Home » C++ » Read file-contents into a string in C++ [duplicate]

Read file-contents into a string in C++ [duplicate]

Posted by: admin November 30, 2017 Leave a comment

Questions:

Possible Duplicate:
What is the best way to slurp a file into a std::string in c++?

In scripting languages like Perl, it is possible to read a file into a variable in one shot.

    open(FILEHANDLE,$file);
    $content=<FILEHANDLE>;

What would be the most efficient way to do this in C++?

Answers:

Like this:

#include <fstream>
#include <string>

int main(int argc, char** argv)
{

  std::ifstream ifs("myfile.txt");
  std::string content( (std::istreambuf_iterator<char>(ifs) ),
                       (std::istreambuf_iterator<char>()    ) );

  return 0;
}

The statement

  std::string content( (std::istreambuf_iterator<char>(ifs) ),
                       (std::istreambuf_iterator<char>()    ) );

can be split into

std::string content;
content.assign( (std::istreambuf_iterator<char>(ifs) ),
                (std::istreambuf_iterator<char>()    ) );

which is useful if you want to just overwrite the value of an existing std::string variable.

Questions:
Answers:

The most efficient, but not the C++ way would be:

   FILE* f = fopen(filename, "r");

   // Determine file size
   fseek(f, 0, SEEK_END);
   size_t size = ftell(f);

   char* where = new char[size];

   rewind(f);
   fread(where, sizeof(char), size, f);

   delete[] where;

#EDIT – 2

Just tested the std::filebuf variant also. Looks like it can be called the best C++ approach, even though it’s not quite a C++ approach, but more a wrapper. Anyway, here is the chunk of code that works almost as fast as plain C does.

   std::ifstream file(filename, std::ios::binary);
   std::streambuf* raw_buffer = file.rdbuf();

   char* block = new char[size];
   raw_buffer->sgetn(block, size);
   delete[] block;

I’ve done a quick benchmark here and the results are following. Test was done on reading a 65536K binary file with appropriate (std::ios:binary and rb) modes.

[==========] Running 3 tests from 1 test case.
[----------] Global test environment set-up.
[----------] 4 tests from IO
[ RUN      ] IO.C_Kotti
[       OK ] IO.C_Kotti (78 ms)
[ RUN      ] IO.CPP_Nikko
[       OK ] IO.CPP_Nikko (106 ms)
[ RUN      ] IO.CPP_Beckmann
[       OK ] IO.CPP_Beckmann (1891 ms)
[ RUN      ] IO.CPP_Neil
[       OK ] IO.CPP_Neil (234 ms)
[----------] 4 tests from IO (2309 ms total)

[----------] Global test environment tear-down
[==========] 4 tests from 1 test case ran. (2309 ms total)
[  PASSED  ] 4 tests.

Questions:
Answers:

The most efficient is to create a buffer of the correct size and then read the file into the buffer.

#include <fstream>
#include <vector>

int main()
{
    std::ifstream       file("Plop");
    if (file)
    {
        /*
         * Get the size of the file
         */
        file.seekg(0,std::ios::end);
        std::streampos          length = file.tellg();
        file.seekg(0,std::ios::beg);

        /*
         * Use a vector as the buffer.
         * It is exception safe and will be tidied up correctly.
         * This constructor creates a buffer of the correct length.
         * Because char is a POD data type it is not initialized.
         *
         * Then read the whole file into the buffer.
         */
        std::vector<char>       buffer(length);
        file.read(&buffer[0],length);
    }
}

Questions:
Answers:

There should be no \0 in text files.

#include<iostream>
#include<fstream>

using namespace std;

int main(){
  fstream f(FILENAME, fstream::in );
  string s;
  getline( f, s, '
#include<iostream> #include<fstream> using namespace std; int main(){ fstream f(FILENAME, fstream::in ); string s; getline( f, s, '\0'); cout << s << endl; f.close(); } 
'); cout << s << endl; f.close(); }

Questions:
Answers:

This depends on a lot of things, such as what is the size of the file, what is its type (text/binary) etc. Some time ago I benchmarked the following function against versions using streambuf iterators – it was about twice as fast:

unsigned int FileRead( std::istream & is, std::vector <char> & buff ) {
    is.read( &buff[0], buff.size() );
    return is.gcount();
}

void FileRead( std::ifstream & ifs, string & s ) {
    const unsigned int BUFSIZE = 64 * 1024; // reasoable sized buffer
    std::vector <char> buffer( BUFSIZE );

    while( unsigned int n = FileRead( ifs, buffer ) ) {
        s.append( &buffer[0], n );
    }
}

Questions:
Answers:

maybe not the most efficient, but reads data in one line:

#include<iostream>
#include<vector>
#include<iterator>

main(int argc,char *argv[]){
  // read standard input into vector:
  std::vector<char>v(std::istream_iterator<char>(std::cin),
                     std::istream_iterator<char>());
  std::cout << "read " << v.size() << "chars\n";
}

Questions:
Answers:

Here’s an iterator-based method.

ifstream file("file", ios::binary);
string fileStr;

istreambuf_iterator<char> inputIt(file), emptyInputIt
back_insert_iterator<string> stringInsert(fileStr);

copy(inputIt, emptyInputIt, stringInsert);