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Is_open Function in C++

In this article, you will learn the is_open function in C++ with its syntax and example.

What is the is_open function?

In C++, the is_open() function determines whether a file stream is open. It accepts a file stream object as input and returns a bool indicating whether the stream is open.

For example:

The is_open() function is defined in the <fstream> header and can be called on ifstream, of stream, and stream objects to check their state. It returns true if the file stream object is associated with an open file and false otherwise.

It is helpful to check if a file stream was successfully opened before trying to read from or write to it. It can prevent bugs and exceptions caused by operating on a stream that failed to open a file.

The is_open() function is a convenient way to verify that a file stream is ready for IO operations. It spares you from having to keep track of the open state separately. It's a simple but essential stream state-checking tool for C++ programs working with files.


Here is an example of how to declare the is_open() function in C++:

To use is_open():

  • Include the <fstream> header defining the file stream classes.
  • Create an ifstream, of stream, or stream object to represent the file stream.
  • Call the is_open() method on the stream object. It will return a bool.
  • Optionally assign the return value to a bool variable for later use.

The is_open() method is accessed through the .Operator on the stream object. No parameters are needed.

Some key points:

  • is_open() is a public member function of the file stream classes.
  • It returns a bool indicating the state of the stream.
  • It does not take any arguments - the stream object it is called on is implicitly passed.
  • It can be called multiple times to check the state as needed.

So, in C++, is_open() function is declared by simply calling it on a file stream object; no special syntax is required. The <fstream> header makes the function accessible.

Return Value

The is_open() function returns a bool value indicating whether the file stream is open.

  • It returns true if the file stream is associated with an open file. It means that it is ready for reading from or writing to the file.
  • It returns false if the file stream is not currently associated with an open file. It may indicate an error opening the file or the stream being closed.

For example:

Here, we check the return value of the is_open() function to determine whether we can proceed with reading from the stream or if an error opens the file.

The key advantages of the bool return value are:

  • It indicates the open/closed state with true/false.
  • It can be tested in conditional statements like if/else.
  • The state can be stored in a variable for later use.


The is_open() function does not directly throw any exceptions in C++. However, it may expose exceptions that were thrown early when opening the file stream.


  • Is_open() itself will never throw an exception. It always returns a bool value.
  • However, if an exception was thrown when opening the stream, is_open() will return false.

So, is_open() doesn't rethrow the exception but allows detecting a previous error.

For example:

Here, the is_open() exposes the fact an exception was thrown trying to open "invalidfile.txt" without throwing the exception again.

Some reasons is_open() could return false, indicating a previous exception:

  • File not found
  • Permission denied
  • Invalid filename
  • Disk error

Data Races

Here are the key points:

  • Don't directly share ifstream objects between threads.
  • Pass references to ifstream to each thread.
  • Use mutexes/locks to synchronize access.
  • Allow only one thread to access the stream at a time.
  • Avoid simultaneous read/write operations from multiple threads.
  • Use local streams for each thread instead of global.
  • Carefully design multi-threaded programs to prevent concurrent stream access.
  • Ensure stream access is synchronized using references, locks, etc.
  • Assume stream operations are not atomic - interleaves cause bugs.



File is open for writing.
File is open for reading.
It is a sample text.

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