Can immer::vector support incomplete types with sizes known at compile time?

[aimoonchen]

test code:

#include <immer/box.hpp>
#include <immer/map.hpp>
#include <immer/vector.hpp>

#include <cstdio>
#include <string>

template <typename T>
struct size_traits {
    static constexpr std::size_t value = sizeof(T);
};

template <std::size_t Size>
constexpr auto derive_bl() {
    constexpr std::size_t space = 264;
    constexpr std::size_t full_elems = space / Size;
    if constexpr (full_elems >= 256) return 8;
    else if constexpr (full_elems >= 128) return 7;
    else if constexpr (full_elems >= 64) return 6;
    else if constexpr (full_elems >= 32) return 5;
    else if constexpr (full_elems >= 16) return 4;
    else if constexpr (full_elems >= 8) return 3;
    else if constexpr (full_elems >= 4) return 2;
    else if constexpr (full_elems >= 2) return 1;
    else return 0;
}

struct Value;

template <>
struct size_traits<Value> {
    static constexpr std::size_t value = 16;
};

using ValueVector =
    immer::vector<Value, immer::default_memory_policy, immer::default_bits, derive_bl<size_traits<Value>::value>()>;
using ValueVectorPtr = immer::box<ValueVector>;

struct Value {
    int data;
    ValueVectorPtr vec;
};

static_assert(sizeof(Value) == size_traits<Value>::value);

int main() {
    printf("sizeof(Value) = %zu\n", sizeof(Value));

    Value root{42, {}};
    auto vec = ValueVector{}.push_back(Value{1, {}}).push_back(Value{2, {}});
    root.vec = ValueVectorPtr{vec};

    printf("root.vec.get().size() = %zu\n", root.vec.get().size());
}

[arximboldi]

Not officially, but potentially yes. What are you trying to achieve?

As you have deducted, deriving BL (which only needs the size) is the reason really why the type is required to be complete.

[aimoonchen]

I want to define the Value type like this:

struct Value;

using ValueVector = immer::vector<Value, immer::default_memory_policy>;
using ValueVectorPtr = immer::box<ValueVector>;

struct Value {
    int data;
    ValueVectorPtr vec;
};