I'm trying to read an entire stream (multiple lines) into a string.
I'm using this code, and it works, but it's offending my sense of style... Surely there's an easier way? Maybe using stringstreams?
void Obj::loadFromStream(std::istream & stream)
{
std::string s;
std::streampos p = stream.tellg(); // remember where we are
stream.seekg(0, std::ios_base::end); // go to the end
std::streamoff sz = stream.tellg() - p; // work out the size
stream.seekg(p); // restore the position
s.resize(sz); // resize the string
stream.read(&s[0], sz); // and finally, read in the data.
Actually, a const reference to a string would do as well, and that may make things easier...
const std::string &s(... a miracle occurs here...)
How about
std::istreambuf_iterator<char> eos;
std::string s(std::istreambuf_iterator<char>(stream), eos);
(could be a one-liner if not for MVP)
post-2011 edit, this approach is now spelled
std::string s(std::istreambuf_iterator<char>(stream), {});
I'm late to the party, but here is a fairly efficient solution:
std::string gulp(std::istream &in)
{
std::string ret;
char buffer[4096];
while (in.read(buffer, sizeof(buffer)))
ret.append(buffer, sizeof(buffer));
ret.append(buffer, in.gcount());
return ret;
}
I did some benchmarking, and it turns out that the std::istreambuf_iterator technique (used by the accepted answer) is actually much slower. On gcc 4.4.5 with -O3, it's about a 4.5x difference on my machine, and the gap becomes wider with lower optimization settings.
You could do
std::string s;
std::ostringstream os;
os<<stream.rdbuf();
s=os.str();
but I don't know if it's more efficient.
Alternative version:
std::string s;
std::ostringstream os;
stream>>os.rdbuf();
s=os.str();
You can try using something from algorithms. I have to get ready for work but here's a very quick stab at things (there's got to be a better way):
copy( istreambuf_iterator<char>(stream), istreambuf_iterator<char>(), back_inserter(s) );
Well, if you are looking for a simple and 'readable' way to do it. I would recomend add/use some high level framework on your project. For that I's always use Poco and Boost on all my projects. In this case, with Poco:
string text;
FileStream fstream(TEXT_FILE_PATH);
StreamCopier::copyToString(fstream, text);
Perhaps this 1 line C++11 solution:
std::vector<char> s{std::istreambuf_iterator<char>{in},{}};
What about to use getline with delimiter? The next code helps me to read whole std::cin into string on ubuntu with g++-10.
#include <iostream>
#include <string>
using namespace std;
int main() {
string s;
getline(cin, s, {}); //the whole stream into variable s
return 0;
}
Related
This question already has answers here:
How do I read an entire file into a std::string in C++?
(23 answers)
Closed 6 years ago.
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?
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.
As to why I don't like the latter as well: first, because it's longer and harder to read. Second, because it requires that you initialize the contents of the string with data you don't care about, then immediately write over that data (yes, the time to initialize is usually trivial compared to the reading, so it probably doesn't matter, but to me it still feels kind of wrong). Third, in a text file, position X in the file doesn't necessarily mean you'll have read X characters to reach that point -- it's not required to take into account things like line-end translations. On real systems that do such translations (e.g., Windows) the translated form is shorter than what's in the file (i.e., "\r\n" in the file becomes "\n" in the translated string) so all you've done is reserved a little extra space you never use. Again, doesn't really cause a major problem but feels a little wrong anyway.
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>());
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!!!
}
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');
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
}
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();
}
}
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;
}
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
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 *.
This question already has answers here:
How do I read an entire file into a std::string in C++?
(23 answers)
Closed 6 years ago.
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?
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.
As to why I don't like the latter as well: first, because it's longer and harder to read. Second, because it requires that you initialize the contents of the string with data you don't care about, then immediately write over that data (yes, the time to initialize is usually trivial compared to the reading, so it probably doesn't matter, but to me it still feels kind of wrong). Third, in a text file, position X in the file doesn't necessarily mean you'll have read X characters to reach that point -- it's not required to take into account things like line-end translations. On real systems that do such translations (e.g., Windows) the translated form is shorter than what's in the file (i.e., "\r\n" in the file becomes "\n" in the translated string) so all you've done is reserved a little extra space you never use. Again, doesn't really cause a major problem but feels a little wrong anyway.
