How to read a line from a text file in c/c++? [duplicate]

Question

After exhaustive googling and visiting many forums, I am yet to find a good comprehensive answer for this question. A lot of the forums suggest using the get line istream& getline (char* s, streamsize n ) function. My question is what if I don't know what the length of each line is and cannot predict what the size may be? Also what is it's equivalent in C?

Is there any specific function in c /c++ to read one single line each time from a text file ?

Explanation , with Code snippets will help me a lot.

Answer 1

In C++, you can use the global function std::getline, it takes a string and a stream and an optional delimiter and reads 1 line until the delimiter specified is reached. An example:

#include <string>
#include <iostream>
#include <fstream>

int main() {
    std::ifstream input("filename.txt");
    std::string line;

    while( std::getline( input, line ) ) {
        std::cout<<line<<'\n';
    }

    return 0;
}

This program reads each line from a file and echos it to the console.

For C you're probably looking at using fgets, it has been a while since I used C, meaning I'm a bit rusty, but I believe you can use this to emulate the functionality of the above C++ program like so:

#include <stdio.h>

int main() {
    char line[1024];
    FILE *fp = fopen("filename.txt","r");

    //Checks if file is empty
    if( fp == NULL ) {                       
        return 1;
    }

    while( fgets(line,1024,fp) ) {
        printf("%s\n",line);
    }

    return 0;
}

With the limitation that the line can not be longer than the maximum length of the buffer that you're reading in to.