ostream::write not writing entire struct? - c++

Question

I'm trying to export various values, such as ints and simple structs, to a binary file. Here's some code:
#include &ltiostream&gt
#include &ltfstream&gt
#include &ltcstdint&gt
using namespace std;
template&ltclass T&gt void writeToStream(ostream& o, T& val)
{
o.write((char*)&val, sizeof(T));
cout &lt&lt o.tellp() &lt&lt 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.

Answer 2

I have cleaned up your code as follows. Though I do not know why the old code output two bytes, the new code does output four.
#include <iostream>
#include <fstream>
#include <cstdint>
using std::cout;
using std::endl;
using std::uint16_t;
using std::ostream;
using std::ofstream;
using std::ios;
template <class T> void writeToStream(ostream& o, T& val)
{
o.write(reinterpret_cast<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);
// Just let the stream "test" pass out of scope.
// It closes automatically.
//test.close();
return 0;
}
(My standard library lacks cstdint, so I used short rather than uint16_t, but I doubt that this matters.)
The std::ofstream type is derived from std::ostream. The writeToStream() function is happier, or at least more regular and more general, if passed a plain std::ostream. Also, for information: to issue using namespace std; is almost never recommended in C++.
Good luck.