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>());
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!!!
}
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');
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
}
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();
}
}
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;
}
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
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 *.
Apparently boost::asio::async_read doesn't like strings, as the only overload of boost::asio::buffer allows me to create const_buffers, so I'm stuck with reading everything into a streambuf.
Now I want to copy the contents of the streambuf into a string, but it apparently only supports writing to char* (sgetn()), creating an istream with the streambuf and using getline().
Is there any other way to create a string with the streambufs contents without excessive copying?
I don't know whether it counts as "excessive copying", but you can use a stringstream:
std::ostringstream ss;
ss << someStreamBuf;
std::string s = ss.str();
Like, to read everything from stdin into a string, do
std::ostringstream ss;
ss << std::cin.rdbuf();
std::string s = ss.str();
Alternatively, you may also use a istreambuf_iterator. You will have to measure whether this or the above way is faster - i don't know.
std::string s((istreambuf_iterator<char>(someStreamBuf)),
istreambuf_iterator<char>());
Note that someStreamBuf above is meant to represent a streambuf*, so take its address as appropriate. Also note the additional parentheses around the first argument in the last example, so that it doesn't interpret it as a function declaration returning a string and taking an iterator and another function pointer ("most vexing parse").
It's really buried in the docs...
Given boost::asio::streambuf b, with size_t buf_size ...
boost::asio::streambuf::const_buffers_type bufs = b.data();
std::string str(boost::asio::buffers_begin(bufs),
boost::asio::buffers_begin(bufs) + buf_size);
Another possibility with boost::asio::streambuf is to use boost::asio::buffer_cast<const char*>() in conjunction with boost::asio::streambuf::data() and boost::asio::streambuf::consume() like this:
const char* header=boost::asio::buffer_cast<const char*>(readbuffer.data());
//Do stuff with header, maybe construct a std::string with std::string(header,header+length)
readbuffer.consume(length);
This won't work with normal streambufs and might be considered dirty, but it seems to be the fastest way of doing it.
For boost::asio::streambuf you may find a solution like this:
boost::asio::streambuf buf;
/*put data into buf*/
std::istream is(&buf);
std::string line;
std::getline(is, line);
Print out the string :
std::cout << line << std::endl;
You may find here: http://www.boost.org/doc/libs/1_49_0/doc/html/boost_asio/reference/async_read_until/overload3.html
One can also obtain the characters from asio::streambuf using std::basic_streambuf::sgetn:
asio::streambuf in;
// ...
char cbuf[in.size()+1]; int rc = in.sgetn (cbuf, sizeof cbuf); cbuf[rc] = 0;
std::string str (cbuf, rc);
The reason you can only create const_buffer from std::string is because std::string explicitly doesn't support direct pointer-based writing in its contract. You could do something evil like resize your string to a certain size, then const_cast the constness from c_str() and treat it like a raw char* buffer, but that's very naughty and will get you in trouble someday.
I use std::vector for my buffers because as long as the vector doesn't resize (or you are careful to deal with resizing), you can do direct pointer writing just fine. If I need some of the data as a std::string, I have to copy it out, but the way I deal with my read buffers, anything that needs to last beyond the read callback needs to be copied out regardless.
I didn't see an existing answer for reading exactly n chars into a std::stringstream, so here is how that can be done:
std::stringstream ss;
boost::asio::streambuf sb;
const auto len = 10;
std::copy_n(boost::asio::buffers_begin(sb.data()), len,
std::ostream_iterator<decltype(ss)::char_type>(ss));
Compiler explorer
A simpler answer would be to convert it in std::string and manipulate it some what like this
std::string buffer_to_string(const boost::asio::streambuf &buffer)
{
using boost::asio::buffers_begin;
auto bufs = buffer.data();
std::string result(buffers_begin(bufs), buffers_begin(bufs) + buffer.size());
return result;
}
Giving a very concise code for the task.
I mostly don't like answers that say "You don't want X, you want Y instead and here's how to do Y" but in this instance I'm pretty sure I know what tstenner wanted.
In Boost 1.66, the dynamic string buffer type was added so async_read can directly resize and write to a string buffer.
