std::ifstream in cpp

Posted by chunyang on October 30, 2019
TAGS: #cpp

Introduction

In this blog and next blog, we will introduce the functions in C++ that we read from and write to files. It is the basic functionality when we learn programming.

A simple example

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

/* We assume there are 3 lines in test.txt
 * 1.3
 * 4
 * hello
 */

int main(int argc, char* argv[]) {

    std::string path = "test.txt";
    std::ifstream input(path, std::ios::in);
    double d1; input >> d1;
    input.get(); /* skip \n */
    char d2;
    char d3[2];
    input.get(d3, 2); // read count - 1 character */
    std::cout << "get(& char) d2: " << d3 << "\n";
    d2 = d3[0];
    input.putback(d2);
    input.get(d2);
    std::cout << "get(*ptr, count) d2: " << d2 << "\n";
    input.unget();
    input.read(reinterpret_cast<char*>(&d2), sizeof(d2));
    std::cout << "read(char*, count) d2: " << d2 << " gcount() = " << input.gcount() << "\n";


    input.get(); /* skip \n */
    std::array<char, 10> arr;
    input.getline(&arr[0], 10); /* default delimit: \n */
    std::cout << d1 << " " << d2 << " " << &arr[0] << std::endl;
    std::cout << std::boolalpha << "good = " << input.good() << " eof = " << input.eof() << std::endl;
    std::cout << "last: " << input.get() << std::endl;
    std::cout << std::boolalpha << "good = " << input.good() << " eof = " << input.eof() << std::endl;

    input.clear(); /* you should clear first, otherwise following code will fail */
    input.seekg(0, std::ios_base::seekdir::beg);
    std::cout << std::boolalpha << "good = " << input.good() << " eof = " << input.eof() << std::endl;
    std::string x;
    input >> d1 >> d2;
    input.get();
    std::getline(input, x);
    std::cout << "d1: " << d1 << " d2: " << d2 << " x: " << x << std::endl;
}

Compile and run:

g++ -std=c++11 program.cpp -o program
./program

Common interfaces

Open & close

  • From constructor
  • open(path)
  • close()
  • is_open()

Open mode:

  • std::ios::app
  • std::ios::ate
  • std::ios::trunc
  • std::ios::binary
  • std::ios::in
  • std::ios::out
  • good()
  • eof()
  • bad()
  • fail()
  • rdstate()
  • setstate()
  • clear()

State(iostate):

  • std::ios::goodbit
  • std::ios::badbit
  • std::ios::failbit
  • std::ios::eofbit

Read data

  • operator>>
  • get
    • get(char& c)
    • get(char* arr, count): read upto count - 1
    • get(char* arr, count, delimit): read upto count - 1
  • peek()
  • read(char*, size)
  • getline(char*, size, delimit): read upto size - 1
    • consume delimit
    • set an null at the next position of successful read.
  • std::getline(stream, string, delimit)
  • seek
    • seekg(pos)
    • seekg(pos, refer)
  • tellg()
  • gcount()

Direction:

  • std::ios::seek_dir::beg
  • std::ios::seek_dir::cur
  • std::ios::seek_dir::end

Un-read data

  • putback(char c)
  • unget()

Tricks

Copy content of a file into a vector. Each line should not contain spaces.

You can use to split words and convert them to upper case.

#include <algorithm>
#include <fstream>
#include <iterator>
#include <iostream>
#include <string>
#include <vector>

int main(int argc, char* argv[]) {
    std::ifstream ifs("test.txt");
    std::vector<std::string> result;
    copy(std::istream_iterator<std::string>(ifs),
         std::istream_iterator<std::string>(),
         std::back_inserter(result));
    std::for_each(result.begin(), result.end(),
                  [](const std::string& s) { std::cout << s << std::endl; });
    auto x = [](std::string& str)->std::string {
        for (char& c : str) {
            c = ::toupper(c);
        }
        return str;
    };
    std::transform(begin(result), end(result), result.begin(), x);
    std::for_each(result.begin(), result.end(),
                  [](const std::string& s) { std::cout << s << std::endl; });
    return 0;
}

References


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