C++: if file line length exceeds array (buffer) length

Suppose we have implemented the following scenario:

view plainprint?

1. // read input file line by line  
2. // allocate 256 characters for each line  
3.   
4. ifstream input_file("some_file.txt");  
5. const int BUF_SIZE=256;  
6. char buf[BUF_SIZE];  
7. string s, strCurString;  
8.   
9. if (!input_file.is_open())  
10. {  
11.    cerr << "File some_file.txt coudl not be open!" << endl;  
12.    getch();  
13.    exit(EXIT_FAILURE);  
14. }  
15.   
16. while(!input_file.eof()) {  
17.    input_file.getline(buf, BUF_SIZE);  
18.    strCurString = buf;  
19.    s += strCurString;  
20. }  
21.   
22. cout << "File contents: " << endl << s << endl;  

// read input file line by line
// allocate 256 characters for each line

ifstream input_file("some_file.txt");
const int BUF_SIZE=256;
char buf[BUF_SIZE];
string s, strCurString;

if (!input_file.is_open())
{
   cerr << "File some_file.txt coudl not be open!" << endl;
   getch();
   exit(EXIT_FAILURE);
}

while(!input_file.eof()) {
   input_file.getline(buf, BUF_SIZE);
   strCurString = buf;
   s += strCurString;
}

cout << "File contents: " << endl << s << endl;

But what if the current file length exceeds BUF_SIZE? Well, in this case the while loop will never end, becoming an infinite loop. Why? Simply, because in the input file stream object a special bit (failbit) will be set, saying that the last getline() operation has failed (in this case not due to the end of a file, but due to the buffer length exceeding). In this case all subsequent calls to getline() will fail to read anything (can be seen by calling input_file.gcount(), which constantly returns 0 (zero) after the last getline() call that led to setting a failbit).


To overcome this, we can use a trick found here:

view plainprint?

1. // read input file line by line  
2. // allocate 256 characters for each line  
3.   
4. ifstream input_file("some_file.txt");  
5. const int BUF_SIZE=256;  
6. char buf[BUF_SIZE];  
7. string s, strCurString;  
8.   
9. if (!input_file.is_open())  
10. {  
11.    cerr << "File some_file.txt coudl not be open!" << endl;  
12.    getch();  
13.    exit(EXIT_FAILURE);  
14. }  
15.   
16. while(!input_file.eof()) {  
17.    input_file.getline(buf, BUF_SIZE);  
18.   
19.    // remember about failbit when amount of  
20.    // characters in the current line is  
21.    // more than BUF_SIZE  
22.    if (input_file.fail() && !input_file.eof())  
23.        // clear up the failbit and   
24.        // continue reading the input file  
25.       input_file.clear();  
26.    strCurString = buf;  
27.    s += strCurString;  
28. }  
29.   
30. cout << "File contents: " << endl << s << endl;  

// read input file line by line
// allocate 256 characters for each line

ifstream input_file("some_file.txt");
const int BUF_SIZE=256;
char buf[BUF_SIZE];
string s, strCurString;

if (!input_file.is_open())
{
   cerr << "File some_file.txt coudl not be open!" << endl;
   getch();
   exit(EXIT_FAILURE);
}

while(!input_file.eof()) {
   input_file.getline(buf, BUF_SIZE);

   // remember about failbit when amount of
   // characters in the current line is
   // more than BUF_SIZE
   if (input_file.fail() && !input_file.eof())
       // clear up the failbit and 
       // continue reading the input file
      input_file.clear();
   strCurString = buf;
   s += strCurString;
}

cout << "File contents: " << endl << s << endl;