Writing the binary file using wfstream

Question

I want to write a struct data in a binary file using wfstram class.

Why The output file is empty?

The following is my code.

#include "stdafx.h"
#include <string>
#include <fstream>

using namespace std;

struct bin_data
{
    wstring ch;
    size_t id;
};

int main()
{

    wfstream   f(L"test_bin_file.txt", ios::out | ios::binary);
    bin_data *d = new bin_data;
    d->id = 100;
    d->ch = L"data100";
    f.write((wchar_t*)&d, sizeof(struct bin_data));
    f.close();

    return 0;
}

If you expect you could save string content into a file, you're wrong. You'll get only pointer/counter saved — Severin Pappadeux
– Severin Pappadeux, Commented Jul 25, 2016 at 20:18
Write id first, it is of fixed length, f.write((char*)&d->id, sizeof(size_t)); Then write string length, again fixed record and last content of the string using d->ch.data() — Severin Pappadeux
– Severin Pappadeux, Commented Jul 25, 2016 at 20:38
The purpose of existence for std::wfstream is to translate between Unicode code points, represented by wchar_t on the program side, and UTF-8, GB18030, or other kinds of narrow multibyte encoding, represented by chars, on the filesystem side. Giving it an ios::binary does not change that — Cubbi
– Cubbi, Commented Nov 9, 2017 at 20:18

Severin Pappadeux · Accepted Answer · 2016-07-25 21:05:23Z

2

I don't like much working with wide streams when dealing with binary data - binary are ultimately bytes, so you don't have to worry about much about char vs wchar. Code below writes 30 bytes on x64 - 8 for id, 8 for length and 14 (7*2) for string itself

int main() {
ofstream  f(L"QQQ.txt", ios::out | ios::binary);

bin_data *d = new bin_data;
d->id = 100;
d->ch = L"data100";

// ID first
f.write((char*)&d->id, sizeof(d->id));
// then string length
auto l = d->ch.length();
f.write((char*)&l, sizeof(l));
// string data
f.write((char*)d->ch.data(), l*sizeof(wchar_t));

f.close();

return 0;
}

answered Jul 25, 2016 at 21:05

Severin Pappadeux

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