zpp::bits

A modern C++20 binary serialization library, with just one header file.

This library is a successor to zpp::serializer. The library tries to be simpler for use, but has more or less similar API to its predecessor.

Motivation

Provide a single, simple header file, that would enable one to:

Enable save & load any STL container / string / utility into and from a binary form, in a zero overhead approach.
Enable save & load any object, by adding only a few lines to any class, without breaking existing code.
Enable save & load the dynamic type of any object, by a simple one-liner.

The Difference From zpp::serializer

It is simpler
Performance improvements
Almost everything is constexpr
More flexible with serialization of the size of variable length types, opt out from serializing size.
Opt-in for zpp::throwing if header is found.
More friendly towards freestanding (no exception runtime support).
Breaks compatibility with anything lower than C++20 (which is why the original library is left intact).
Better naming for utility classes and namespaces, for instance zpp::bits is more easily typed than zpp::serializer.
For now, dropped support for polymorphic serialization, seeking a more modern way to do it.

For most types, enabling serialization is just one line. Here is an example of a person class with name and age:

struct person
{
    // Add this line to your class with the number of members:
    using serialize = zpp::bits::members<2>; // Two members

    std::string name;
    int age{};
};

Most of the time types we serialize can work with structured binding, and this library takes advantage of that, but you need to provide the number of members in your class for this to work using the method above.

If your data members or default constructor are private, you need to become friend with zpp::bits::access like so:

struct private_person
{
    // Add this line to your class.
    friend zpp::bits::access;
    using serialize = zpp::bits::members<2>;

private:
    std::string name;
    int age{};
};

To enable save & load of any object, even ones without structured binding, add the following lines to your class

    constexpr static auto serialize(auto & archive, auto & self)
    {
        return archive(self.object_1, self.object_2, ...);
    }

Note that object_1, object_2, ... are the non-static data members of your class.

Here is the example of a person class again with explicit serialization function:

struct person
{
    constexpr static auto serialize(auto & archive, auto & self)
    {
        return archive(self.name, self.age);
    }

    std::string name;
    int age{};
};

Example how to serialize the person into and from a vector of bytes:

// The `data_in_out` utility function creates a vector of bytes, the input and output archives
// and returns them so we can decompose them easily in one line using structured binding like so:
auto [data, in, out] = zpp::bits::data_in_out();

// Serialize a few people:
out(person{"Person1", 25}, person{"Person2", 35});

// Define our people.
person p1, p2;

// We can now deserialize them either one by one `in(p1)` `in(p2)`, or together, here
// we chose to do it together in one line:
in(p1, p2);

This example almost works, we are being warned that we are discarding the return value. we need to check for errors, the library offers multiple ways to do so - a return value based, exception based, or zpp::throwing based.

The return value based way for being most explicit, or if you just prefer return values:

auto [data, in, out] = zpp::bits::data_in_out();

auto result = out(person{"Person1", 25}, person{"Person2", 35});
if (failure(result)) {
    // `result` is implicitly convertible to `std::errc`.
    // handle the error or return/throw exception.
}

person p1, p2;

result = in(p1, p2);
if (failure(result)) {
    // `result` is implicitly convertible to `std::errc`.
    // handle the error or return/throw exception.
}

The exceptions based way using .or_throw() (read this as "succeed or throw" - hence or_throw()):

int main()
{
    try {
        auto [data, in, out] = zpp::bits::data_in_out();

        // Check error using `or_throw()` which throws an exception.
        out(person{"Person1", 25}, person{"Person2", 35}).or_throw();

        person p1, p2;

        // Check error using `or_throw()` which throws an exception.
        in(p1, p2).or_throw();

        return 0;
    } catch (const std::exception & error) {
        std::cout << "Failed with error: " << error.what() << '\n';
        return 1;
    } catch (...) {
        std::cout << "Unknown error\n";
        return 1;
    });
}

Another option is zpp::throwing it turns into two simple co_awaits, to understand how to check for error we provide a full main function:

int main()
{
    return zpp::try_catch([]() -> zpp::throwing<int> {
        auto [data, in, out] = zpp::bits::data_in_out();

        // Check error using `co_await`, which suspends the coroutine.
        co_await out(person{"Person1", 25}, person{"Person2", 35});

        person p1, p2;

        // Check error using `co_await`, which suspends the coroutine.
        co_await in(p1, p2);

        co_return 0;
    }, [](zpp::error error) {
        std::cout << "Failed with error: " << error.message() << '\n';
        return 1;
    }, [](/* catch all */) {
        std::cout << "Unknown error\n";
        return 1;
    });
}

