How to use std::getline() to read a text file into an array of strings in C++?

Question

I am trying to use std::getline() in my project to read in a text file into an array of strings.

Here is my code:

ifstream ifs ( path );
string * in_file;
int count = 0;
while ( !ifs.eof() )
{
    ++count;
    if ( count == 1 )
    {
        in_file = new string[1];
    }
    else
    {
            // Dynamically allocate another space in the stack
    string *old_in_file = in_file;
    in_file = new string[count];
            // Copy over values
    for ( int i = 0 ; i < ( count - 1 ) ; i++ )
    {
        in_file[i] = old_in_file[i];
    }
    delete[] old_in_file;
    }
            // After doing some debugging I know this is the problem what am I 
            // doing wrong with it?
    getline(ifs,in_file[count - 1]);
}

So after doing some decoding I know that the getline() is not placing any value in the array of strings. It seems to place a null string in the array.

The goal is to read in a text file like:

Hello
Bye
See you later

The array will be filled like:

in_file [0] = Hello
in_file [1] = Bye
in_file [2] = See you later

If your assignment permits you to use `std::vector`, you should probably do that instead of `new`'ing and `delete`'ing every iteration. — Steve Howard, Oct 01 '13 at 18:32
Search StackOverflow for "parse getline read file". This has been asked toooo many times. — Thomas Matthews, Oct 01 '13 at 18:33

score 11 · Accepted Answer · edited May 23 '17 at 10:29

Never wrap reading from the stream with the following loop:

while ( !ifs.eof() )

At some websites, you will find an example telling you to do:

while ( ifs.good() )

which is a bit better than the first loop, yet still it is quite error prone and not advisable to do. Have a look at: Why is iostream::eof inside a loop condition considered wrong?

The most common ways of reading the files are either using std::getline when reading by lines:

std::string line;
while ( std::getline(ifs, line) ) {
    if (line.empty())                  // be careful: an empty line might be read
        continue;                      
    ...
}

or simply using >> operator when reading by words or extracting concrete types (e.g. numbers):

std::string word;
while ( ifs >> word ) {               
    ...
}

And to your dynamically allocated C-style array of std::string objects: avoid dynamic allocation as much as possible. Believe me, you don't want to take care of memory management on your own. Prefer using objects with automatic storage duration. Take advantage of what the standard library provides.
As it was pointed out already: use STL containers such as std::vector instead of C-style arrays:

std::ifstream ifs(path);
std::vector<std::string> lines;

std::string line;
while ( std::getline(ifs, line) )
{
    // skip empty lines:
    if (line.empty())
        continue;

    lines.push_back(line);
}

I know this works, but I'm confused as to using the result of `std::getline` as the condition for the `while` loop. How does the stream returned by `std::getline` cause the loop to continue or stop? — user3731622, Mar 23 '16 at 17:25
`while ( std::getline(ifs, line) )` won't work if you need to synchronize access to the stream with locks in a multi-threaded environment — ajeh, Apr 03 '18 at 15:15

score 10 · Answer 2 · answered Oct 01 '13 at 18:33

Why so trouble ?

Simply use std:vector of std::string

std::string str;

std::vector <std::string> vec;

while ( std::getline(ifs,str) )
{
  vec.push_back(str) ;
}

If you really need an array of string

do :

string * in_file = new string[vec.size()];

And copy the elements from vec to in_file

for(size_t i=0;i<vec.size();i++)
 in_file[i] = vec[i];

How to use std::getline() to read a text file into an array of strings in C++?

2 Answers2