How to get the line number from a file in C++?

Question

What would be the best way to get the line number of the current line in a file that I have opened with a ifstream? So I am reading in the data and I need to store

Answer 1

Use std::getline to read each line in one by one. Keep an integer indicating the number of lines you have read: initialize it to zero and each time you call std::getline and it succeeds, increment it.

Answer 2

An inefficient but dead simple way is to have a function that given a stream, it counts the new line characters from the beginning of the stream to the current position.

int getCurrentLine(std::istream& is)
{
    int lineCount = 1;
    is.clear();     // need to clear error bits otherwise tellg returns -1.
    auto originalPos = is.tellg();
    if (originalPos < 0) 
        return -1;
    is.seekg(0);
    char c;
    while ((is.tellg() < originalPos) && is.get(c))
    {
        if (c == '\n') ++lineCount;
    }
    return lineCount;
}

In some code I am working on, I am only interested to know the line number if invalid input is encountered, in which case import is aborted immediately. Since the function is called only once the inefficiency is not really a problem.

The following is a full example:

#include <iostream>
#include <sstream>


int getCurrentLine(std::istream& is)
{
    int lineCount = 1;
    is.clear();     // need to clear error bits otherwise tellg returns -1.
    auto originalPos = is.tellg();
    if (originalPos < 0) 
        return -1;
    is.seekg(0);
    char c;
    while ((is.tellg() < originalPos) && is.get(c))
    {
        if (c == '\n') ++lineCount;
    }
    return lineCount;
}

void ReadDataFromStream(std::istream& s)
{
    double x, y, z;
    while (!s.fail() && !s.eof())
    {
        s >> x >> y >> z;
        if (!s.fail())
            std::cout << x << "," << y << "," << z << "\n";
    }

    if (s.fail())
        std::cout << "Error at line: " << getCurrentLine(s) << "\n";
    else
        std::cout << "Read until line: " << getCurrentLine(s) << "\n";
}

int main(int argc, char* argv[])
{
    std::stringstream s;
    s << "0.0 0.0 0.0\n";
    s << "1.0 ??? 0.0\n";
    s << "0.0 1.0 0.0\n";
    ReadDataFromStream(s);

    std::stringstream s2;
    s2 << "0.0 0.0 0.0\n";
    s2 << "1.0 0.0 0.0\n";
    s2 << "0.0 1.0 0.0";
    ReadDataFromStream(s2);

    return 0;
}

Answer 3

If you don't want to limit yourself to std::getline, then you could use class derived from std::streambuf, and which keeps track of the current line number:

class CountingStreamBuffer : public std::streambuf { /* see below */ };

// open file
std::ifstream file("somefile.txt");

// "pipe" through counting stream buffer
CountingStreamBuffer cntstreambuf(file.rdbuf());
std::istream is(&cntstreambuf);

// sample usage
is >> x >> y >> z;
cout << "At line " << cntstreambuf.lineNumber();
std::getline(is, str);
cout << "At line " << cntstreambuf.lineNumber();

---

Here is a sample implementation of CountingStreamBuffer:

#include <streambuf>

class CountingStreamBuffer : public std::streambuf
{
public:
    // constructor
    CountingStreamBuffer(std::streambuf* sbuf) : 
        streamBuf_(sbuf), 
        lineNumber_(1),
        lastLineNumber_(1),
        column_(0),
        prevColumn_(static_cast<unsigned int>(-1)),
        filePos_(0) 
    {
    }

    // Get current line number
    unsigned int        lineNumber() const  { return lineNumber_; }

    // Get line number of previously read character
    unsigned int        prevLineNumber() const { return lastLineNumber_; }

    // Get current column
    unsigned int        column() const   { return column_; }

    // Get file position
    std::streamsize     filepos() const { return filePos_; }

protected:
    CountingStreamBuffer(const CountingStreamBuffer&);
    CountingStreamBuffer& operator=(const CountingStreamBuffer&);

    // extract next character from stream w/o advancing read pos
    std::streambuf::int_type underflow() 
    { 
        return streamBuf_->sgetc(); 
    }

    // extract next character from stream
    std::streambuf::int_type uflow()
    {
        int_type rc = streamBuf_->sbumpc();

        lastLineNumber_ = lineNumber_;
        if (traits_type::eq_int_type(rc, traits_type::to_int_type('\n'))) 
        {
            ++lineNumber_;
            prevColumn_ = column_ + 1;
            column_ = static_cast<unsigned int>(-1);
        }

        ++column_;
        ++filePos_;
        return rc;
    }

    // put back last character
    std::streambuf::int_type pbackfail(std::streambuf::int_type c)
    {
        if (traits_type::eq_int_type(c, traits_type::to_int_type('\n'))) 
        {
            --lineNumber_;
            lastLineNumber_ = lineNumber_;
            column_ = prevColumn_;
            prevColumn_ = 0;
        }

        --column_;
        --filePos_;

        if (c != traits_type::eof())
            return streamBuf_->sputbackc(traits_type::to_char_type(c));  
        else 
            return streamBuf_->sungetc();
    }

    // change position by offset, according to way and mode  
    virtual std::ios::pos_type seekoff(std::ios::off_type pos, 
                                  std::ios_base::seekdir dir, 
                                  std::ios_base::openmode mode)
    {
        if (dir == std::ios_base::beg 
         && pos == static_cast<std::ios::off_type>(0))
        {
            lastLineNumber_ = 1;
            lineNumber_ = 1;
            column_ = 0;
            prevColumn_ = static_cast<unsigned int>(-1);
            filePos_ = 0;

            return streamBuf_->pubseekoff(pos, dir, mode);
        }
        else
            return std::streambuf::seekoff(pos, dir, mode);
    }

    // change to specified position, according to mode
    virtual std::ios::pos_type seekpos(std::ios::pos_type pos,
                                  std::ios_base::openmode mode)
    {   
        if (pos == static_cast<std::ios::pos_type>(0))
        {
            lastLineNumber_ = 1;
            lineNumber_ = 1;
            column_ = 0;
            prevColumn_ = static_cast<unsigned int>(-1);
            filePos_ = 0;

            return streamBuf_->pubseekpos(pos, mode);
        }
        else
            return std::streambuf::seekpos(pos, mode);
    }


private:
    std::streambuf*     streamBuf_;     // hosted streambuffer
    unsigned int        lineNumber_;    // current line number
    unsigned int        lastLineNumber_;// line number of last read character
    unsigned int        column_;        // current column
    unsigned int        prevColumn_;    // previous column
    std::streamsize     filePos_;       // file position
};

Answer 4

From an ifstream point of view there is no line number. If you read in the file line by line, then you just have to keep track of it yourself.