ifstream and put it in a string

[nvthielen]

Hello C++'s,

I am trying to open a TEXT file and put it in a string.

string line;

ifstream infile;
infile.open("data_01.LIB");
infile.read(line, 20);

but it gif me a response the line is not a valid, variable.

[AbstractionAnon]

Two problems.

1) You don't check that the open of the file succeeded.
Always use if (file.is_open()) to check.

2) You can't read directly into line. std:string is not a Plain Old Data (POD) data type.
It keeps information about the string in the object. If you read directly into the object, you wipe out that information. You want getline().

[coder777]

Depending on what you are trying to read it might be a good idea to use std::getline(...). See:

https://cplusplus.com/reference/string/string/getline/?kw=getline

Note that this will read the data until a '\n' is reached. But you might use another delimiter.

[seeplus]

To read the whole file into a string, then:

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

int main() {
	if (std::ifstream ifs { "data_01.LIB" }) {
		std::string data((std::istreambuf_iterator<char>(ifs)), std::istreambuf_iterator<char>());

		// use data here
		std::cout << data << '\n';
	} else
		std::cout << "Cannot open file\n";
}

[code amended as per below comment]

[Duthomhas]

The most canonical way to read a whole file into a string is to use a stringstream and some RVO:

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

std::string file_to_string( const std::string & filename, bool binary = false )
{
  std::ifstream f( filename, binary ? std::ios::binary : 0 );
  if (!f) throw std::runtime_error( "file_to_string: " + filename );

  std::ostringstream ss;
  ss << f.rdbuf();
  return ss.str();
}

Enjoy!

[Cubbi]

@seeplus you wouldn't need noskipws if you used istreambuf_iterator

[seeplus]

True. Thanks. I've changed the code above... :) :)

[nvthielen]

c++ : seeplus , thnx I works fine now.

Tnx a lot