fstream pointer is not pointing

Question

I am trying to make a pointer to fstream so I can use it in all of my class methods :

class P
{
    private:
        fstream *fs;

    public:
        P()
        {
            fstream fs(filepath, openmode);
            this->fs = &fs;
        }
};

But it seem to be not pointing to it at all e.g if I write:

fs->is_open()

it will return false, whereas if I write:

fs.is_open()

it will return true.

What is causing this? I also tried to change the pointer to another like fstr but that didn't work either.

fsteam fs(..)? Please don't re-write your code by hand. Please copy-and-paste what you have actually written and handed to a compiler. There's no point in us debugging something that doesn't actually exist... — sarnold
– sarnold, Commented Jan 22, 2012 at 5:16
Things might improve if your typing was more careful. fsteam and fstream are not the same thing. The code doesn't compile: filepath and openmode are not visibly declared. That makes it hard for us to guess what you're really doing. — Jonathan Leffler
– Jonathan Leffler, Commented Jan 22, 2012 at 5:18

Remy Lebeau · Accepted Answer · 2012-01-22 06:01:40Z

4

To accomplish what you are asking for, you need to use the new operator, like this:

SmartIO::SmartIO(const char * filepath , Mode mode)
    : fs(NULL), buffer(NULL), offset(0)
{
    switch (mode)
    {
        case Mode::inText:
        {
            fs = new fstream(filepath,fstream::in|fstream::ate);
            break;
        }
        case Mode::inBinary:
        {
            fs = new fstream(filepath,fstream::in|fstream::binary|fstream::ate);
            break;
        }
    }

    if ((fs) && (fs->is_open()))
    {
        buffer = new std:vector<char>(fs->tellg(), 0);
        fs->seekg(0, ios::beg);
        fs->read(buffer->data(), buffer->size());
    }
    else
    {
        printf( "cant open the file!");
    }
}

SmartIO::~SmartIO()
{
    delete fs;
    delete buffer;
}

edited Jan 22, 2012 at 6:01

answered Jan 22, 2012 at 5:26

Remy Lebeau

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1 Comment

Loki Astari Over a year ago

FAIL: Why not use an object over a pointer. If you really must use a pointer (and their are reason to do so) you must also explain the rule of three otherwise the obvious beginner will just break the code in other ways. But in most situations an object is preferable.

Alok Save · Accepted Answer · 2012-01-22 05:17:46Z

4

You are pointing your member pointer fs to an locally created fstream object, which does not exist once its local scope ends, what you are left with is a dangling pointer.

Your this->fs now points to something that does not exist.

Whenever you use this dangling pointer to operate on the stream, it results in Undefined Behavior.

answered Jan 22, 2012 at 5:17

Alok Save

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Comments

Kerrek SB · Accepted Answer · 2012-01-22 15:06:45Z

2

You need to create the actual stream object dynamically. Something like this might be an approach:

class Foo
{
    std::istream * p;
    bool must_clean;
public:
    Foo() : p(nullptr), must_clean(false)
    {
        if (...)      { p = new std::ifstream("somefile.txt"); must_clean = true; }
        else if (...) { p = new std::istringstream(global_buf); must_clean = true; }
        else          { p = &std::cin; }
    }
    ~Foo() { if (must_clean) delete p; }
    Foo(Foo const&) = delete;
    Foo & operator=(Foo const&) = delete);
};

You can decide whether you want an istream, ostream or iostream pointer.

edited Jan 22, 2012 at 15:06

answered Jan 22, 2012 at 5:32

Kerrek SB

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2 Comments

Alok Save Over a year ago

+1, To counter the unexplained, unneeded Downvote.This is the way how it should be done.

Loki Astari Over a year ago

Come on. You know better. "Rule of three". PS. no down vote yet.

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