In the docs on the README, the Reflective programming section suggests in the second example that rfl::to_view() can be used to iterate over the fields at compile-time, with a relevant example using view.apply(...)...
However, I am unable to use it in a constexpr method forced to execute at compile-time, here is a contrived demo that reproduces the issue. It fails to compile because it complains that rfl::to_view() cannot be used in a constant expression:
#include <rfl.hpp>
struct Pod {
std::uint16_t foo; // 2
std::uint32_t bar; // 4
float baz; // 4
};
template <typename Struct>
constexpr std::size_t get_packed_size() {
std::size_t size = 0;
Struct unused{};
const auto view = rfl::to_view(unused);
view.apply([&](const auto& field) {
size += sizeof(decltype(*field.value()));
});
return size;
}
int main() {
static_assert(
get_packed_size<Pod>() ==
sizeof(std::uint16_t) + sizeof(std::uint32_t) + sizeof(float)
);
}
As a counter-example, if one instead defines the function as non-constexpr, and uses assert() instead of static_assert(), then the program both compiles and runs successfully without any assertion failures:
#include <cassert>
#include <rfl.hpp>
struct Pod {
std::uint16_t foo; // 2
std::uint32_t bar; // 4
float baz; // 4
};
template <typename Struct>
std::size_t get_packed_size() {
std::size_t size = 0;
Struct unused{};
const auto view = rfl::to_view(unused);
view.apply([&](const auto& field) {
size += sizeof(decltype(*field.value()));
});
return size;
}
int main() {
assert(
get_packed_size<Pod>() ==
sizeof(std::uint16_t) + sizeof(std::uint32_t) + sizeof(float)
);
}By the way, it would be nice if reflect-cpp provided a nicer way than this for me to iterate over all the fields of an object and get their types. The default-constructed variable unused is only provided here because I couldn't work out how else to ask reflect-cpp to iterate the fields of a struct for me without providing it with an object of that struct's type first. For my purposes, I only care about their types, not their values, in this context...
I suspect the issue I am experiencing here might be highly related to that mentioned in #220, which if I'm not mistaken, also seems to rest on the inability for rfl::to_view() to be used in a constant expression. I peeked at your function declarations and I noted that rfl::to_view() and other functions it calls are not declared constexpr, that might be required since I don't think constexpr can be deduced in general?
In the docs on the README, the Reflective programming section suggests in the second example that
rfl::to_view()can be used to iterate over the fields at compile-time, with a relevant example usingview.apply(...)...However, I am unable to use it in a
constexprmethod forced to execute at compile-time, here is a contrived demo that reproduces the issue. It fails to compile because it complains thatrfl::to_view()cannot be used in a constant expression:As a counter-example, if one instead defines the function as non-constexpr, and uses
assert()instead ofstatic_assert(), then the program both compiles and runs successfully without any assertion failures:By the way, it would be nice if reflect-cpp provided a nicer way than this for me to iterate over all the fields of an object and get their types. The default-constructed variable
unusedis only provided here because I couldn't work out how else to ask reflect-cpp to iterate the fields of a struct for me without providing it with an object of that struct's type first. For my purposes, I only care about their types, not their values, in this context...I suspect the issue I am experiencing here might be highly related to that mentioned in #220, which if I'm not mistaken, also seems to rest on the inability for
rfl::to_view()to be used in a constant expression. I peeked at your function declarations and I noted thatrfl::to_view()and other functions it calls are not declaredconstexpr, that might be required since I don't thinkconstexprcan be deduced in general?