Question

I’m trying to export various values, such as ints and simple structs, to a binary file. Here’s some code:

#include <iostream>
#include <fstream>
#include <cstdint>
using namespace std;

template<class T> void writeToStream(ostream& o, T& val)
{
    o.write((char*)&val, sizeof(T));
    cout << o.tellp() << endl;  //always outputs 4
}

struct foo {
    uint16_t a, b;
};

int main()
{
    foo myFoo = {42, 42};
    ofstream test("test.txt", ios::binary);
    writeToStream(test, myFoo);
    test.close();
}

The program should generate an output file 4 bytes long. But when I open it, it’s only 2 bytes long. If I change myFoo.a and myFoo.b to contain values of 256 or more (requires more than 1 byte to store), then the file becomes 4 bytes long. I’m using the Visual Studio 11 Developer Preview on Win7; I haven’t checked to see if the same happens on other systems or compilers. How can I make it output correctly for values of a or b under 256?

Answer 1

A file can only be read back by a program that understands the format in which it was stored. Notepad++ has no understanding of the format in which your file was stored, so it has no ability to read it back and render it sensibly. Either write the file in a format Notepad++ understands, such as ASCII text, or only read the file with a program that understand the format you wrote it in.