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I need to read a whole file into memory and place it in a C++ std::string.

If I were to read it into a char[], the answer would be very simple:

std::ifstream t; int length; t.open("file.txt"); // open input file t.seekg(0, std::ios::end); // go to the end length = t.tellg(); // report location (this is the length) t.seekg(0, std::ios::beg); // go back to the beginning buffer = new char[length]; // allocate memory for a buffer of appropriate dimension t.read(buffer, length); // read the whole file into the buffer t.close(); // close file handle // ... Do stuff with buffer here ...

Now, I want to do the exact same thing, but using a std::string instead of a char[]. I want to avoid loops, i.e. I don't want to:

std::ifstream t; t.open("file.txt"); std::string buffer; std::string line; while(t){ std::getline(t, line); // ... Append line to buffer and go on } t.close()

Any ideas?

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  • 3
    There will always be a loop involved, but it can be implicit as part of the standard library. Is that acceptable? Why are you trying to avoid loops? Commented Apr 8, 2010 at 19:22
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    I believe that the poster knew that reading bytes involved looping. He just wanted an easy, perl-style gulp equivalent. That involved writing little code. Commented Jan 5, 2012 at 10:57
  • This code is buggy, in the event that the std::string doesn't use a continuous buffer for its string data (which is allowed): stackoverflow.com/a/1043318/1602642 Commented Jul 30, 2013 at 18:34
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    @ChrisDesjardins: (1) Your link is outdated (C++11 made it contiguous) and (2) even if it wasn't, std::getline(istream&, std::string&) would still do the right thing. Commented Nov 23, 2015 at 13:50
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    Side note for anyone looking at this code: The code presented as an example for reading into char[] does not null-terminate the array (read does not do this automatically), which may not be what you expect. Commented Apr 11, 2016 at 1:00

9 Answers 9

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1142

There are a couple of possibilities. One I like uses a stringstream as a go-between:

std::ifstream t("file.txt"); std::stringstream buffer; buffer << t.rdbuf();

Now the contents of "file.txt" are available in a string as buffer.str().

Another possibility (though I certainly don't like it as well) is much more like your original:

std::ifstream t("file.txt"); t.seekg(0, std::ios::end); size_t size = t.tellg(); std::string buffer(size, ' '); t.seekg(0); t.read(&buffer[0], size);

Officially, this isn't required to work under the C++98 or 03 standard (string isn't required to store data contiguously) but in fact it works with all known implementations, and C++11 and later do require contiguous storage, so it's guaranteed to work with them.

Starting with C++ 17, you can simplify the latter a bit though, by using std::filesystem::file_size to get the size of the file:

auto size = std::filesystem::file_size(filename); std::string content(size, '\0'); std::ifstream in(filename); in.read(&content[0], size);

Personally, I prefer the first version, at least for small files. The last is generally more efficient if the file is very large. The second is more complex and less readable, so I like it the least, but it does provide compatibility with older (pre-C++ 17) compilers, if you need that.

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43 Comments

Important note for some, at least on my implementation, the three-liner works at least as good as the C fopen alternative for files under 50KB. Past that, it seems to lose performance fast. In which case, just use the second solution.
make sure to #include <sstream>
Should also check to see if the file has opened, e.g., if (!t) std::cerr << "Error opening file." << std::endl;. Of course, don't forget to close the file as well when you are done.
Most of the time, you're fine not testing whether the file has opened (the other operations will simply fail). As a rule, you should avoid printing out error messages on the spot, unless you're sure that fits with the rest of the program -- if you must do something, throwing an exception is usually preferable. You should almost never explicitly close a file either -- the destructor will do that automatically.
Wouldn't constructing an empty string and then calling reserve(size) on it be more efficient?
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Update: Turns out that this method, while following STL idioms well, is actually surprisingly inefficient! Don't do this with large files. (See: http://insanecoding.blogspot.com/2011/11/how-to-read-in-file-in-c.html)

You can make a streambuf iterator out of the file and initialize the string with it:

#include <string> #include <fstream> #include <streambuf> std::ifstream t("file.txt"); std::string str((std::istreambuf_iterator<char>(t)), std::istreambuf_iterator<char>());

Not sure where you're getting the t.open("file.txt", "r") syntax from. As far as I know that's not a method that std::ifstream has. It looks like you've confused it with C's fopen.

Edit: Also note the extra parentheses around the first argument to the string constructor. These are essential. They prevent the problem known as the "most vexing parse", which in this case won't actually give you a compile error like it usually does, but will give you interesting (read: wrong) results.

