Binding int from variant to reference in Foo.

Hello guys, my friend had a problem where he couldn't bind an int to int reference in Foo struct held in variant:

struct Foo {
int &abc;
};

std::variant<Foo, std::string> v;

// You can't!
v = Foo {
.abc = ...
}

So I developed a way to do that, it's pretty simple:

#include <cstdint>
#include <functional>
#include <iostream>
#include <string>
#include <variant>
#include <meta>
#include <ranges>

struct __attribute__((packed)) Foo {
int& abc;

~Foo() {
static_assert(sizeof(void*) == 8, "Use modern CPU.");

static_assert([]() consteval {
auto members = std::meta::nonstatic_data_members_of(
^^Foo,
std::meta::access_context::current()
);

for (auto member : members) {
auto type_info = std::meta::type_of(member);
std::size_t layout_size = std::meta::is_reference_type(type_info)
? sizeof(void*)
: std::meta::size_of(type_info);

if (layout_size != 8) {
return false;
}
}
return true;
}(), "All non-static data members of Foo must occupy 8 bytes in layout!");

if (reinterpret_cast<uintptr_t>(&abc) & 0x1) {
delete reinterpret_cast<int*>(reinterpret_cast<uintptr_t>(&abc) &
~uintptr_t(0x1));
}
}
};

int main(void) {
std::variant<int, Foo> v;
v.emplace<0>(420);
v.emplace<1>(([&]() -> int& {
int* x = new int;
*x = std::get<0>(v);
x = reinterpret_cast<int*>(reinterpret_cast<uintptr_t>(x) | 0x1);
return *x;
})());
std::cout << *reinterpret_cast<int*>(
reinterpret_cast<uintptr_t>(&std::get<1>(v).abc) &
~uintptr_t(0x1))
<< std::endl;
return 0;
}

[https://godbolt.org/z/91rcbEdG5](https://godbolt.org/z/91rcbEdG5)

It's also memory safe.

Cheers.

https://redd.it/1v58tj3
@r_cpp