SIGABRT in binary read/write - c++

Question

I wrote a very small code snippet and have already gotten the following error:
malloc: *** error for object 0x100100080: pointer being freed was not allocated
Problem is, I have no idea what pointer the compiler's talking about. I pass a variable in by address to the read/write functions, but I never freed it as far as I know. Where's the error in my code? I ran it with Leaks and Zombies, but got nothing.
Here's my program:
#include <iostream>
#include <fstream>
#include <string>
#include <vector>
#include <algorithm>
using namespace std;
class Bank
{
private:
string __name;
public:
Bank()
{
__name = "";
}
Bank(string name)
{
__name = name;
}
string getName() const { return __name; }
};
int main (int argc, char * const argv[])
{
Bank bank("Bank of America");
Bank bank2;
cout << "Bank1: " << bank.getName() << endl;
string filename = bank.getName() + ".bank";
ofstream fout(filename.c_str(), ios::binary);
if (fout.good())
fout.write((char *)&bank, sizeof(bank));
fout.close();
ifstream fin(filename.c_str(), ios::binary);
if (fin.good())
fin.read((char *)&bank2, sizeof(bank2));
fin.close();
cout << "Bank2: " << bank2.getName() << endl;
return 0;
}

Answer 1

You can't read an object that contains a std::string (or anything that's not Plain Ol' Data) with fin.read()--
The object is read and written as a stream of bytes, but std:string contains a pointer to memory that is stored elsewhere and is not written with your fout.write() and is not initialized properly with your fin.read()
It is because it is not initialized properly with your fin.read() that you are getting the heap error; when the object goes out of scope, the destructor of the improperly initialized std::string is being called, and trying to free memory that it doesn't own.
You probably want to write a custom i/o method for your object and save or load it piece-by-piece. For a shortcut to doing this, use the Boost serialization library.

Answer 2

Because your Bank class contains a std::string, you can't read/write it as binary like you are thinking. A std::string has internal pointers. If you write it as binary, you are just going to be writing pointers and not the actual string contents. Likewise, when you read the string, you are going to be reading a pointer. In this case, you end up making both your bank and bank2 objects have strings which point to the same memory, so when that memory is freed it gets freed twice.
You'll need to have some other way of writing your bank data to a file. In this case, a simple ASCII file with the bank name would be fine.

Answer 3

You cannot do what you are doing, simply because std::string cannot be copied like that. Internally a string object allocates memory and a simple copy of the outer structure doesn't do what you expect.
You need to serialize this structure properly.

Answer 4

Don't use underscores, please
Pass objects by reference: Bank(string& name), please
This is evil: fout.write((char *)&bank, sizeof(bank));
You may want to write << and >> ostream operators of your Bank class.
For example:
friend std::ostream& operator<<(std::ostream &out, const Bank& b);
friend std::istream& operator>>(std::istream &out, const Bank& b);

Answer 5

Members functions write of ostream and read of istream are specifically designed to input and output binary data. If you do want to manipulate binary data, use the following:
ifstream fin(filename.c_str(), ios::in|ios::binary|ios::ate);
size = fin.tellg();
memblock = new char [size];
fin.seekg(0, ios::beg);
if (fin.good()){
fin.read(memblock, size);
fin.close();
}
delete[] memblock;