I have a map with pointers to objects of a class, and I'm trying to output them all to a binary file and then read them back. I'm not convinced I have the syntax correct because when I am reading them back they are broken, i.e. don't seem to have any of the data they were given before they went into the file. The line before this I iterate through the data and output them all so I know they are ok before they are printed
This is the code that outputs the account into the binary file.
for (it = accounts.begin(); it != accounts.end(); it++)
{
outFile.write((char*)&(*it).second, sizeof(Account));
}
Anyone know if this is ok?
EDIT: And it wasn't
for (it = accounts.begin(); it != accounts.end(); it++)
{
outFile.write((char*)&(*(it->second)), sizeof(Account));
}
That's not how you serialize objects for several reasons:
if the object is polymorphic, most(all?) implementations will have a pointer to a virtual table inside the object, so you write that, but when you read it back, it is no longer valid.
other pointers to members are written, but don't make sense when you read them back.
Serialization is not as easy as writing the bytes of the object to file.
The easy solution is to use an existing library - google protocol buffers come to mind.
Another is to implement it yourself, but that's hard, especially if you want support on multiple platforms/operating systems.
It looks like you are trying to write memory addresses cast to char* to a file. These numbers will have no relevance whatsoever. Presumably you want to serialize the objects pointed at by those pointers. So first you need to find a means to serialize and de-serialize objects of that type, then write those to a file by re-referencing the pointers held in the map.
You will see adresses of memory and no more. You have to write much more complex method. See Google protobuf encoding for some thoughts.
Related
I would like to know what happens when I write:
object.write((char*)&class_object, sizeof(class_object));
// or
object.read((char*)&class_object, sizeof(class_object));
From what I read so far, the class_object is converted to a pointer. But I don't know how it manages to convert data carried by the object into binary. What does the binary actually represent?
I am a beginner.
EDIT
Could you please explain what really happens when we write the above piece of code? I mean, what actually happens when we write (char*)*S, say where S is the object of a class that I have declared?
Imagine it this way, the class instance is just some memory chunk resting in your RAM, if you convert your class to a char pointer:
SomeClass someClassInstance;
char* data = reinterpret_cast<char*>(&someClassInstance);
It will point to the same data in your memory but it will be treated as a byte array in your program.
If you convert it back:
SomeClass* instance = reinterpret_cast<SomeClass*>(data);
It will be treated as the class again.
So in order to write your class to a file and later reconstruct it, you can just write the data to some file which will be sizeof(SomeClass) in size and later read the file and convert the raw bytes to the class instance.
However, keep in mind that you can only do this if your class is POD (Plain Old Data)!
In practice, your code won't work and is likely to yield undefined behavior, at least when your class or struct is not a POD (plain old data) and contains pointers or virtual functions (so has some vtable).
The binary file would contain the bit representation of your object, and this is not portable to another computer, or even to another process running the same program (notably because of ASLR) unless your object is a POD.
See also this answer to a very similar question.
You probably want some serialization. Since disks and file accesses are a lot slower (many dozen of thousands slower) than the CPU, it is often wise to use some more portable data representation. Practically speaking, you should consider some textual representation like e.g. JSON, XML, YAML etc.... Libraries such as jsoncpp are really easy to use, and you'll need to code something to transform your object into some JSON, and to create some object from a JSON.
Remember also that data is often more costly and more precious than code. The point is that you often want some old data (written by a previous version of your program) to be read by a newer version of your program. And that might not be trivial (e.g. if you have added or changed the type of some field in your class).
You could also read about dynamic software updating. It is an interesting research subject. Be aware of databases.
Read also about parsing techniques, notably about recursive descent parsers. They are relevant.
I have a vector with pointers to a base class object so i can manage objects derived from that class.
vector <Product*> products;
i am trying to write these objects to a file while iterating through the vector
but i am not sure if this works correctly.
void Inventory :: saveProductsToFile()
{
ofstream outfile;
outfile.open("inventory.dat",ios::binary);
list <Product*> :: iterator it;
for(it=products.begin(); it!=products.end(); it++)
outfile.write((char*)*(it),sizeof(Product));
}
The file is created but i have no idea if i'm saving the actual objects themselves or their
addresses.Is this correct or is there another way?
This is how the file looks like:
ˆFG " H*c \Âõ(œ##pFG h*c b'v b#
You code can work. You cannot serialize polymorphic objects in
that way. For starters, you're writing the hidden vptr out
to disk; when you reread the data, it will not be valid. And
you're only writing out the data in the base class (Product),
because that's what sizeof(Product) evaluates to. And
finally, just writing a byte image of anything but a char[]
will probably mean that you won't be able to reread the data
some time in the future (after a compiler upgrade, or a machine
upgrade, or whatever).
What you have to do is to define a format (binary or text) for
the file, and write that. For the basic types, you can start
with something existing, like XDR or Protocol buffers, but
neither of these work that well with polymorphic types. For
polymorphic types, you have to start by defining how you
identify the type in question when rereading. This can be
tricky: there's nothing in std::type_info which helps, so you
need some means of establishing a relationship between your
(derived) types and the identifier. Then every derived class
must implement a write function, which first writes its type,
then writes its data out, one element by one. When reading, you
read the type, look up the appropriate read function for that
type in a map, and call that function, which then reads the data
one by one.
Finally, I might point out that all successful serialization
schemes I've seen depend on generated code. You describe your
types in a separate file, or in special markup (in a specially
marked comment in the C++), and have a program which reads that,
and generates the necessary code (and often the actual classes
you use).
Thats not how you "serialize" data. Like this the pointers are only valid during runtime or until you delete them (depending on what happens/stops first). Like this you wouldn't be able to restore your data, because after the program has stopped everything from your former memory becomes invalid. You would have to store the actual values from your class.
I am working on a C++ application that uses a library that is written in C (StormLib). The library has a function to read a file to a void* buffer(I am guessing a char[]). To which I would like to send to a different library to be processed. Hopefully it can be done with something like boost::iostreams::stream_buffer or boost::asio::streambuf to store the file as to be read by whatever method needs.
I have tried simply passing in a istream (that has a boost::asio::streambuf open) to the function and it gives me a BADACCESS as it tried to execute
memcpy((theFile),(myiStream),(full size of the file))
I would basically like a sort of "bag of bits" object that can be easily moved to different methods for conversion of the data in a structured fashion but I do not know how I should implement it.
What do you want to do with the data once you have it back in C++? If you just want the raw data, then you could (for example) just create a std::vector<char>, resize so it's big enough to hold all the data*, and then pass a pointer to its first element.
* How you determine "big enough" is a different question...
I have written an object to file but when I read the object from the file, it is not getting the correct value. If 1 object is considered as 1 record and if only 1 record is there in the file, it's getting the value. If I write more records (many objects of the same type) then it's not getting the value.
There could be a couple of problems. Of the top of my head it might be an issue with how you are interfacing with the file, as the comments above indicate. Or it might be a serialization issue.
I recently learned that one cannot simply write a class to a file because its not guaranteed to be contiguous memory (other reasons too, but the moral I took was don't do it).
Instead one needs to serialize the object/class before writing it to the file.
A couple of web resources that I found useful for this are:
http://functionx.com/cpp/articles/serialization.htm (I liked this one a lot).
http://functionx.com/cpp/articles/serialization.htm
Is it possible to serialize and deserialize a class in C++?
I have a list of objects that I would like to store in a file as small as possible for later retrieval. I have been carefully reading this tutorial, and am beginning (I think) to understand, but have several questions. Here is the snippet I am working with:
static bool writeHistory(string fileName)
{
fstream historyFile;
historyFile.open(fileName.c_str(), ios::binary);
if (historyFile.good())
{
list<Referral>::iterator i;
for(i = AllReferrals.begin();
i != AllReferrals.end();
i++)
{
historyFile.write((char*)&(*i),sizeof(Referral));
}
return true;
} else return false;
}
Now, this is adapted from the snippet
file.write((char*)&object,sizeof(className));
taken from the tutorial. Now what I believe it is doing is converting the object to a pointer, taking the value and size and writing that to the file. But if it is doing this, why bother doing the conversions at all? Why not take the value from the beginning? And why does it need the size? Furthermore, from my understanding then, why does
historyFile.write((char*)i,sizeof(Referral));
not compile? i is an iterator (and isn't an iterator a pointer?). or simply
historyFile.write(i,sizeof(Referral));
Why do i need to be messing around with addresses anyway? Aren't I storing the data in the file? If the addresses/values are persisting on their own, why can't i just store the addresses deliminated in plain text and than take their values later?
And should I still be using the .txt extension? < edit> what should I use instead then? I tried .dtb and was not able to create the file. < /edit> I actually can't even seem to get file to open without errors with the ios::binary flag. I'm also having trouble passing the filename (as a string class string, converted back by c_str(), it compiles but gives an error).
Sorry for so many little questions, but it all basically sums up to how to efficiently store objects in a file?
What you are trying to do is called serialization. Boost has a very good library for doing this.
What you are trying to do can work, in some cases, with some very important conditions. It will only work for POD types. It is only guaranteed to work for code compiled with the same version of the compiler, and with the same arguments.
(char*)&(*i)
says to take the iterator i, dereference it to get your object, take the address of it and treat it as an array of characters. This is the start of what is being written to the file. sizeof(Referral) is the number of bytes that will be written out.
An no, an iterator is not necessarily a pointer, although pointers meet all the requirements for an iterator.
Question #1 why does ... not compile?
Answer: Because i is not a Referral* -- it's a list::iterator ;; an iterator is an abstraction over a pointer, but it's not a pointer.
Question #2 should I still be using the .txt extension?
Answer: probably not. .txt is associated by many systems to the MIME type text/plain.
Unasked Question: does this work?
Answer: if a Referral has any pointers on it, NO. When you try to read the Referrals from the file, the pointers will be pointing to the location on memory where something used to live, but there is no guarantee that there is anything valid there anymore, least of all the thing that the pointers were pointing to originally. Be careful.
isn't an iterator a pointer?
An iterator is something that acts like a pointer from the outside. In most (perhaps all) cases, it is actually some form of object instead of a bare pointer. An iterator might contain a pointer as an internal member variable that it uses to perform its job, but it just as well might contain something else or additional variables if necessary.
Furthermore, even if an iterator has a simple pointer inside of it, it might not point directly at the object you're interested in. It might point to some kind of bookkeeping component used by the container class which it can then use to get the actual object of interest. Fortunately, we don't need to care what those internal details actually are.
So with that in mind, here's what's going on in (char*)&(*i).
*i returns a reference to the object stored in the list.
& takes the address of that object, thus yielding a pointer to the object.
(char*) casts that object pointer into a char pointer.
That snippet of code would be the short form of doing something like this:
Referral& r = *i;
Referral* pr = &r;
char* pc = (char*)pr;
Why do i need to be messing around
with addresses anyway?
And why does it need the size?
fstream::write is designed to write a series of bytes to a file. It doesn't know anything about what those bytes mean. You give it an address so that it can write the bytes that exist starting wherever that address points to. You give it a size so that it knows how many bytes to write.
So if I do:
MyClass ExampleObject;
file.write((char*)ExampleObject, sizeof(ExampleObject));
Then it writes all the bytes that exist directly within ExampleObject to the file.
Note: As others have mentioned, if the object you want to write has members that dynamically allocate memory or otherwise make use of pointers, then the pointed to memory will not be written by a single simple fstream::write call.
will serialization give a significant boost in storage efficiency?
In theory, binary data can often be both smaller than plain-text and faster to read and write. In practice, unless you're dealing with very large amounts of data, you'll probably never notice the difference. Hard drives are large and processors are fast these days.
And efficiency isn't the only thing to consider:
Binary data is harder to examine, debug, and modify if necessary. At least without additional tools, but even then plain-text is still usually easier.
If your data files are going to persist between different versions of your program, then what happens if you need to change the layout of your objects? It can be irritating to write code so that a version 2 program can read objects in a version 1 file. Furthermore, unless you take action ahead of time (like by writing a version number in to the file) then a version 1 program reading a version 2 file is likely to have serious problems.
Will you ever need to validate the data? For instance, against corruption or against malicious changes. In a binary scheme like this, you'd need to write extra code. Whereas when using plain-text the conversion routines can often help fill the roll of validation.
Of course, a good serialization library can help out with some of these issues. And so could a good plain-text format library (for instance, a library for XML). If you're still learning, then I'd suggest trying out both ways to get a feel for how they work and what might do best for your purposes.
What you are trying to do (reading and writing raw memory to/from file) will invoke undefined behaviour, will break for anything that isn't a plain-old-data type, and the files that are generated will be platform dependent, compiler dependent and probably even dependent on compiler settings.
C++ doesn't have any built-in way of serializing complex data. However, there are libraries that you might find useful. For example:
http://www.boost.org/doc/libs/1_40_0/libs/serialization/doc/index.html
Did you have already a look at boost::serialization, it is robust, has a good documentation, supports versioning and if you want to switch to an XML format instead of a binary one, it'll be easier.
Fstream.write simply writes raw data to a file. The first parameter is a pointer to the starting address of the data. The second parameter is the length (in bytes) of the object, so write knows how many bytes to write.
file.write((char*)&object,sizeof(className));
^
This line is converting the address of object to a char pointer.
historyFile.write((char*)i,sizeof(Referral));
^
This line is trying to convert an object (i) into a char pointer (not valid)
historyFile.write(i,sizeof(Referral));
^
This line is passing write an object, when it expects a char pointer.