previously could not be serialized at all, so no existing archive is affected. Loading reconstructs the value with `detail::stack_construct`, which placement-constructs the object through `load_construct_data` before the value is read. This is unlike the pre-2017 implementation, which serialized into unconstructed storage (`detail::stack_allocate`), so the reliability concern that motivated the earlier restriction does not apply here. Closes issue #121.
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The value of an
optional<T>is now handled like a single element of a standard library container: whenTis default constructible, it is serialized in place, exactly as before; otherwise, construction is routed throughsave_construct_data/load_construct_dataso thatTcan be reconstructed on load. This letsoptional<T>be serialized for types that cannot or should not provide a default constructor (const-qualified members, third party classes, and so on).The default-constructible path is left byte for byte unchanged, so archives written by earlier versions keep the same layout. The construct-data path writes an extra
item_versionplus whateversave_construct_dataemits, which is new wire content only for types that previously could not be serialized at all, so no existing archive is affected.Loading reconstructs the value with
detail::stack_construct, which placement-constructs the object throughload_construct_databefore the value is read. This is unlike the pre-2017 implementation, which serialized into unconstructed storage (detail::stack_allocate), so the reliability concern that motivated the earlier restriction does not apply here.Closes issue #121.