I'm using leveldb to store key-value pairs of integer and MyClass objects. Actually, a key can contain more then one of theses objects. The problem I have appears when retrieving the data from the database. It compiles, however the values of the MyClass members are not the one I put into the database.
std::string value;
leveldb::Slice keySlice = ANYKEY;
levelDBObj->Get(leveldb::ReadOptions(), keySlice, &value);
The std::string
value1
can now contain only one MyClass object or more. So how do I get them?
I already tried the following which didn't work;
1.) directly typecasting and memcpy
std::vector<MyClass> vObjects;
MyClass* obj = (MyClass*)malloc( value.size());
memcpy((void*)obj, (void*) (value.c_str()), value.size());
MyClass dummyObj;
int numValues = value.size()/sizeof(MyClass);
for( int i=0; i<numValues; ++i) {
dummyObj = *(obj+i);
vObjects.push_back(dummyObj);
}
2.) reinterpret_cast to void pointer
MyClass* obj = (MyClass*)malloc( value.size());
const void* vobj = reinterpret_cast<const void*>( value.c_str() );
int numValues = value.size()/sizeof(MyClass);
for( int i=0; i<numValues; ++i) {
const MyClass dummyObj = *(reinterpret_cast<const MyClass*>(vobj)+i);
vObjects.push_back(dummyObj);
}
MyClass is a collection of several public members, e.g. unsigned int and unsigned char and it has a stable size.
I know that there are similar problems with only one object. But in my case the vector can contain more then one and it comes from the leveldb database.
EDIT: SOLUTION
I wrote (de)serialization method for MyClass which then made it working. Thanks for the hint!
void MyClass::serialize( char* outBuff ) {
memcpy(outBuff, (const void*) &aVar, sizeof(aVar));
unsigned int c = sizeof(aVar);
memcpy(outBuff+c, (const void*) &bVar, sizeof(bVar));
c += sizeof(bVAr);
/* and so on */
}
void MyClass::deserialize( const char* inBuff ) {
memcpy((void*) &aVar, inBuff, sizeof(aVar));
unsigned int c = sizeof(aVar);
memcpy((void*) &aVar, inBuff+c, sizeof(aVar));
c += sizeof(aVar);
/* and so on */
}
The get
method is as follows (put
analogously):
int getValues(leveldb::Slice keySlice, std::vector<MyObj>& values) const {
std::string value;
leveldb::Status status = levelDBObj->Get(leveldb::ReadOptions(), keySlice, &value);
if (!status.ok()) {
values.clear();
return -1;
}
int nValues = value1.size()/sizeof(CHit);
MyObj dummyObj;
for( int i=0; i<nValues; ++i) {
dummyObj.deserialize(value.c_str()+i*sizeof(MyObj));
values.push_back(dummyObj);
}
return 0;
}
You have to serialize your class... otherwise, you're just taking some memory and writing it in leveldb. Whatever you get back is not only going to be different, but it will probably be completely useless too. Check out this question for more info on serialization: How do you serialize an object in C++?
LevelDB does support multiple objects under one key, however, try to avoid doing that unless you have a really good reason. I would recommend that you hash each object with a unique hash (see Google's CityHash if you want a hashing function) and store the serialized objects with their corresponding hash. If your objects is a collection in itself, then you have to serialize all of your objects to an array of bytes and have some method that allows you to determine where each object begins/ends.
Update
A serializable class would look something like this: