Ytukyg

I have a class like this:

struct SomeClass{



    SomeClass(){}



    SomeClass(const SomeClass& rhs):

    _m1(rhs._m1),

    _m2(rhs._m2),

    _m3(rhs._m3),

    _m4(rhs._m4){}



    SomeClass& operator=(const SomeClass& rhs){

    // same as above

    return *this;

    }



    int _m1;

    std::vector<int> _m2;

    std::vector<int> _m3;

    int _m4;

};

At some point in my program, I want to save the data stored in a SomeClass object for later use:

    SomeClass someObj = arr->getBest(); // arr is a pointer to AnotherClass,

                                        // in which different SomeClass

                                        // objects are initialized and then

                                        // involved in various 

                                        // computations in AnotherClass, 

                                        // finally the best one SomeClass 

                                        // object will be save here

    fwrite(&someObj, sizeof(SomeClass), 1, saveFile);

After the file being saved to saveFile, I try to read it and get an error:

    SomeClass readingSomeObj;

    fread(&readingSomeObj, sizeof(SomeClass), 1, savedFile));

Compiler complaints "Access violation reading location 0x...". By watching the locals in VS, I see that the other two fields of readingSomeObj, namely _m1 and _m4 have values in them but not _m2 and _m3, in VS it says "unable to read memory".

It further relates the problem to the SomeClass, specifically this line _m2(rhs._m2),.

But when I was saving the someObj in the line fwrite(&someObj, sizeof(SomeClass), 1, saveFile); I did see the two members _m2 and _m3 of someObj have values (by hovering my mouse over &someObj).

I can't figure out what was going wrong. My guess is problem with constructors of SomeClass, but still as a newbie to C++, I don't know how to correct it. In other parts of my program, I used the empty constructor of SomeClass to initialize a member of AnotherClass. But everything runs fine except for this saving/reading part. I am looking for your generous help and thanks a million!

You can't serialize vectors like this. They have memory allocated, so do something like:

• Store your integers to the file


• Store the size of the first vector


• Store the content of your first vector


• Do the same for the second vector

Then when you read your file:

• Read your two integers


• Read your first vector size, allocate the vector


• Read the values for your first vector


• Do the same for the second vector.

You can also use a serialization library like Boost.Serialization that takes care of pieces of this in a more reliable way.

You've made a ton of assumptions about your data type that do not hold. The most egregious is that all its bytes are stored in a "row" in memory, all next to each other, which is the only way a block-wise copy-and-read could ever work.

This may hold for PODs (modulo alignment and endianness considerations) but it won't work for complex datatypes like vector, that point to "real" data actually stored elsewhere (via dynamic allocation).

Your deserialised vectors consist pretty much solely of pointer values that are long-since invalid.

Your serialisation routine will have to be more intelligent than this. Ultimately there's no "one" way to do it. You can define your own format or you can use some off-the-shelf serialisation solution that already has a format that supports standard containers.

You could Google "C++ class serialisation" for some ideas.