Why is cin.failbit always set even when input is valid?

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I was trying to write a program that asks to input a char array using cin.getline() and if given input is bigger than array length array gets extended.

I used cin.failbit to see if user's input was too long. But nothing went right. So after debugging I figured out that the problem lies in failbit.

So I wrote a simple program to see what was wrong about it and it turned out that somehow cin.failbit always returns true when in if-statement even when input seems valid.

int main () {

    char name[256];



    std::cout << "Enter your name: ";

    std::cin.getline (name,256, 'n');



    std::cout << "characters read: " << std::cin.gcount() << std::endl;

    if (std::cin.failbit){

        std::cin.clear();

        std::cout << "failedn";

    }

}

For example, when input is "qwer" program outputs that 5 characters have been read (cin.gcount), so it should be fine but it also outputs "fail" meaning that failbit flag is set. But I believe it shouldn't be in this case.

program output here

So can anyone explain why failbit appears to be set permanently?

Any help will be highly appreciated

edited Jan 3 at 20:33

scohe001

8,19212442

asked Jan 3 at 20:28

keptt

@fran please answer in answer not comments.

– Yakk - Adam Nevraumont
Jan 3 at 20:33

FWIW you can use if (!std::cin) instead to check if the stream is in a failure state.

– NathanOliver
Jan 3 at 20:33

3

I was mistaken, there is no failbit member function. It's checking that the value of the failbit bit mask isn't zero. I was thinking of the fail() member function. std::cin.failbit is a constant that indicates which bit represents a failed state, it's not checking the state of the stream.

– François Andrieux
Jan 3 at 20:33

thanks for replies. I considered using cin.fail() or !cin but they are not invoked when user's input is longer than the len of the array. User's input just gets cut and all characters past the size of array remain in the buffer (at least that is what I observed)

– keptt
Jan 3 at 20:42

1

@keptt No, failbit is not a flag in the stateful sense. It indicates which bit is associated with that flag. It cannot be set or cleared, it's a constant. The phrase "the failbit flag" means "the bit indicated by the bitmask failbit".

– François Andrieux
Jan 3 at 21:04

|
show 3 more comments

I was trying to write a program that asks to input a char array using cin.getline() and if given input is bigger than array length array gets extended.

I used cin.failbit to see if user's input was too long. But nothing went right. So after debugging I figured out that the problem lies in failbit.

So I wrote a simple program to see what was wrong about it and it turned out that somehow cin.failbit always returns true when in if-statement even when input seems valid.

int main () {

    char name[256];



    std::cout << "Enter your name: ";

    std::cin.getline (name,256, 'n');



    std::cout << "characters read: " << std::cin.gcount() << std::endl;

    if (std::cin.failbit){

        std::cin.clear();

        std::cout << "failedn";

    }

}

program output here

So can anyone explain why failbit appears to be set permanently?

Any help will be highly appreciated

edited Jan 3 at 20:33

scohe001

8,19212442

asked Jan 3 at 20:28

keptt

@fran please answer in answer not comments.

– Yakk - Adam Nevraumont
Jan 3 at 20:33

FWIW you can use if (!std::cin) instead to check if the stream is in a failure state.

– NathanOliver
Jan 3 at 20:33

3

I was mistaken, there is no failbit member function. It's checking that the value of the failbit bit mask isn't zero. I was thinking of the fail() member function. std::cin.failbit is a constant that indicates which bit represents a failed state, it's not checking the state of the stream.

– François Andrieux
Jan 3 at 20:33

thanks for replies. I considered using cin.fail() or !cin but they are not invoked when user's input is longer than the len of the array. User's input just gets cut and all characters past the size of array remain in the buffer (at least that is what I observed)

– keptt
Jan 3 at 20:42

1

@keptt No, failbit is not a flag in the stateful sense. It indicates which bit is associated with that flag. It cannot be set or cleared, it's a constant. The phrase "the failbit flag" means "the bit indicated by the bitmask failbit".

– François Andrieux
Jan 3 at 21:04

|
show 3 more comments

I was trying to write a program that asks to input a char array using cin.getline() and if given input is bigger than array length array gets extended.