Following KeithB's point in the comments, here's a way to do it that allocates all the memory up front (rather than relying on the string class's automatic reallocation):

#include <string> #include <fstream> #include <streambuf> std::ifstream t("file.txt"); std::string str; t.seekg(0, std::ios::end); str.reserve(t.tellg()); t.seekg(0, std::ios::beg); str.assign((std::istreambuf_iterator<char>(t)), std::istreambuf_iterator<char>());
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26 Comments

open is definitely a method of ifstream, however the 2nd parameter is wrong. cplusplus.com/reference/iostream/ifstream/open
@KeithB If efficiency is important, you could find the file length the same was as in the char* example and call std::string::reserve to preallocate the necessary space.
No sure why people are voting this up, here is a quick question, say I have a 1MB file, how many times will the "end" passed to the std::string constructor or assign method be invoked? People think these kind of solutions are elegant when in fact they are excellent examples of HOW NOT TO DO IT.
Benchmarked: both Tyler's solutions take about 21 seconds on a 267 MB file. Jerry's first takes 1.2 seconds and his second 0.5 (+/- 0.1), so clearly there's something inefficient about Tyler's code.
The insanecoding blog post is benchmarking solutions to a slightly different problem: it is reading the file as binary not text, so there's no translation of line endings. As a side effect, reading as binary makes ftell a reliable way to get the file length (assuming a long can represent the file length, which is not guaranteed). For determining the length, ftell is not reliable on a text stream. If you're reading a file from tape (e.g., a backup), the extra seeking may be a waste of time. Many of the blog post implementations don't use RAII and can therefore leak if there's an error.
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I think best way is to use string stream. simple and quick !!!

#include <fstream> #include <iostream> #include <sstream> //std::stringstream int main() { std::ifstream inFile; inFile.open("inFileName"); //open the input file std::stringstream strStream; strStream << inFile.rdbuf(); //read the file std::string str = strStream.str(); //str holds the content of the file std::cout << str << "\n"; //you can do anything with the string!!! }
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8 Comments

Simple and quick, right! insanecoding.blogspot.com/2011/11/how-to-read-in-file-in-c.html
Remember to close the stream afterwards...
@YngveSneenLindal Or let the destructor do it automatically - take advantage of C++!
@YngveSneenLindal are you sure you need to close the stream afterwards? Apparently the fstream's memory allocation should be deallocated once fstream is destroyed (out of the scope)? Although it can be good to use .close() for error checking?
Why post this when it was already in the answer by Jerry Coffin from years before?
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You may not find this in any book or site, but I found out that it works pretty well:

#include <fstream> // ... std::string file_content; std::getline(std::ifstream("filename.txt"), file_content, '\0');
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6 Comments

Casting eof to (char) is a bit dodgy, suggesting some kind of relevance and universality which is illusory. For some possible values of eof() and signed char, it will give implementation-defined results. Directly using e.g. char(0) / '\0' would be more robust and honestly indicative of what's happening.
@TonyD. Good point about converting eof() to a char. I suppose for old-school ascii character sets, passing any negative value (msb set to 1) would work. But passing \0 (or a negative value) won't work for wide or multi-byte input files.
This will only work, as long as there are no "eof" (e.g. 0x00, 0xff, ...) characters in your file. If there are, you will only read part of the file.
@OlafDietsche There shouldn't be 0x00 in an ASCII file (or I wouldn't call it ASCII file). 0x00 appears to me like a good option to force the getline() to read the whole file. And, I must admit that this code is as short as easy to read although the higher voted solutions look much more impressive and sophisticated.
@Scheff After revisiting this answer, I don't know, how I reached to that conclusion and comment. Maybe I thought, that (char) ifs.eof() has some meaning. eof() returns false at this point, and the call is equivalent to std::getline(ifs, s, 0);. So it reads until the first 0 byte, or the end of file, if there's no 0 byte.
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Try one of these two methods:

string get_file_string(){ std::ifstream ifs("path_to_file"); return string((std::istreambuf_iterator<char>(ifs)), (std::istreambuf_iterator<char>())); } string get_file_string2(){ ifstream inFile; inFile.open("path_to_file");//open the input file stringstream strStream; strStream << inFile.rdbuf();//read the file return strStream.str();//str holds the content of the file }
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3

I figured out another way that works with most istreams, including std::cin!

std::string readFile() { stringstream str; ifstream stream("Hello_World.txt"); if(stream.is_open()) { while(stream.peek() != EOF) { str << (char) stream.get(); } stream.close(); return str.str(); } }
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1

If you happen to use glibmm you can try Glib::file_get_contents.

#include <iostream> #include <glibmm.h> int main() { auto filename = "my-file.txt"; try { std::string contents = Glib::file_get_contents(filename); std::cout << "File data:\n" << contents << std::endl; catch (const Glib::FileError& e) { std::cout << "Oops, an error occurred:\n" << e.what() << std::endl; } return 0; }
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1 Comment

Imho: Although this works, providing a "glib" solution, which is the non-platform-independent equivalent of pandora's chest, might confuse enormously, even more, if there's a simple CPP-standard solution to it.
0

I could do it like this:

void readfile(const std::string &filepath,std::string &buffer){ std::ifstream fin(filepath.c_str()); getline(fin, buffer, char(-1)); fin.close(); }

If this is something to be frowned upon, please let me know why

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2 Comments

char(-1) is probably not a portable way to denote EOF. Also, getline() implementations are not required to support the "invalid" EOF pseudo-character as a delimiter character, I think.
@reddish it indeed isn't, in modern C++ it is better to use std::char_traits<char>::eof(). If someone is still using an ancient compiler... <cstdio> contains EOF macro.
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I don't think you can do this without an explicit or implicit loop, without reading into a char array (or some other container) first and ten constructing the string. If you don't need the other capabilities of a string, it could be done with vector<char> the same way you are currently using a char *.

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1 Comment

Well, to be fair, all of the answers above do include a loop in some way, be it as a boilerplate or behind the scenes...

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