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Showing content from https://www.tutorialspoint.com/cpp_standard_library/cpp_fstream_is_open.htm below:

C++ Fstream Library - is_open Function

Returns whether the stream is currently associated to a file. If streams can be associated to files by a successful call to member open or directly on construction, and disassociated by calling close or on destruction. The file association of a stream is kept by its internal stream buffer: Internally, then the function calls rdbuf()->is_open().

Following is the declaration for fstream::is_open.

bool is_open();

bool is_open() const;

It returns true, if a file is open and associated with this stream object or else returns false.

Strong guarantee − if an exception is thrown, there are no changes in the stream.

• Accesses the fstream object.

• Concurrent access to the same stream may introduce data races.

In below example explains about fstream is_open function.

#include <iostream>     
#include <fstream>      

int main () {
   std::fstream fs;
   fs.open ("test.txt");
   if (fs.is_open()) {
      fs << "lorem ipsum";
      std::cout << "Operation successfully performed\n";
      fs.close();
   } else {
      std::cout << "Error opening file";
   }
   return 0;
}

fstream.htm

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