I tested the first answer and got a compiler error when compiling using "g++ -std=c++11"
What worked for me was:
#include <string>
#include <boost/asio.hpp>
#include <sstream>
//other code ...
boost::asio::streambuf response;
//more code
std::ostringstream sline;
sline << &response; //need '&' or you a compiler error
std::string line = sline.str();
This compiled and ran.
I think it's more like:
streambuf.commit( number_of_bytes_read );
istream istr( &streambuf );
string s;
istr >> s;
I haven't looked into the basic_streambuf code, but I believe that should be just one copy into the string.
This question already has answers here:
How do I read an entire file into a std::string in C++?
(23 answers)
Closed 6 years ago.
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?
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.
As to why I don't like the latter as well: first, because it's longer and harder to read. Second, because it requires that you initialize the contents of the string with data you don't care about, then immediately write over that data (yes, the time to initialize is usually trivial compared to the reading, so it probably doesn't matter, but to me it still feels kind of wrong). Third, in a text file, position X in the file doesn't necessarily mean you'll have read X characters to reach that point -- it's not required to take into account things like line-end translations. On real systems that do such translations (e.g., Windows) the translated form is shorter than what's in the file (i.e., "\r\n" in the file becomes "\n" in the translated string) so all you've done is reserved a little extra space you never use. Again, doesn't really cause a major problem but feels a little wrong anyway.
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>());
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!!!
}
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');
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
}
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();
}
}
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;
}
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
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 *.
I currently do this, and the conversion to std::string at the end take 98% of the execution time. There must be a better way!
std::string
file2string(std::string filename)
{
std::ifstream file(filename.c_str());
if(!file.is_open()){
// If they passed a bad file name, or one we have no read access to,
// we pass back an empty string.
return "";
}
// find out how much data there is
file.seekg(0,std::ios::end);
std::streampos length = file.tellg();
file.seekg(0,std::ios::beg);
// Get a vector that size and
std::vector<char> buf(length);
// Fill the buffer with the size
file.read(&buf[0],length);
file.close();
// return buffer as string
std::string s(buf.begin(),buf.end());
return s;
}
Being a big fan of C++ iterator abstraction and the algorithms, I would love the following to be the fasted way to read a file (or any other input stream) into a std::string (and then print the content):
#include <algorithm>
#include <fstream>
#include <iostream>
#include <iterator>
#include <string>
int main()
{
std::string s(std::istreambuf_iterator<char>(std::ifstream("file")
>> std::skipws),
std::istreambuf_iterator<char>());
std::cout << "file='" << s << "'\n";
}
This certainly is fast for my own implementation of IOStreams but it requires a lot of trickery to actually get it fast. Primarily, it requires optimizing algorithms to cope with segmented sequences: a stream can be seen as a sequence of input buffers. I'm not aware of any STL implementation consistently doing this optimization. The odd use of std::skipws is just to get reference to the just created stream: the std::istreambuf_iterator<char> expects a reference to which the temporary file stream wouldn't bind.
Since this probably isn't the fastest approach, I would be inclined to use std::getline() with a particular "newline" character, i.e. on which isn't in the file:
std::string s;
// optionally reserve space although I wouldn't be too fuzzed about the
// reallocations because the reads probably dominate the performances
std::getline(std::ifstream("file") >> std::skipws, s, 0);
This assumes that the file doesn't contain a null character. Any other character would do as well. Unfortunately, std::getline() takes a char_type as delimiting argument, rather than an int_type which is what the member std::istream::getline() takes for the delimiter: in this case you could use eof() for a character which never occurs (char_type, int_type, and eof() refer to the respective member of char_traits<char>). The member version, in turn, can't be used because you would need to know ahead of time how many characters are in the file.
BTW, I saw some attempts to use seeking to determine the size of the file. This is bound not to work too well. The problem is that the code conversion done in std::ifstream (well, actually in std::filebuf) can create a different number of characters than there are bytes in the file. Admittedly, this isn't the case when using the default C locale and it is possible to detect that this doesn't do any conversion. Otherwise the best bet for the stream would be to run over the file and determine the number of characters being produced. I actually think that this is what would be needed to be done when the code conversion could something interesting although I don't think it actually is done. However, none of the examples explicitly set up the C locale, using e.g. std::locale::global(std::locale("C"));. Even with this it is also necessary to open the file in std::ios_base::binary mode because otherwise end of line sequences may be replaced by a single character when reading. Admittedly, this would only make the result shorter, never longer.
