Is there a one-liner to read in a file to a string in C++?

Question

I need a quick easy way to get a string from a file in standard C++. I can write my own, but just want to know if there is already a standard way, in C++.

Equivalent of this if you know Cocoa:

NSString *string = [NSString stringWithContentsOfFile:file];

Answer 1

We can do it but it's a long line :

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

using namespace std;

int main()
{
    // The one-liner
    string fileContents(istreambuf_iterator<char>(ifstream("filename.txt")), istreambuf_iterator<char>());

    // Check result
    cout << fileContents;
}

Edited : use "istreambuf_iterator" instead of "istream_iterator"

Answer 2

Its almost possible with an istream_iterator (3 lines!)

#include <iostream>
#include <fstream>
#include <iterator>
#include <string>
#include <sstream>

using namespace std;

int main()
{
    ifstream file("filename.txt");
    string fileContents;

    copy(istreambuf_iterator<char>(file),
              istreambuf_iterator<char>(),
              back_inserter(fileContents));
}

Edited - got rid of intermediate string stream, now copies straight into the string, and now using istreambuf_iterator, which ignores whitespace (thanks Martin York for your comment).

Answer 3

The standard C++ library doesn't provide a function to do this.

Answer 4

Best I can do is 5 lines:

#include <fstream>
#include <vector>
using namespace std;

ifstream f("filename.txt");
f.seekg(0, ios::end);
vector<char> buffer(f.tellg());
f.seekg(0, ios::beg);
f.read(&buffer[0], buffer.size());

Answer 5

How about:

#include <fstream>
#include <sstream>
#include <iostream>

using namespace std;

int main( void )
{
  stringstream os(stringstream::out);
  os << ifstream("filename.txt").rdbuf();
  string s(os.str());
  cout << s << endl;
}

Answer 6

If you do it like the following (but properly wrapped up nicely unlike below), you can read in the file without worrying about a 0x1A byte in the file (for example) cutting the reading of the file short. The previously suggested methods will choke on a 0x1A (for example) in a file.

#include <iostream>
#include <cstdio>
#include <vector>
#include <cstdlib>
using namespace std;

int main() {
    FILE* in = fopen("filename.txt", "rb");
    if (in == NULL) {
        return EXIT_FAILURE;
    }
    if (fseek(in, 0, SEEK_END) != 0) {
        fclose(in);
        return EXIT_FAILURE;
    }
    const long filesize = ftell(in);
    if (filesize == -1) {
        fclose(in);
        return EXIT_FAILURE;
    }
    vector<unsigned char> buffer(filesize);
    if (fseek(in, 0, SEEK_SET) != 0 || fread(&buffer[0], sizeof(buffer[0]), buffer.size(), in) != buffer.size() || ferror(in) != 0) {
        fclose(in);
        return EXIT_FAILURE;
    }
    fclose(in);
}

But, yeh, it's not an already-implemented 1-liner though.

Edit: 0x1A wasn't a good example as ios_base::binary will cover that. However, even then C++ streams often give me trouble when reading in png files all at once with .read(). Using the C way works better. Just can't remember a good example to show why. It was probably with .read()ing a binary file in blocks in a loop instead that can be a problem with C++ streams. So, disregard this post.

Answer 7

std::string temp, file; std::ifstream if(filename); while(getline(if, temp)) file += temp;

It's not a short or single-statement line, but it is one line and it's really not that bad.