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Allow variant deserialization to make types with private constructors #339

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4 changes: 4 additions & 0 deletions include/boost/serialization/access.hpp
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Original file line number Diff line number Diff line change
Expand Up @@ -129,6 +129,10 @@ class access {
// class doesn't have a class-specific placement new defined.
::new(t)T;
}
template<class T>
static T *construct_r(void * t) {
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@KJTsanaktsidis KJTsanaktsidis Feb 6, 2026

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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();

return ::new(t)T;
}
template<class T, class U>
static T & cast_reference(U & u){
return static_cast<T &>(u);
Expand Down
20 changes: 14 additions & 6 deletions include/boost/serialization/std_variant.hpp
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Expand Up @@ -122,11 +122,19 @@ struct variant_impl
// with an implementation that de-serialized to the address of the
// aligned storage included in the variant.
using type = mp::front<Seq>;
type value;
ar >> BOOST_SERIALIZATION_NVP(value);
v = std::move(value);
struct ValueData {
alignas(type) unsigned char data[sizeof(type)];
type *ptr_;
void * mem() { return static_cast<void *>(data); }
type * ptr() { return ptr_; }
ValueData() : ptr_(access::construct_r<type>(mem())) {}
~ValueData() { ptr()->~type(); }
} value;

ar >> BOOST_SERIALIZATION_NVP(*value.ptr());
v = std::move(*value.ptr());
type * new_address = & std::get<type>(v);
ar.reset_object_address(new_address, & value);
ar.reset_object_address(new_address, value.ptr());
return;
}
//typedef typename mpl::pop_front<S>::type type;
Expand All @@ -149,7 +157,7 @@ struct variant_impl<0, Seq>

template<class Archive, class... Types>
void load(
Archive & ar,
Archive & ar,
std::variant<Types...>& v,
const unsigned int version
){
Expand Down Expand Up @@ -188,7 +196,7 @@ void serialize(Archive &, std::monostate &, const unsigned int /*version*/)

namespace boost {
namespace serialization {

template<class... Types>
struct tracking_level<
std::variant<Types...>
Expand Down
16 changes: 12 additions & 4 deletions include/boost/serialization/variant.hpp
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Expand Up @@ -159,11 +159,19 @@ struct variant_impl {
// with an implementation that de-serialized to the address of the
// aligned storage included in the variant.
typedef typename mpl::front<S>::type head_type;
head_type value;
ar >> BOOST_SERIALIZATION_NVP(value);
v = std::move(value);;
struct ValueData {
alignas(head_type) unsigned char data[sizeof(head_type)];
head_type *ptr_;
void * mem() { return static_cast<void *>(data); }
head_type * ptr() { return ptr_; }
ValueData() : ptr_(access::construct_r<head_type>(mem())) {}
~ValueData() { ptr()->~head_type(); }
} value;

ar >> BOOST_SERIALIZATION_NVP(*value.ptr());
v = std::move(*value.ptr());
head_type * new_address = & get<head_type>(v);
ar.reset_object_address(new_address, & value);
ar.reset_object_address(new_address, value.ptr());
return;
}
typedef typename mpl::pop_front<S>::type type;
Expand Down
20 changes: 14 additions & 6 deletions include/boost/serialization/variant2.hpp
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Expand Up @@ -92,11 +92,19 @@ struct variant_impl
// with an implementation that de-serialized to the address of the
// aligned storage included in the variant.
using type = mp11::mp_front<Seq>;
type value;
ar >> BOOST_SERIALIZATION_NVP(value);
v = std::move(value);
struct ValueData {
alignas(type) unsigned char data[sizeof(type)];
type *ptr_;
void * mem() { return static_cast<void *>(data); }
type * ptr() { return ptr_; }
ValueData() : ptr_(access::construct_r<type>(mem())) {}
~ValueData() { ptr()->~type(); }
} value;

ar >> BOOST_SERIALIZATION_NVP(*value.ptr());
v = std::move(*value.ptr());
type * new_address = & variant2::get<type>(v);
ar.reset_object_address(new_address, & value);
ar.reset_object_address(new_address, value.ptr());
return;
}
//typedef typename mpl::pop_front<S>::type type;
Expand All @@ -119,7 +127,7 @@ struct variant_impl<Seq>

template<class Archive, class... Types>
void load(
Archive & ar,
Archive & ar,
variant2::variant<Types...> & v,
const unsigned int version
){
Expand Down Expand Up @@ -158,7 +166,7 @@ void serialize(Archive &ar, variant2::monostate &, const unsigned int /*version*

namespace boost {
namespace serialization {

template<class... Types>
struct tracking_level<
variant2::variant<Types...>
Expand Down
40 changes: 34 additions & 6 deletions test/test_variant.cpp
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Expand Up @@ -57,6 +57,32 @@ namespace boost {
#include "A.hpp"
#include "A.ipp"

class V {
private:
friend class boost::serialization::access;
int m_i;
V() :
m_i(0)
{}
template<class Archive>
void serialize(Archive& ar, unsigned /*version*/)
{
ar & BOOST_SERIALIZATION_NVP(m_i);
}
public:
V(int i) :
m_i(i)
{}
int get_i() const
{
return m_i;
}
bool operator==(const V& other) const
{
return m_i == other.m_i;
}
};

class are_equal
: public boost::static_visitor<bool>
{
Expand Down Expand Up @@ -156,6 +182,8 @@ void test(Variant & v)
test_type(v);
v = std::string("we can't stop here, this is Bat Country");
test_type(v);
v = V(67);
test_type(v);
}

#include <boost/serialization/variant.hpp>
Expand All @@ -166,10 +194,10 @@ void test(Variant & v)

int test_boost_variant(){
std::cerr << "Testing boost_variant\n";
boost::variant<bool, int, float, double, A, std::string> v;
boost::variant<bool, int, float, double, A, std::string, V> v;
test(v);
const A a;
boost::variant<bool, int, float, double, const A *, std::string> v1 = & a;
boost::variant<bool, int, float, double, const A *, std::string, V> v1 = & a;
test_type(v1);
return EXIT_SUCCESS;
}
Expand All @@ -180,10 +208,10 @@ int test_boost_variant(){

int test_boost_variant2(){
std::cerr << "Testing boost_variant2\n";
boost::variant2::variant<bool, int, float, double, A, std::string> v;
boost::variant2::variant<bool, int, float, double, A, std::string, V> v;
test(v);
const A a;
boost::variant2::variant<bool, int, float, double, const A *, std::string> v1 = & a;
boost::variant2::variant<bool, int, float, double, const A *, std::string, V> v1 = & a;
test_type(v1);
return EXIT_SUCCESS;
}
Expand All @@ -194,10 +222,10 @@ int test_boost_variant2(){
#include <variant>
int test_std_variant(){
std::cerr << "Testing Std Variant\n";
std::variant<bool, int, float, double, A, std::string> v;
std::variant<bool, int, float, double, A, std::string, V> v;
test(v);
const A a;
std::variant<bool, int, float, double, const A *, std::string> v1 = & a;
std::variant<bool, int, float, double, const A *, std::string, V> v1 = & a;
test_type(v1);
return EXIT_SUCCESS;
}
Expand Down