How do I reset the state of an output stream after an error has occurred?

Question

Ryan McCombe · Accepted Answer

Resetting the state of an output stream after an error has occurred is essential to continue using the stream. C++ provides methods to handle and clear the stream state.

Detecting the Error State

First, you need to detect if an error has occurred. You can check the state of the stream using methods like fail() or bad():

1#include <iostream>
2
3int main() {
4  std::cout.setstate(std::ios::failbit);  
5
6  if (std::cout.fail()) {
7    std::cerr << "Stream is in a fail state";  
8  }
9}

1Stream is in a fail state

Clearing the Error State

To clear the error state and restore the stream to a usable state, use the clear() method:

1#include <iostream>
2
3int main() {
4  std::cout.setstate(std::ios::failbit);  
5
6  if (std::cout.fail()) {
7    std::cerr << "Stream is in a fail state\n";
8    std::cout.clear();  
9  }
10
11  if (std::cout) {
12    std::cout << "Stream state has been reset";  
13  }
14}

1Stream is in a fail state
2Stream state has been reset

Preserving Existing Flags

Sometimes, you might want to preserve certain flags like std::ios::eofbit while clearing others. You can do this by passing specific flags to clear():

1#include <iostream>
2
3int main() {
4  // Set the failbit and eofbit for the
5  // std::cout stream
6  std::cout.setstate(
7    std::ios::failbit | std::ios::eofbit
8  );
9
10  // Check if the stream is in a fail state
11  // and log a message
12  if (std::cout.fail()) {
13    std::cerr << "Stream is in a fail state\n";
14
15    // Clear only the failbit while keeping
16    // the eofbit set
17    std::cout.clear(std::ios::eofbit);
18  }
19
20  // Check if the EOF bit is still set
21  // and log a message
22  if (std::cout.eof()) {
23    std::cerr << "EOF bit is still set";
24  }
25}

1Stream is in a fail state
2EOF bit is still set

Practical Example

Here's a practical example where you might need to reset the stream state:

1#include <iostream>
2#include <limits>  // For std::numeric_limits
3
4int main() {
5  int value;
6
7  while (true) {
8    std::cout << "Enter an integer: ";
9    std::cin >> value;
10
11    if (std::cin.fail()) {
12      std::cerr << "Invalid input, "
13        "please enter an integer.\n";
14
15       // Clear the error flag on cin
16      std::cin.clear();
17
18      // Discard invalid input
19      std::cin.ignore(
20        std::numeric_limits<std::streamsize>::max(),
21        '\n'
22      );  
23    } else {
24      // Discard any extraneous input
25      std::cin.ignore(
26        std::numeric_limits<std::streamsize>::max(),
27        '\n'
28      );
29      break; // Exit the loop if input is valid
30    }
31  }
32
33  std::cout << "You entered: " << value;
34}

1Enter an integer: a
2Invalid input, please enter an integer.
3Enter an integer: 5
4You entered: 5

This example demonstrates how to handle invalid input in std::cin and reset the stream state to allow for re-entry.

By understanding how to detect and clear stream states, you can handle errors more gracefully and ensure your programs remain robust and user-friendly.

Output Streams

Resetting Stream State

Detecting the Error State

Clearing the Error State

Preserving Existing Flags

Practical Example

Output Streams

Professional C++

Questions & Answers