Okay so I have fstreams that I swap between output and input. Is there anyway I can tell which openmode the fstream is in?

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Last Post by deceptikon

Do you want the open mode that was passed to the constructor or open member function? Or do you want to know dynamically whether the last operation was a read or a write?

Ultimately, you'd need to maintain a flag providing this information, which could be stored in the fstream object using xalloc/iword/pword, if you so choose. Though a separate variable would be simpler, in my opinion. Just wrap it up in another class and call it good. For example to remember the open mode:

#include <fstream>
#include <ios>
#include <iostream>
#include <string>

using namespace std;

class File {
    ios::open_mode _mode;
    fstream _stream;
    File(const string& filename, ios::openmode mode)
        : _mode(mode), _stream(filename.c_str(), mode)

    ios::openmode mode() const
        return _mode;

    fstream& get() {
        return _stream;

    operator fstream&()
        return get(); 

int main()
    File fs("test.txt", ios::in);
    string s;

    while (getline(fs.get(), s)) {
        cout << s << '\n';

    cout << "Opened in read mode: " << (fs.mode() & ios::in ? "YES" : "NO") << '\n';
    cout << "Opened in write mode: " << (fs.mode() & ios::out ? "YES" : "NO") << '\n';

Remembering the last operation is a little more tedious because it needs to be updated every time you perform a read or a write, but it's no more difficult.

Edited by deceptikon


I want to know dynamically whether the last operation was a read or a write. I would rather not make a class for this.


The solution is the same: you need to save that information somewhere on every read or write. If you don't want to use a separate variable or wrap everything up in a class, then look up the xalloc/iword/pword member functions provided by ios_base.

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