Is it possible to serialize and deserialize a class in C++?
I've been using Java for 3 years now, and serialization / deserialization is fairly trivial in that language. Does C++ have similar features? Are there native libraries that handle serialization?
An example would be helpful.
The Boost::serialization library handles this rather elegantly. I've used it in several projects. There's an example program, showing how to use it, here.
The only native way to do it is to use streams. That's essentially all the Boost::serialization library does, it extends the stream method by setting up a framework to write objects to a text-like format and read them from the same format.
For built-in types, or your own types with operator<< and operator>> properly defined, that's fairly simple; see the C++ FAQ for more information.
Boost is a good suggestion. But if you would like to roll your own, it's not so hard.
Basically you just need a way to build up a graph of objects and then output them to some structured storage format (JSON, XML, YAML, whatever). Building up the graph is as simple as utilizing a marking recursive decent object algorithm and then outputting all the marked objects.
I wrote an article describing a rudimentary (but still powerful) serialization system. You may find it interesting: Using SQLite as an On-disk File Format, Part 2.
I recommend Google protocol buffers. I had the chance to test drive the library on a new project and it's remarkably easy to use. The library is heavily optimized for performance.
Protobuf is different than other serialization solutions mentioned here in the sense that it does not serialize your objects, but rather generates code for objects that are serialization according to your specification.
As far as "built-in" libraries go, the << and >> have been reserved specifically for serialization.
You should override << to output your object to some serialization context (usually an iostream) and >> to read data back from that context. Each object is responsible for outputting its aggregated child objects.
This method works fine so long as your object graph contains no cycles.
If it does, then you will have to use a library to deal with those cycles.
I'm using the following template to implement serialization:
template <class T, class Mode = void> struct Serializer
{
template <class OutputCharIterator>
static void serializeImpl(const T &object, OutputCharIterator &&it)
{
object.template serializeThis<Mode>(it);
}
template <class InputCharIterator>
static T deserializeImpl(InputCharIterator &&it, InputCharIterator &&end)
{
return T::template deserializeFrom<Mode>(it, end);
}
};
template <class Mode = void, class T, class OutputCharIterator>
void serialize(const T &object, OutputCharIterator &&it)
{
Serializer<T, Mode>::serializeImpl(object, it);
}
template <class T, class Mode = void, class InputCharIterator>
T deserialize(InputCharIterator &&it, InputCharIterator &&end)
{
return Serializer<T, Mode>::deserializeImpl(it, end);
}
template <class Mode = void, class T, class InputCharIterator>
void deserialize(T &result, InputCharIterator &&it, InputCharIterator &&end)
{
result = Serializer<T, Mode>::deserializeImpl(it, end);
}
Here T is the type you want to serialize Mode is a dummy type to differentiate between different kinds of serialization, eg. the same integer can be serialized as little endian, big endian, varint, etc.
By default the Serializer delegates the task to the object being serialized. For built in types you should make a template specialization of the Serializer.
Convenience function templates are also provided.
For example little endian serialization of unsigned integers:
struct LittleEndianMode
{
};
template <class T>
struct Serializer<
T, std::enable_if_t<std::is_unsigned<T>::value, LittleEndianMode>>
{
template <class InputCharIterator>
static T deserializeImpl(InputCharIterator &&it, InputCharIterator &&end)
{
T res = 0;
for (size_t i = 0; i < sizeof(T); i++)
{
if (it == end) break;
res |= static_cast<T>(*it) << (CHAR_BIT * i);
it++;
}
return res;
}
template <class OutputCharIterator>
static void serializeImpl(T number, OutputCharIterator &&it)
{
for (size_t i = 0; i < sizeof(T); i++)
{
*it = (number >> (CHAR_BIT * i)) & 0xFF;
it++;
}
}
};
Then to serialize:
std::vector<char> serialized;
uint32_t val = 42;
serialize<LittleEndianMode>(val, std::back_inserter(serialized));
To deserialize:
uint32_t val;
deserialize(val, serialized.begin(), serialized.end());
Due to the abstract iterator logic, it should work with any iterator (eg. stream iterators), pointer, etc.
Here is a simple serializer library I knocked up. It's header only, c11 and has examples for serializing basic types. Here's one for a map to class.
https://github.com/goblinhack/simple-c-plus-plus-serializer
#include "c_plus_plus_serializer.h"
class Custom {
public:
int a;
std::string b;
std::vector c;
friend std::ostream& operator<<(std::ostream &out,
Bits my)
{
out << bits(my.t.a) << bits(my.t.b) << bits(my.t.c);
return (out);
}
friend std::istream& operator>>(std::istream &in,
Bits my)
{
in >> bits(my.t.a) >> bits(my.t.b) >> bits(my.t.c);
return (in);
}
friend std::ostream& operator<<(std::ostream &out,
class Custom &my)
{
out << "a:" << my.a << " b:" << my.b;
out << " c:[" << my.c.size() << " elems]:";
for (auto v : my.c) {
out << v << " ";
}
out << std::endl;
return (out);
}
};
static void save_map_key_string_value_custom (const std::string filename)
{
std::cout << "save to " << filename << std::endl;
std::ofstream out(filename, std::ios::binary );
std::map< std::string, class Custom > m;
auto c1 = Custom();
c1.a = 1;
c1.b = "hello";
std::initializer_list L1 = {"vec-elem1", "vec-elem2"};
std::vector l1(L1);
c1.c = l1;
auto c2 = Custom();
c2.a = 2;
c2.b = "there";
std::initializer_list L2 = {"vec-elem3", "vec-elem4"};
std::vector l2(L2);
c2.c = l2;
m.insert(std::make_pair(std::string("key1"), c1));
m.insert(std::make_pair(std::string("key2"), c2));
out << bits(m);
}
static void load_map_key_string_value_custom (const std::string filename)
{
std::cout << "read from " << filename << std::endl;
std::ifstream in(filename);
std::map< std::string, class Custom > m;
in >> bits(m);
std::cout << std::endl;
std::cout << "m = " << m.size() << " list-elems { " << std::endl;
for (auto i : m) {
std::cout << " [" << i.first << "] = " << i.second;
}
std::cout << "}" << std::endl;
}
void map_custom_class_example (void)
{
std::cout << "map key string, value class" << std::endl;
std::cout << "============================" << std::endl;
save_map_key_string_value_custom(std::string("map_of_custom_class.bin"));
load_map_key_string_value_custom(std::string("map_of_custom_class.bin"));
std::cout << std::endl;
}
Output:
map key string, value class
============================
save to map_of_custom_class.bin
read from map_of_custom_class.bin
m = 2 list-elems {
[key1] = a:1 b:hello c:[2 elems]:vec-elem1 vec-elem2
[key2] = a:2 b:there c:[2 elems]:vec-elem3 vec-elem4
}