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Showing content from https://cplusplus.com/reference/ios/basic_ios/clear/ below:

public member function

<ios> <iostream>

std::basic_ios::clear 
void clear (iostate state = goodbit);

Set error state flags

Sets a new value for the stream's internal error state flags.

The current value of the flags is overwritten: All bits are replaced by those in state; If state is goodbit (which is zero) all error flags are cleared.

In the case that no stream buffer is associated with the stream when this function is called, the badbit flag is automatically set (no matter the value for that bit passed in argument state).

Note that changing the state may throw an exception, depending on the latest settings passed to member exceptions.

The current state can be obtained with member function rdstate.



Parameters
state
An object of type ios_base::iostate that can take as value any combination of the following state flag member constants:

iostate value
(member constants) indicates functions to check state flags good() eof() fail() bad() rdstate() goodbit No errors (zero value iostate) true false false false goodbit eofbit End-of-File reached on input operation false true false false eofbit failbit Logical error on i/o operation false false true false failbit badbit Read/writing error on i/o operation false false true true badbit eofbit, failbit and badbit are member constants with implementation-defined values that can be combined (as if with the bitwise OR operator).


goodbit is zero, indicating that none of the other bits is set.

Return Value none

Example 
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// clearing errors
#include <iostream>     // std::cout
#include <fstream>      // std::fstream

int main () {
  char buffer [80];
  std::fstream myfile;

  myfile.open ("test.txt",std::fstream::in);

  myfile << "test";
  if (myfile.fail())
  {
    std::cout << "Error writing to test.txt\n";
    myfile.clear();
  }

  myfile.getline (buffer,80);
  std::cout << buffer << " successfully read from file.\n";

  return 0;
}

In this example, myfile is open for input operations, but we perform an output operation on it, so failbit is set. The example calls then clear in order to remove the flag and allow further operations like getline to be attempted on myfile.

Data races Modifies the stream object.
Concurrent access to the same stream object may cause data races.

Exception safetyBasic guarantee: if an exception is thrown, the stream is in a valid state.
It throws an exception of member type failure if the resulting error state flag is not goodbit and member exceptions was set throw for that state.

See also
basic_ios::fail
Check whether failbit or badbit is set (public member function)
basic_ios::good
Check whether state of stream is good (public member function)
basic_ios::bad
Check whether badbit is set (public member function)
basic_ios::eof
Check whether eofbit is set (public member function)
basic_ios::rdstate
Get error state flags (public member function)

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