fstream issue with ! operator

[canilao]

Hi All,

So I ran into an issue with using the ! operator with fstream:

// Make sure the file is ready to be read before we move on.
1: fstream fileStream;
2: fileStream.open("c:\\some_file.txt");

// Try to open until success.
3: while(!fileStream) fileStream.open("c:\\some_file.txt");

// Close the file since we know its complete written to disk.
4: fileStream.close();

The idea is that I do not want to move on until the file is readable.
What I was seeing was if the call to open() on line 2 failed and the
call to open on line 3 succeeded, the ! operator on line 3 still
always returned false!? On the other hand, if the call to open() on
line 2 succeeded, the ! operator on line 3 return true...

So it seems if open fails the first time, the state checked by the !
operator never changes?

The way I got around it was either using c-style fopen or re-writing
line 3 like this:

3: while(!fileStream.is_open()) fileStream.open("c:\
\some_file.txt");

Is this how fstream works?

Thanks,
Chris Anilao

 

[progmanos]

Hi All,

So I ran into an issue with using the ! operator with fstream:

// Make sure the file is ready to be read before we move on.
1:     fstream fileStream;
2:     fileStream.open("c:\\some_file.txt");

// Try to open until success.
3:     while(!fileStream) fileStream.open("c:\\some_file.txt");

// Close the file since we know its complete written to disk.
4:     fileStream.close();

The idea is that I do not want to move on until the file is readable.
What I was seeing was if the call to open() on line 2 failed and the
call to open on line 3 succeeded, the ! operator on line 3 still
always returned false!?  On the other hand, if the call to open() on
line 2 succeeded, the ! operator on line 3 return true...

So it seems if open fails the first time, the state checked by the !
operator never changes?

The way I got around it was either using c-style fopen or re-writing
line 3 like this:

3:     while(!fileStream.is_open()) fileStream.open("c:\
\some_file.txt");

Is this how fstream works?

Thanks,
Chris Anilao

I think so. I'm not sure what applying the not operator to the
fstream object itself would do. However, you are correct in modifying
the code to use the is_open object since it returns a boolean value
depending on whether the object "is associated with a file"(
http://www.cplusplus.com/reference/iostream/fstream/is_open.html ).

 

[Martin York]

I'm not sure what applying the not operator to the
fstream object itself would do. However, you are correct in modifying
the code to use the is_open object since it returns a boolean value

Well actually no is_open() is not correct.
If used in a boolean context an fstream object will convert itself to
bool by testing its internal flags.


So if '!fileStream' is returning false then you will not be able to
read from the file even if it opened correctly. This is because some
other flag is indicating an error state (and this flag will be checked
when other methods are invoked on the object).

You should reset the error flags before attepting to re-open the file.

See: http://www.cplusplus.com/reference/iostream/ios_base/iostate.html
For what else can go wrong.

#include <fstream>
#include <iostream>

int main()
{
std::fstream data;
data.open("./plop");

while(!data)
{
std::cout << "Failed" << std::endl;
// Pause here
std::cout << "Retrying" << std::endl;

// Put the state of the object back into its original state.
data.clear();

// Retry opening the file.
data.open("./plop");
if (data.is_open())
{
std::cout << "Open" << std::endl;
if (!data)
{
std::cout << "Reading from this file will fail." <<
std::endl;
}
}
}

std::cout << "Finished" << std::endl;
}

 

[Eric Pruneau]

Hi All,

So I ran into an issue with using the ! operator with fstream:

// Make sure the file is ready to be read before we move on.
1: fstream fileStream;
2: fileStream.open("c:\\some_file.txt");

// Try to open until success.
3: while(!fileStream) fileStream.open("c:\\some_file.txt");

// Close the file since we know its complete written to disk.
4: fileStream.close();

The idea is that I do not want to move on until the file is readable.
What I was seeing was if the call to open() on line 2 failed and the
call to open on line 3 succeeded, the ! operator on line 3 still
always returned false!? On the other hand, if the call to open() on
line 2 succeeded, the ! operator on line 3 return true...

So it seems if open fails the first time, the state checked by the !
operator never changes?

The way I got around it was either using c-style fopen or re-writing
line 3 like this:

3: while(!fileStream.is_open()) fileStream.open("c:\
\some_file.txt");

Is this how fstream works?

Thanks,
Chris Anilao

Consider your trying to open a read-only file.

while(!fileStream.is_open())
fileStream.open("c:\\some_file.txt");

is an infinite loop...

 

[canilao]

Well actually no is_open() is not correct.
If used in a boolean context an fstream object will convert itself to
bool by testing its internal flags.

So if '!fileStream' is returning false then you will not be able to
read from the file even if it opened correctly. This is because some
other flag is indicating an error state (and this flag will be checked
when other methods are invoked on the object).

You should reset the error flags before attepting to re-open the file.

See:http://www.cplusplus.com/reference/iostream/ios_base/iostate.html
For what else can go wrong.

#include <fstream>
#include <iostream>

int main()
{
std::fstream data;
data.open("./plop");

while(!data)
{
std::cout << "Failed" << std::endl;
// Pause here
std::cout << "Retrying" << std::endl;

// Put the state of the object back into its original state.
data.clear();

// Retry opening the file.
data.open("./plop");
if (data.is_open())
{
std::cout << "Open" << std::endl;
if (!data)
{
std::cout << "Reading from this file will fail." <<
std::endl;
}
}
}

std::cout << "Finished" << std::endl;

}

Thanks!!! This helps out alot!

- Chris

 

[progmanos]

Well actually no is_open() is not correct.
If used in a boolean context an fstream object will convert itself to
bool by testing its internal flags.

So if '!fileStream' is returning false then you will not be able to
read from the file even if it opened correctly. This is because some
other flag is indicating an error state (and this flag will be checked
when other methods are invoked on the object).

You should reset the error flags before attepting to re-open the file.

See:http://www.cplusplus.com/reference/iostream/ios_base/iostate.html
For what else can go wrong.

#include <fstream>
#include <iostream>

int main()
{
    std::fstream    data;
    data.open("./plop");

    while(!data)
    {
        std::cout << "Failed" << std::endl;
        // Pause here
        std::cout << "Retrying" << std::endl;

        // Put the state of the object back into its original state.
        data.clear();

        // Retry opening the file.
        data.open("./plop");
        if (data.is_open())
        {
            std::cout << "Open" << std::endl;
            if (!data)
            {
                std::cout << "Reading from this file will fail." <<
std::endl;
            }
        }
    }

    std::cout << "Finished" << std::endl;



}- Hide quoted text -

- Show quoted text -

What could cause an error state?

 

[Martin York]

What could cause an error state?

See:http://www.cplusplus.com/reference/iostream/ios_base/iostate.html
For what else can go wrong.