ifstream not reading EOF character

问题:

I am creating a program (In C++) that takes an ASCII file and reads a few values from each line until it reaches the end of the file. I am using ifstream to read the file, and I have never had problems with it stopping when I use the ifstream.eof() method. This time, however, even though it found the eof character in my test case, when I analyzed my other files, it is infinite looping because it never finds the eof character. Is this a coding issue, or an issue with my files?

string line = "";
unsigned long pos = 0;
ifstream curfile(input.c_str());
getline(curfile, line);
int linenumber = 0;
cout<<"About to try to read the file"<<endl;
if (!curfile.good())
    cout<<"Bad file read"<<endl;
while (!curfile.eof())
{

    cout<<"Getting line "<<linenumber<<endl;
    linenumber++;
    pos = line.find_first_of(' ');
    line = line.substr(pos+1, line.size()-1);
    pos = line.find_first_of(' ');
    current.push_back(atof(line.substr(0, pos).c_str()));
    for (int i = 0; i<4; i++)
    {
        pos = line.find_first_of(' ');
        line = line.substr(pos+1, line.size()-1);
    }
    pos = line.find_first_of(' ');
    dx.push_back(atof(line.substr(0, pos).c_str()));
    pos = line.find_first_of(' ');
    line = line.substr(pos+1, line.size()-1);
    pos = line.find_first_of(' ');
    dy.push_back(atof(line.substr(0, pos).c_str()));
    getline(curfile, line);
}

EDIT: When I first run the loop, currentfile.good() returns false...what am I doing that causes it to return that?

回答1:

First thing is first, you shouldn't check like that. eof() doesn't return true until after a failed read. But you can do better (and easier)!

check the stream state with the implicit conversion to void* which can be used in a bool context. Since most of the read operations on streams return a reference to the stream, you can write some very consice code like this:

std::string line;
while(std::getline(currentfile, line)) {
    // process line
}

Basically what it is doing is saying "while I could successfully extract a line from currentfile, do the following", which is what you really meant to say anyway ;-);

Like I said, this applies to most stream operations, so you can do things like this:

int x;
std::string y;
if(std::cin >> x >> y) {
    // successfully read an integer and a string from cin!
}

EDIT: The way I would rewrite your code is like this:

string line;
unsigned long pos = 0;
int linenumber = 0;

ifstream curfile(input.c_str());

std::cout << "About to try to read the file" << std::endl;
while (std::getline(curfile, line)) {

    std::cout << "Getting line " << linenumber << std::endl;
    linenumber++;

    // do the rest of the work with line
}

回答2:

Do not do it like that.

EOF is not the only thing you'll encounter while reading. There's a bunch of errors you might get, and so the best is to simply test the stream itself:

while(currentfile)
{
    // read somehow
}

If you're reading lines, then, the simplest way is:

std::string line;
while(std::getline(currentfile, line))
{
    // use line
}

回答3:

Your first call to getline is triggering one of the fail-bits on the ifstream object. That is why if you do a check for a fail-bit using ios::good(), you never enter your read loop. I would check to see what the value of line is ... it's probably empty, meaning you're having another issue reading your file, like maybe permissions problems, etc.

回答4:

The problem is here:

if (!curfile.good())
    cout<<"Bad file read"<<endl;   // OK you print bad.
while (!curfile.eof())             // But the loop is still entered.
                                   // Another reason to **NEVER** to use 
                                   // while (file.eof()) // as bad does not mean eof
                                                         // though eof is bad

Try this:

void readFile(std::istream& str)
{   
    std::string     line;
    while(std::getline(str, line))
    {
        std::stringstream   lineStream(line);
        std::string         ignoreWord;
        int                 number[3];

        lineStream >> ignoreWord   // reads one space seporated word
                   >> number[0]    // reads a number
                   >> ignoreWord >> ignoreWord >> ignoreWords  // reads three words 
                   >> number[1]    // reads a number
                   >> number[2];   // reads a number

        current.push_back(number[0]);
        dx.push_back(number[1]);
        dy.push_back(number[2]);
    }   
}