std::string from file input

[Alexandros Frantzis]

Hello,
I am trying to read a big file into a string. AFAIK std::string can read
in words or "lines" from a stream. So one option is to continuously
append all the "lines" from the file to the string.

The second option is to use the istream::read(Ch *p,streamsize n)
function and then create a std::string from 'buf'. However this is a
waste of time and space because (if I am not mistaken) the std::string
constructor will make a copy of the character array pointed to by buf.

Does anyone know any other (fast) way of doing this?

Thanks in advance,
Alexandros Frantzis

 

[Mike Wahler]

Hello,
I am trying to read a big file into a string. AFAIK std::string can read
in words or "lines" from a stream. So one option is to continuously
append all the "lines" from the file to the string.

The second option is to use the istream::read(Ch *p,streamsize n)
function and then create a std::string from 'buf'. However this is a
waste of time and space because (if I am not mistaken) the std::string
constructor will make a copy of the character array pointed to by buf.

Does anyone know any other (fast) way of doing this?

Thanks in advance,
Alexandros Frantzis

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

int main()
{
std::string filename("myfile");

std::ifstream ifs(filename.c_str());
if(!ifs)
{
std::cerr << "Cannot open file '" << filename << "'\n";
return EXIT_FAILURE;
}

std:stringstream oss;
oss << ifs.rdbuf();

if(!ifs && !ifs.eof())
std::cerr << "Error reading file '" << filename << "'\n";

std::string contents(oss.str());
std::cout << "file_contents:\n" << contents << '\n';

return 0;
}


-Mike

 

[Thomas Matthews]

Hello,
I am trying to read a big file into a string. AFAIK std::string can read
in words or "lines" from a stream. So one option is to continuously
append all the "lines" from the file to the string.

The second option is to use the istream::read(Ch *p,streamsize n)
function and then create a std::string from 'buf'. However this is a
waste of time and space because (if I am not mistaken) the std::string
constructor will make a copy of the character array pointed to by buf.

Does anyone know any other (fast) way of doing this?

Thanks in advance,
Alexandros Frantzis

I would never read an entire file into a string, since
files are notorisly bigger than a computers memory.

One method is to use a std::vector and ifstream::read().
The std::vector will expand as needed. There are ways to
optimize this by reserving extra space.

Is there a need to read the entire file into memory?

--
Thomas Matthews

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Other sites:
http://www.josuttis.com -- C++ STL Library book

 

[Jerry Coffin]

Hello,
I am trying to read a big file into a string. AFAIK std::string can read
in words or "lines" from a stream. So one option is to continuously
append all the "lines" from the file to the string.

The second option is to use the istream::read(Ch *p,streamsize n)
function and then create a std::string from 'buf'. However this is a
waste of time and space because (if I am not mistaken) the std::string
constructor will make a copy of the character array pointed to by buf.

The usual way is to read from the stream's buffer into a stringstream:

std:stringstream buffer;
buffer << input.rdbuf();

at this point, buffer.str() is an std::string containing the entire
contents of the file.