Constructing input and output archives together and separately from data:

// Create both a vector of bytes, input and output archives.
auto [data, in, out] = zpp::bits::data_in_out();

// Create just the input and output archives, and bind them to the
// existing vector of bytes.
std::vector<std::byte> data;
auto [in, out] = zpp::bits::in_out(data);

// Create all of them separately
std::vector<std::byte> data;
zpp::bits::in in(data);
zpp::bits::out out(data);

// When you need just data and in/out
auto [data, in] = zpp::bits::data_in();
auto [data, out] = zpp::bits::data_out();

Archives can be constructed from either one of the byte types:

// Either one of these work with the below.
std::vector<std::byte> data;
std::vector<char> data;
std::vector<unsigned char> data;
std::string data;

// Automatically works with either `std::byte`, `char`, `unsigned char`.
zpp::bits::in in(data);
zpp::bits::out out(data);

You can also use fixed size data objects such as std::array and view types such as std::span similar to the above. You just need to make sure there is enough size since they are non resizable.

When using a vector, it automatically grows to the right size, however, you can also output and input from a span, in which case your memory size is limited by the memory span:

zpp::bits::in in(std::span{pointer, size});
zpp::bits::out out(std::span{pointer, size});

Query the position of in and out using position(), in other words the bytes read and written respectively:

std::size_t bytes_read = in.position();
std::size_t bytes_written = out.position();

Reset the position backwards or forwards, or to the beginning:

in.reset(); // reset to beginning.
in.reset(position); // reset to position.

out.reset(); // reset to beginning.
out.reset(position); // reset to position.

Serializing STL containers and strings, first stores a 4 byte size, then the elements:

std::vector v = {1,2,3,4};
out(v);
in(v);

The reason why the default size type is of 4 bytes (i.e std::uint32_t) is that most programs almost never reach a case of a container being more than ~4 billion items, and it may be unjust to pay the price of 8 bytes size by default.

For specific size types that are not 4 bytes, use zpp::serializer::size_is<SizeType>():

std::vector<int> v = {1,2,3,4};
out(zpp::bits::sized<std::uint16_t>(v));
in(zpp::bits::sized<std::uint16_t>(v));

Make sure that the size type is large enough for the serialized object, otherwise less items will be serialized, according to conversion rules of unsigned types.

You can also choose to not serialize the size at all, like so:

std::vector<int> v = {1,2,3,4};
out(zpp::bits::unsized(v));
in(zpp::bits::unsized(v));

Serialization using argument dependent lookup is also possible, using both the automatic member serialization way or with fully defined serialization functions.

With automatic member serialization:

namespace my_namespace
{
struct adl
{
    int x;
    int y;
};

constexpr auto serialize(const adl & adl) -> zpp::bits::members<2>;
} // namespace my_namespace

With fully defined serialization functions:

namespace my_namespace
{
struct adl
{
    int x;
    int y;
};

constexpr auto serialize(auto & archive, adl & adl)
{
    return archive(adl.x, adl.y);
}

constexpr auto serialize(auto & archive, const adl & adl)
{
    return archive(adl.x, adl.y);
}
} // namespace my_namespace

If you know your type is serializaeble just as raw bytes, you can opt in and optimize its serialization to a mere memcpy:

struct point
{
    int x;
    int y;

    constexpr static auto serialize(auto & archive, auto & self)
    {
        // Serialize as bytes, instead of serializing each
        // member separately. The overall result is the same, but this may be
        // faster sometimes.
        return archive(zpp::bits::as_bytes(self));
    }
};

It's also possible to do this directly from a vector or span of trivially copyable types, this time we use bytes instead of as_bytes because we convert the contents of the vector to bytes rather than the vector object itself (the data the vector points to rather than the vector object):

std::vector<point> points;
out(zpp::bits::bytes(points));
in(zpp::bits::bytes(points));

However in this case the size is not serialized, this may be extended in the future to also support serializing the size similar to other view types. If you need to serialize as bytes and want the size, as a workaround it's possible to cast to std::span<std::byte>.

This should cover most of the basic stuff, more documentation may come in the future.

Final Words

I wish that you find this library useful. Please feel free to submit any issues, make suggestions for improvements, etc.

felixguendling / zpp_bits

zpp::bits

Motivation

The Difference From zpp::serializer

Contents

Final Words

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