Input error

For the error of input: Just remember C language return EOF and NULL, and C++ return all false.


	Number, non-white	character, white	string

C	scanf return EOF	getchar return EOF	fgets return NULL

c++	cin return false	cin return false	cin.getline return false

When you can’t continue input, you need to know if it’s end of file or fail. In C language, you can use feof and ferror function, in C++, you can use cin.eof(), cin.fail() or cin.bad() three functions. cin.bad() means serious system error happens, and input buffer can’t be consistent and can’t be recoveried anymore.

When failbit or badbit are set. fail() return false, So you need to judge it by bad() again if this information is important for you. Usually, bad() is not use very often.

eofbit is interesting topic. when you reach(not read) the "logical end-of-file", it will not be set, It’s set by a read function, not by position Instead, You can use std::ifstream::peek() to check for the "logical end-of-file".

eof is set by last read EOF position When you read EOF position, failbit is set and eof is set too. So If EOF is set, failbit must set too. If only failbit is set, it means that input error happen. Under these two conditions, istream will return false by operator bool.

In http://en.cppreference.com/w/cpp/io/basic_ios/operator_bool . You can see eofbit are true and failbit are false, operator bool return true. I don’t’ know when it will happen. eof() function returns "true" after the program attempts to read past the end of the file. But when read action set eofbit, it will set failbit at the sametime, so bool operator will return false because failbit are set.

How do you know if you read an incorrect input word, or if you are at the end of the file? This is when you use cin.eof() or feof() in C language.

you can use clear() and setstate() to set the states, Why? It’s just depends on what the program is trying to accomplish. It provides a means for you to change the state. setstate() is different with clear(), clear() clear all the status bit. but setstate just set specific bit.

You can use exceptions() method to control if exceptions will throw, when the eofbit, failbit and badbit is set.

setstate() is to provide a means for input and output functions to change the state. For end user, we don’t use it very often. We just use clear() more.

for rdstate() it will return all the bit value, then you can use & to certain bit( such as failbit) to check if the bit has been set. But here, you can use fail() directly, so we don’t use this function very often.

std::ifstream is;
is.open ("test.txt");
if ( (is.rdstate() & std::ifstream::failbit ) != 0 )
std::cerr << "Error␣opening␣’test.txt’\n";

clear() function is very important, when cin.fail() return true, you have to use clear(), otherwise all he below cin operation will not work. See below example.

char ch, str[20];
cin.getline(str, 5);
cout<<"flag1:"<<cin.good()<<endl;    // check good bit
cin.clear();   //without, cin>>ch below will not work at all
cout<<"flag1:"<<cin.good()<<endl;    // check good bit again
cin>>ch;
cout<<"str:"<<str<< "␣ch:" <<ch<<endl;

input:
12345[Enter]
output:
flag1:0 // good()return false
flag2:1 // good()return true after clear().
str:1234 ch:5

1.1.15.3 Input error