The other approaches using the extraction from std::streambuf* (i.e. those involving rdbuf()) all require that the resulting content is copied at some point. Given that the file may actually be very large this may not be an option. Without the copy this could very well be the fastest approach, however. To avoid the copy, it would be possible to create a simple custom stream buffer which takes a reference to a std::string as constructor argument and directly appends to this std::string:
#include <fstream>
#include <iostream>
#include <string>
class custombuf:
public std::streambuf
{
public:
custombuf(std::string& target): target_(target) {
this->setp(this->buffer_, this->buffer_ + bufsize - 1);
}
private:
std::string& target_;
enum { bufsize = 8192 };
char buffer_[bufsize];
int overflow(int c) {
if (!traits_type::eq_int_type(c, traits_type::eof()))
{
*this->pptr() = traits_type::to_char_type(c);
this->pbump(1);
}
this->target_.append(this->pbase(), this->pptr() - this->pbase());
this->setp(this->buffer_, this->buffer_ + bufsize - 1);
return traits_type::not_eof(c);
}
int sync() { this->overflow(traits_type::eof()); return 0; }
};
int main()
{
std::string s;
custombuf sbuf(s);
if (std::ostream(&sbuf)
<< std::ifstream("readfile.cpp").rdbuf()
<< std::flush) {
std::cout << "file='" << s << "'\n";
}
else {
std::cout << "failed to read file\n";
}
}
At least with a suitably chosen buffer I would expect the version to be the fairly fast. Which version is the fastest will certainly depend on the system, the standard C++ library being used, and probably a number of other factors, i.e. you want to measure the performance.
You can try this:
#include <fstream>
#include <sstream>
#include <string>
int main()
{
std::ostringstream oss;
std::string s;
std::string filename = get_file_name();
if (oss << std::ifstream(filename, std::ios::binary).rdbuf())
{
s = oss.str();
}
else
{
// error
}
// now s contains your file
}
You can also just use oss.str() directly if you like; just make sure you have some sort of error check somewhere.
No guarantee that it's the most efficient; you probably can't beat <cstdio> and fread. As #Benjamin pointed out, the string stream only exposes the data by copy, so you could instead read directly into the target string:
#include <string>
#include <cstdio>
std::FILE * fp = std::fopen("file.bin", "rb");
std::fseek(fp, 0L, SEEK_END);
unsigned int fsize = std::ftell(fp);
std::rewind(fp);
std::string s(fsize, 0);
if (fsize != std::fread(static_cast<void*>(&s[0]), 1, fsize, fp))
{
// error
}
std::fclose(fp);
(You might like to use a RAII wrapper for the FILE*.)
Edit: The fstream-analogue of the second version goes like this:
#include <string>
#include <fstream>
std::ifstream infile("file.bin", std::ios::binary);
infile.seekg(0, std::ios::end);
unsigned int fsize = infile.tellg();
infile.seekg(0, std::ios::beg);
std::string s(fsize, 0);
if (!infile.read(&s[0], fsize))
{
// error
}
Edit: Yet another version, using streambuf-iterators:
std::ifstream thefile(filename, std::ios::binary);
std::string s((std::istreambuf_iterator<char>(thefile)), std::istreambuf_iterator<char>());
(Mind the aditional parentheses to get the correct parsing.)
Ironically, the example for string::reserve is reading a file into a string. You don't want to read the file into one buffer and then have to allocate/copy into another one.
Here's the example code:
// string::reserve
#include <iostream>
#include <fstream>
#include <string>
using namespace std;
int main ()
{
string str;
size_t filesize;
ifstream file ("test.txt",ios::in|ios::ate);
filesize=file.tellg();
str.reserve(filesize); // allocate space in the string
file.seekg(0);
for (char c; file.get(c); )
{
str += c;
}
cout << str;
return 0;
}
I don't know how efficient it is, but here is a simple (to read) way, by just setting the EOF as the delimiter:
string buffer;
ifstream fin;
fin.open("filename.txt");
if(fin.is_open()) {
getline(fin,buffer,'\x1A');
fin.close();
}
The efficiency of this obviously depends on what's going on internally in the getline algorithm, so you could take a look at the code in the standard libraries to see how it works.