通过取消引用迭代器将Vector写入文件

Question

I am writing the contents of vector to a file through iterator as fallows

for(vector<Student>::iterator itr=vec.begin();itr!=vec.end();itr++){
    out.write((char*)itr,sizeof *itr);
}

It's giving compile time error as no suitable conversion function exists.But when I dereferenced itr and taken its address as below

out.write( (char*)&(*itr),sizeof *itr);

its working fine. So why here I can't use itr directly? Why I need to dereference it and take its address again?

Answer 1

You can't just byte-by-byte serialise complex objects like this, but your attempt doesn't even try to do that!

Your first example attempts to take an iterator , cast it to a char* (which is completely nonsensical) then write sizeof(Student) worth of bytes starting at that nonsensical pointer into a file.

Your second example bypasses the helpful compiler errors telling you about your mistake, and forces the cast. Sure, you get a result, but it's a total nonsense result and your program could even crash.

Firstly, you should be attempting to serialise the Student objects, not the iterators that point to them, and secondly, unless they are very simple objects and you know what you're doing, you're still going to need much more intelligent serialisation logic than just writing their component bytes into a file. Learn about alignment, padding, endianness, indirection....