C++ What's wrong during reading a file one line at a time?

const char *filename = "test.txt";
ifstream fin(filename);
char line[80];
while(!fin.eof()){
int i = 0;
fin.getline(line,80);
while(line[i] != '\0'){
std::cout << line[i];
i++;
}
// if(fin.fail() && !fin.eof()){
// fin.clear();
// if(fin.peek() == '\0')
// fin.get();
}
}
i want to know why fin.fail() is set to 1?
but if using:
string line;
while(getline(fin,line))
std::cout << line;
it works. fin.fail() is not set to 1. What makes the difference?
i want to reading one line by line from a file. if using char line[81] or char * line = new char [81]; i must add the comment line or it will loop for ever. i don't understand 1. What reason sets the fin.fail() to 1.
2.if i complie the code as follows ,it still doesn't work.
while(!fin.eof()){
int i = 0;
fin.getline(line,80);
while(line[i] != '\0'){
std::cout << line[i];
i++;
}
fin.clear();
}
regardless of i has any value,(i assign i to 0.).line[i] always equals to '\0'. That is to say it didn't execute the "fin.getline(line,80);". it is the result of he fin.fail() is set to 1.
Thank you.Any help is precious to me.

He found one bug - now you ask another different question???