I used cin.failbit to see if user's input was too long. But nothing went right. So after debugging I figured out that the problem lies in failbit.

So I wrote a simple program to see what was wrong about it and it turned out that somehow cin.failbit always returns true when in if-statement even when input seems valid.

int main () {

    char name[256];



    std::cout << "Enter your name: ";

    std::cin.getline (name,256, 'n');



    std::cout << "characters read: " << std::cin.gcount() << std::endl;

    if (std::cin.failbit){

        std::cin.clear();

        std::cout << "failedn";

    }

}

program output here

So can anyone explain why failbit appears to be set permanently?

Any help will be highly appreciated

edited Jan 3 at 20:33

scohe001

8,19212442

asked Jan 3 at 20:28

keptt

I was trying to write a program that asks to input a char array using cin.getline() and if given input is bigger than array length array gets extended.

I used cin.failbit to see if user's input was too long. But nothing went right. So after debugging I figured out that the problem lies in failbit.

So I wrote a simple program to see what was wrong about it and it turned out that somehow cin.failbit always returns true when in if-statement even when input seems valid.

int main () {

    char name[256];



    std::cout << "Enter your name: ";

    std::cin.getline (name,256, 'n');



    std::cout << "characters read: " << std::cin.gcount() << std::endl;

    if (std::cin.failbit){

        std::cin.clear();

        std::cout << "failedn";

    }

}

program output here

So can anyone explain why failbit appears to be set permanently?

Any help will be highly appreciated

c++ arrays cin getline

edited Jan 3 at 20:33

scohe001

8,19212442

asked Jan 3 at 20:28

keptt

edited Jan 3 at 20:33

scohe001

8,19212442

asked Jan 3 at 20:28

keptt

edited Jan 3 at 20:33

scohe001

8,19212442

edited Jan 3 at 20:33

scohe001

8,19212442

edited Jan 3 at 20:33

scohe001

8,19212442

asked Jan 3 at 20:28

keptt

asked Jan 3 at 20:28

keptt

asked Jan 3 at 20:28

keptt

@fran please answer in answer not comments.

– Yakk - Adam Nevraumont
Jan 3 at 20:33

FWIW you can use if (!std::cin) instead to check if the stream is in a failure state.

– NathanOliver
Jan 3 at 20:33

3

I was mistaken, there is no failbit member function. It's checking that the value of the failbit bit mask isn't zero. I was thinking of the fail() member function. std::cin.failbit is a constant that indicates which bit represents a failed state, it's not checking the state of the stream.

– François Andrieux
Jan 3 at 20:33

thanks for replies. I considered using cin.fail() or !cin but they are not invoked when user's input is longer than the len of the array. User's input just gets cut and all characters past the size of array remain in the buffer (at least that is what I observed)

– keptt
Jan 3 at 20:42

1

@keptt No, failbit is not a flag in the stateful sense. It indicates which bit is associated with that flag. It cannot be set or cleared, it's a constant. The phrase "the failbit flag" means "the bit indicated by the bitmask failbit".

– François Andrieux
Jan 3 at 21:04

|
show 3 more comments

@fran please answer in answer not comments.

– Yakk - Adam Nevraumont
Jan 3 at 20:33

FWIW you can use if (!std::cin) instead to check if the stream is in a failure state.

– NathanOliver
Jan 3 at 20:33

3

I was mistaken, there is no failbit member function. It's checking that the value of the failbit bit mask isn't zero. I was thinking of the fail() member function. std::cin.failbit is a constant that indicates which bit represents a failed state, it's not checking the state of the stream.

– François Andrieux
Jan 3 at 20:33

thanks for replies. I considered using cin.fail() or !cin but they are not invoked when user's input is longer than the len of the array. User's input just gets cut and all characters past the size of array remain in the buffer (at least that is what I observed)

– keptt
Jan 3 at 20:42

1

@keptt No, failbit is not a flag in the stateful sense. It indicates which bit is associated with that flag. It cannot be set or cleared, it's a constant. The phrase "the failbit flag" means "the bit indicated by the bitmask failbit".

