Storing arbitrary bytes in RocksDB

Question

RocksDB states that it can store arbitrary data but the API only supports std::string types. I want to store std::vector<T> values and it appears if I want to do this then I must convert it into a std::string.

Is there a less brittle way to storing arbitrary types?

Answer 1

I tend to use the following to pack/unpack structures/classes to std::string using a template to automatically adjust to their size.

template <typename T>
std::string Pack(const T* data)
{
    std::string d(sizeof(T), L'\0');
    memcpy(&d[0], data, d.size());
    return d;
}

template <typename T>
std::unique_ptr<T> Unpack(const std::string& data)
{
    if (data.size() != sizeof(T))
        return nullptr;

    auto d = std::make_unique<T>();
    memcpy(d.get(), data.data(), data.size());
    return d;
}

So the following client code can pack and unpack a structure into the database:

    // Test structure
    BOB b = {};
    b.a = 12;
    b.b = 144;
    b.c[0] = 's';
    b.c[1] = '\0';

    // Write to the db
    status = pDb->Put(rocksdb::WriteOptions(), key, Pack(&b));

    // Read from db with same key
    std::string result;
    status = pDb->Get(rocksdb::ReadOptions(), key, &result);
    std::unique_ptr<BOB> pBob = Unpack<BOB>(result);

    if (b.a == pBob->a && b.b == pBob->b && b.c[0] == pBob->c[0])
    {
        printf("Structure matches!\n");
    }
    else
    {
        printf("Structure doesn't match!\n");
    }