Allow variant deserialization to make types with private constructors #339
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If a type has a private no-args constructor, but has friended boost::serialization::access, it can usually be deserialised as normal. However, it _can't_ be deserialized out of a variant, because the variant tries to invoke the default constructor. Fix this by constructing it through boost::serialization::access, which is designed to model "classes that can't be created with no arguments except through deserialization"
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KJTsanaktsidis
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Feb 6, 2026
| ::new(t)T; | ||
| } | ||
| template<class T> | ||
| static T *construct_r(void * t) { |
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Hm, had to add a new overload of construct that actually returned the placement-new result, because of pointer provenance issues.
alignas(T) unsigned char mem[sizeof(T)];
T *ptr = reinterpret_cast<T*>(mem);
::new(ptr)T;
ptr->stuff();
Apparently is no bueno because the compiler can assume that ptr is NOT the same thing as the in-place-constructed T. Need to instead do something like
alignas(T) unsigned char mem[sizeof(T)];
T *ptr = ::new(mem)T;
ptr->stuff();
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If a type has a private no-args constructor, but has friended boost::serialization::access, it can usually be deserialised as normal. However, it can't be deserialized out of a variant, because the variant tries to invoke the default constructor.
Fix this by constructing it through boost::serialization::access, which is designed to model "classes that can't be created with no arguments except through deserialization"