– François Andrieux
Jan 3 at 21:04

@fran please answer in answer not comments.

– Yakk - Adam Nevraumont
Jan 3 at 20:33

FWIW you can use if (!std::cin) instead to check if the stream is in a failure state.

– NathanOliver
Jan 3 at 20:33

I was mistaken, there is no failbit member function. It's checking that the value of the failbit bit mask isn't zero. I was thinking of the fail() member function. std::cin.failbit is a constant that indicates which bit represents a failed state, it's not checking the state of the stream.

– François Andrieux
Jan 3 at 20:33

thanks for replies. I considered using cin.fail() or !cin but they are not invoked when user's input is longer than the len of the array. User's input just gets cut and all characters past the size of array remain in the buffer (at least that is what I observed)

– keptt
Jan 3 at 20:42

@keptt No, failbit is not a flag in the stateful sense. It indicates which bit is associated with that flag. It cannot be set or cleared, it's a constant. The phrase "the failbit flag" means "the bit indicated by the bitmask failbit".

– François Andrieux
Jan 3 at 21:04

|
show 3 more comments

1 Answer
1

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std::cin.failbit is a constant that indicates which of the error bits represents stream failure. It is not an indication of the stream's current state. To check if that bit is set, use the member function std::cin.fail() instead.

However, if the stream failed to read due to reaching the end of the stream fail() will return false leading you to believe that it succeeded. Prefer std::cin.good() to check if the last operation succeeded.

answered Jan 3 at 20:39

François Andrieux

16.4k32850

Documentation link

– user4581301
Jan 3 at 20:41

I appreciate the answer but for me the part about failbit just interferes with getline description link 4th block here states: "The failbit flag is set if the function extracts no characters, or if the delimiting character is not found once (n-1) characters have already been written to s". So for me it's confusing

– keptt
Jan 3 at 20:57

@keptt The error state is stored in a bitset. failbit is a constant that indicates which of the bits represents failure. For example, it's value might be the constant 8 indicating that the 4th bit indicates failure. In that instance, your if would be essentially if(8) and doesn't account for the current state of your stream. An example of a use for failbit might be std::cin.rdstate() & std::ios_base::failbit to see if the failbit is 1 in the bitset returned by rdstate().

– François Andrieux
Jan 3 at 20:59

thanks, I guess I get it now. I tried to act on your advise and used cin.good() - it worked just fine so far. Not to be too annoying but would you mind me asking about dealing with failbit? If it's a constant then it should have some kind of default value meaning if I type smth like - if(cin.failbit == default_value) it would mean that no errors that can trigger failbit were detected and everything is fine. Is it like that?

– keptt
Jan 3 at 21:17

@keptt No. failbit has nothing to do with cin's state. Imagine it's a global constant (because that's what it is). There is no useful information you can extract from it, it's a number and it's always that number for the lifetime of your program. Imagine everywhere you see failbit that it instead just says 4, or 16 or whatever value it has. It cannot change during the lifetime of your program. It's only use is as a bitmask to help you interpret the value returned by rdstate(). The fact that std::cin.failbit compiles is misleading. failbit is a static constexpr.

– François Andrieux
Jan 3 at 21:19

|
show 1 more comment

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answered Jan 3 at 20:39

François Andrieux

16.4k32850

Documentation link

– user4581301
Jan 3 at 20:41

I appreciate the answer but for me the part about failbit just interferes with getline description link 4th block here states: "The failbit flag is set if the function extracts no characters, or if the delimiting character is not found once (n-1) characters have already been written to s". So for me it's confusing

– keptt
Jan 3 at 20:57

@keptt The error state is stored in a bitset. failbit is a constant that indicates which of the bits represents failure. For example, it's value might be the constant 8 indicating that the 4th bit indicates failure. In that instance, your if would be essentially if(8) and doesn't account for the current state of your stream. An example of a use for failbit might be std::cin.rdstate() & std::ios_base::failbit to see if the failbit is 1 in the bitset returned by rdstate().

