ifstream.read() of a file

Question

So I made an output file that does this:

   ofstream outf;
   outf.open(filename);
   string str = "This is test!";
   int lengthofstr = str.length();
   outf.write((char*)&str, sizeof(lengthofstr));
   outf.close();

After that has been completely, with another program (exe) I read in the file:

    void readem(char* filename, ifstream &inf){

    inf.open(filename);

        string a;

        if (inf.is_open()){
            inf.read((char*)&a, sizeof(int));
            cout << a << endl;

        }
        else
            cout << "file is not opening!" << endl;

        inf.close();
    }

The c++ exe breaks as soon as i try to run it. Im trying to return the string. If someone could point me to a tutorial/post/question/ or explain to me how the ifstream read works, thank you ahead of time XD

Answer 1

You can't write a std::string like this at all! The content of the file may contain the characters you expect it to include (because std::string is likely to use the small string optimization) but for bigger string it won't: the std::string object you are writing is just the "control record" of the actual string. To write a std::string you'd probably write two pieces of information:

1. The length of the std::string to be written.
2. The characters of the std::string.

The operations would look something like this:

write(stream, s.size());
write(stream, s.c_str(), s.size());

(the functions write() suitably implement serialization of an integer value and a sequence of characters; just writing bytes is, in my opinion, not the way to go)

Merely writing the characters making up a std::string object isn't any useful.

When reading bytes into the memory occupied by an std::string object you get undefined behavior: the control record part of the string gets overwritten with some content which is unlikely to yield a valid object. Assuming the object is written as above, you could read it (assuming suitable deserializing functions read()) using code like this:

std::string s;
s.resize(read<std::string::size_type>(stream));
read(stream, s.data(), s.size());