I am currently getting into embedded programming with C at work and while there are some things I learned to appreciate about the simplicity of C, two of the things I really miss are generics and stricter type safety. I wondered if C++ can offer a safer way of reading types from arrays of bytes. This, of course, gives me an excuse to work out my C++ template metaprogramming muscles, which have atrophied a little.

API Goal

I want to provide an API that allows us to copy elements from a buffer using static types and compile time bounds checking. Let’s take a look at a concrete example: say I have a fixed size array of 8 bytes, and I want to read the first two bytes as an int16_t, the next four bytes as an int32_t, and the final two bytes as an uint16_t. How can I express this intuitively in C++17? Here’s what I came up with:

uint8_t buf[] = /* an array with compile-time known size*/
auto [i16, i32, u16] = copy_from_buffer<int16_t,uint32_t,uint16_t>(buf);

This feels reasonably intuitive to me. Since we know the size of the buffer at compile time and we know the size of the types we want to read, we can make sure at compile time that the buffer contains enough elements to read the desired types. If it does not, we produce a compile error. On the other hand, it’s fine to read less elements from the buffer than it could potentially produce. So let’s get going.

Pseudocode

Let’s use a little bit of (C++ inspired, but definitely not C++) pseudocode:

fn copy_from_buffer<...Ts, size_t N>(bytes[N]) -> tuple<Ts...> {
    size_t offset = 0;
    tuple<Ts...> copied_values{};
    foreach T in Ts... {
        copied_values [at T] = *(reinterpret_cast<T*>( (&bytes[0]) + offset));
        offset += sizeof(T);
    }
    return copied_values;
}

A good solution is to return a tuple of the desired types so that the structured binding capabilites come for free. That’s the easy part. To assign each element of the tuple, we iterate over the types and reinterpret cast the associated memory to a pointer of that type. Then, we dereference the pointer to copy it into our tuple. While doing so, we keep track of the sizes of the types and accumulate them in an offset variable that we use to find the beginning of the next type in memory. So much for the theory. The problem is that the pseudocode given above is imperative, but C++ template metaprogramming (TMP) isn’t ¹.

Implementation

We’ll start by collecting the puzzle pieces we need and plug them together at the end of this section.

Calculating the Offsets

If we can’t solve the problem imperatively, let’s do it functionally. We begin with the series of types <int16_t,uint32_t,uint16_t> and the first step is to apply sizeof to every type. This gives us <2,4,2>, which we need to transform into the offset sequence <0,2,6>. We can see that the latter is obtained by applying a partial sum to the size-sequence (with the first element being zero). So let’s see how to implement a partial sum first.

template <std::size_t Nelem, typename T, typename... Ts>
constexpr T partial_sum(T const t, Ts const... ts) {
  static_assert(Nelem < 1 + sizeof...(Ts), "Not enough elements to sum");
  static_assert((std::is_same_v<T, Ts> && ...),"All types must be the same");
  if constexpr (Nelem == 0) {
    return 0;
  } else {
    return t + partial_sum<Nelem - 1>(ts...);
  }
}

Here, we have a constexpr function that sums Nelem elements of a given list of numbers. So partial_sum<3>(1,1,1,1) is 3, while partial_sum<1>(1,1,1,1) is 1 and partial_sum<0>(1,1,1,1) is 0. This is the building block we need to transform the sequence of types into the sequence of offsets.

To express a sequence of types in C++ TMP we use std::tuple<Ts...>, but how do we express a sequence of integer numbers? Well, std::integer_sequence of course. It takes the type of the integer and a variadic list of compile time constants of that type. Thus, to pass the sequence <0,2,6> we create a type std::integer_sequence<size_t,0,2,6>. Now we know how to write a metafunction make_offset_sequence that transforms a sequence of types into a sequence of offsets.

// a helper function
template <typename... Ts, size_t... Is>
constexpr auto make_offset_sequence_impl(std::integer_sequence<std::size_t, Is...>) {
  return std::integer_sequence<size_t, (partial_sum<Is>(sizeof(Ts)...))...>();
}
// the actual metafunction with a slim interface
template <typename... Ts>
constexpr auto make_offset_sequence() {
  return make_offset_sequence_impl<Ts...>(std::index_sequence_for<Ts...>());
}

In the helper function we have made use of the fact that we can expand packs in a nested fashion, which is really neat.

Copying the Types

The previous work was most of the heavy lifting, but it’s not quite over. Next, we need a function that extracts an element at a given offset in bytes from a buffer. At this point we pass the buffer as a pointer to bytes and disregard its fixed size. We’ll get back to that later.

template <typename T, size_t OffsetBytes>
constexpr T elem_from_buffer_impl(std::uint8_t const *const buffer) {
  return *(reinterpret_cast<const T *>(buffer + OffsetBytes));
}

This function extracts one element of type T at offset OffsetBytes from a buffer. With this, we have the building block to create our tuple elements:

template <typename... Ts, size_t... Offsets>
constexpr std::tuple<Ts...> from_buffer_impl(uint8_t const *const buffer,
                                             std::index_sequence<Offsets...>) {
  std::tuple<Ts...> tup = {elem_from_buffer_impl<Ts, Offsets>(buffer)...};

  return tup;
}

template <typename... Ts, size_t N, typename BufType>
constexpr std::tuple<Ts...> copy_from_buffer(BufType const (&buffer)[N]) {
  static_assert(N > 0, "Cannot read from empty buffer");
  static_assert(sizeof...(Ts) > 0,
                "Must read at least one element from the buffer");
  static_assert((sizeof(Ts) + ...) <= N * sizeof(buffer[0]),
                "Not enough elements to read from in fixed size buffer");
  return detail::from_buffer_impl<Ts...>(
      reinterpret_cast<uint8_t const *>(&buffer[0]),
      detail::make_offset_sequence<Ts...>());
}

The latter function is the one we are interested in, but we need a helper function again to pass an index sequence that enumerates the types. The final copy_from_buffer function provides compile time safety with regards to the size of the buffer. It also works with other buffer types than bytes and provides the intuitive interface we were looking for.

Conclusion

So this is it. A safer way of reinterpreting elements from a buffer that provides compile time bounds checking and as much type safety as possible. We might ask if it was really necessary do complicated metaprogramming just to achieve this simple effect ². Couldn’t we have used a memory copy into structs? Maybe. As far as I know, a struct can have padding bytes inserted by the compiler and I would not bet on the memory layout being portable across compilers. There are ways to pack structures, but they are not standardized as far as I am aware ³. So, as always, metaprogramming is best ;)

Feel free to check out the code in this github repo.

Endnotes

c++ c++17 template-metaprogramming

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