– François Andrieux
Jan 3 at 20:59

thanks, I guess I get it now. I tried to act on your advise and used cin.good() - it worked just fine so far. Not to be too annoying but would you mind me asking about dealing with failbit? If it's a constant then it should have some kind of default value meaning if I type smth like - if(cin.failbit == default_value) it would mean that no errors that can trigger failbit were detected and everything is fine. Is it like that?

– keptt
Jan 3 at 21:17

@keptt No. failbit has nothing to do with cin's state. Imagine it's a global constant (because that's what it is). There is no useful information you can extract from it, it's a number and it's always that number for the lifetime of your program. Imagine everywhere you see failbit that it instead just says 4, or 16 or whatever value it has. It cannot change during the lifetime of your program. It's only use is as a bitmask to help you interpret the value returned by rdstate(). The fact that std::cin.failbit compiles is misleading. failbit is a static constexpr.

– François Andrieux
Jan 3 at 21:19

|
show 1 more comment

answered Jan 3 at 20:39

François Andrieux

16.4k32850

Documentation link

– user4581301
Jan 3 at 20:41

I appreciate the answer but for me the part about failbit just interferes with getline description link 4th block here states: "The failbit flag is set if the function extracts no characters, or if the delimiting character is not found once (n-1) characters have already been written to s". So for me it's confusing

– keptt
Jan 3 at 20:57

@keptt The error state is stored in a bitset. failbit is a constant that indicates which of the bits represents failure. For example, it's value might be the constant 8 indicating that the 4th bit indicates failure. In that instance, your if would be essentially if(8) and doesn't account for the current state of your stream. An example of a use for failbit might be std::cin.rdstate() & std::ios_base::failbit to see if the failbit is 1 in the bitset returned by rdstate().

– François Andrieux
Jan 3 at 20:59

thanks, I guess I get it now. I tried to act on your advise and used cin.good() - it worked just fine so far. Not to be too annoying but would you mind me asking about dealing with failbit? If it's a constant then it should have some kind of default value meaning if I type smth like - if(cin.failbit == default_value) it would mean that no errors that can trigger failbit were detected and everything is fine. Is it like that?

– keptt
Jan 3 at 21:17

@keptt No. failbit has nothing to do with cin's state. Imagine it's a global constant (because that's what it is). There is no useful information you can extract from it, it's a number and it's always that number for the lifetime of your program. Imagine everywhere you see failbit that it instead just says 4, or 16 or whatever value it has. It cannot change during the lifetime of your program. It's only use is as a bitmask to help you interpret the value returned by rdstate(). The fact that std::cin.failbit compiles is misleading. failbit is a static constexpr.

– François Andrieux
Jan 3 at 21:19

|
show 1 more comment

answered Jan 3 at 20:39

François Andrieux

16.4k32850

answered Jan 3 at 20:39

François Andrieux

16.4k32850

answered Jan 3 at 20:39

François Andrieux

16.4k32850

answered Jan 3 at 20:39

François Andrieux

16.4k32850

answered Jan 3 at 20:39

François Andrieux

16.4k32850

Documentation link

– user4581301
Jan 3 at 20:41

I appreciate the answer but for me the part about failbit just interferes with getline description link 4th block here states: "The failbit flag is set if the function extracts no characters, or if the delimiting character is not found once (n-1) characters have already been written to s". So for me it's confusing

– keptt
Jan 3 at 20:57

@keptt The error state is stored in a bitset. failbit is a constant that indicates which of the bits represents failure. For example, it's value might be the constant 8 indicating that the 4th bit indicates failure. In that instance, your if would be essentially if(8) and doesn't account for the current state of your stream. An example of a use for failbit might be std::cin.rdstate() & std::ios_base::failbit to see if the failbit is 1 in the bitset returned by rdstate().

– François Andrieux
Jan 3 at 20:59

thanks, I guess I get it now. I tried to act on your advise and used cin.good() - it worked just fine so far. Not to be too annoying but would you mind me asking about dealing with failbit? If it's a constant then it should have some kind of default value meaning if I type smth like - if(cin.failbit == default_value) it would mean that no errors that can trigger failbit were detected and everything is fine. Is it like that?

– keptt
Jan 3 at 21:17

@keptt No. failbit has nothing to do with cin's state. Imagine it's a global constant (because that's what it is). There is no useful information you can extract from it, it's a number and it's always that number for the lifetime of your program. Imagine everywhere you see failbit that it instead just says 4, or 16 or whatever value it has. It cannot change during the lifetime of your program. It's only use is as a bitmask to help you interpret the value returned by rdstate(). The fact that std::cin.failbit compiles is misleading. failbit is a static constexpr.

– François Andrieux
Jan 3 at 21:19

|
show 1 more comment

Documentation link

– user4581301
Jan 3 at 20:41

I appreciate the answer but for me the part about failbit just interferes with getline description link 4th block here states: "The failbit flag is set if the function extracts no characters, or if the delimiting character is not found once (n-1) characters have already been written to s". So for me it's confusing

– keptt
Jan 3 at 20:57

@keptt The error state is stored in a bitset. failbit is a constant that indicates which of the bits represents failure. For example, it's value might be the constant 8 indicating that the 4th bit indicates failure. In that instance, your if would be essentially if(8) and doesn't account for the current state of your stream. An example of a use for failbit might be std::cin.rdstate() & std::ios_base::failbit to see if the failbit is 1 in the bitset returned by rdstate().

– François Andrieux
Jan 3 at 20:59

thanks, I guess I get it now. I tried to act on your advise and used cin.good() - it worked just fine so far. Not to be too annoying but would you mind me asking about dealing with failbit? If it's a constant then it should have some kind of default value meaning if I type smth like - if(cin.failbit == default_value) it would mean that no errors that can trigger failbit were detected and everything is fine. Is it like that?

– keptt
Jan 3 at 21:17

@keptt No. failbit has nothing to do with cin's state. Imagine it's a global constant (because that's what it is). There is no useful information you can extract from it, it's a number and it's always that number for the lifetime of your program. Imagine everywhere you see failbit that it instead just says 4, or 16 or whatever value it has. It cannot change during the lifetime of your program. It's only use is as a bitmask to help you interpret the value returned by rdstate(). The fact that std::cin.failbit compiles is misleading. failbit is a static constexpr.

– François Andrieux
Jan 3 at 21:19

Documentation link

– user4581301
Jan 3 at 20:41

I appreciate the answer but for me the part about failbit just interferes with getline description link 4th block here states: "The failbit flag is set if the function extracts no characters, or if the delimiting character is not found once (n-1) characters have already been written to s". So for me it's confusing

– keptt
Jan 3 at 20:57

@keptt The error state is stored in a bitset. failbit is a constant that indicates which of the bits represents failure. For example, it's value might be the constant 8 indicating that the 4th bit indicates failure. In that instance, your if would be essentially if(8) and doesn't account for the current state of your stream. An example of a use for failbit might be std::cin.rdstate() & std::ios_base::failbit to see if the failbit is 1 in the bitset returned by rdstate().

– François Andrieux
Jan 3 at 20:59

thanks, I guess I get it now. I tried to act on your advise and used cin.good() - it worked just fine so far. Not to be too annoying but would you mind me asking about dealing with failbit? If it's a constant then it should have some kind of default value meaning if I type smth like - if(cin.failbit == default_value) it would mean that no errors that can trigger failbit were detected and everything is fine. Is it like that?

– keptt
Jan 3 at 21:17

@keptt No. failbit has nothing to do with cin's state. Imagine it's a global constant (because that's what it is). There is no useful information you can extract from it, it's a number and it's always that number for the lifetime of your program. Imagine everywhere you see failbit that it instead just says 4, or 16 or whatever value it has. It cannot change during the lifetime of your program. It's only use is as a bitmask to help you interpret the value returned by rdstate(). The fact that std::cin.failbit compiles is misleading. failbit is a static constexpr.

– François Andrieux
Jan 3 at 21:19

|
show 1 more comment

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