C++ standard file writing and reading (ifstream, ofstream)

ttp://blog.csdn.net/a125930123/article/details/53542261

 

 

 

Note: "<<", inserter, input data to the stream

      ">>", extractor, output data from stream,

 

ifstream and ofstream are mainly included in the header file <fstream>. fstream can read and write the open file

ifstream <===> hard disk writes files to memory

ofstream <===> memory writes files to disk

ofstream out("out.txt");

if(out.is_open()) //is_open() returns true (1), which means the opening is successful

{

    out<<"HELLO WORLD!"<<endl;

    out.close();

}

HELLO WORLD! is written in the file out.txt

 

ifstream in("out.txt");

cha buffer[200];

if(in.is_open())

{

    while(!in.eof())

    {

        in.getline(buffer,100)

        cout<<buffer<<endl;

        out.close();

    }

}

 

open a file:

 

 

ofstream out("/root/1.txt"); 

or

ofstream out;

out.open("/root/1.txt");

 

write:

out << "hello, world!!!" << endl;

 

 

Close the file:

 

 

out.clear (); //For the portability and reusability of the code, it is best to take this sentence and clear the flag. Some systems may have problems if they are not cleared.

out.close();

 

//For the order of the above two statements. It depends on whether the file is successfully opened. If the file fails to open. If the file is opened first and then closed directly, there will still be a problem with the status bit. The general logic is to close first, regardless of whether the close is successful or not, then clear clear flag

 

Judgment of file open status (verification of status identifier):

.bad() <===> Error in reading and writing files, such as opening for writing with r, or insufficient disk space, return true

.fail() <===> Same as above, and also returns true for data format read errors

.eof() <===> read the file to the end of the file, return true

.good() <===> is the most general, and returns false if any of the above returns true.

If the above flag is cleared, the .clear() function is called

 

Example complete code:

 

 

 

 

 

#include <iostream>

#include <string>

#include <sstream>

#include <fstream>

 

using namespace std;

#define TO_NUMBER(s, n) (istringstream(s) >> n)     // convert string to numeric

#define TO_STRING(n) (((ostringstream&)(ostringstream() << n)).str())     //The value is converted to a string

 

int main(int argc, char **argv)

{

    string lines;

    ifstream infile(argv[1]);

    ofstream outfile(argv[2]);

 

    if(!infile.is_open())

    {

        cout << argv[ 1] << "Failed to open file" << endl;

        return 0;

 

 

    }

 

    if(!outfile.is_open())

    {

        cout << argv[ 2] << "Failed to open file" << endl;

        return 0;

    }

 

    while(getline(infile,lines))

    {

        if(infile.eof())

        {

            cout << "Failed to read file" << endl;

            break;

        }

 

        istringstream strline(lines);

        string tmp_string;

        int i = 1;

 

        strline>>tmp_string;

        string linename = tmp_string;

        while(strline>>tmp_string)

        {

            outfile<<"# "<<i<<" "<<linename<<" "<<i<<" "<<tmp_string<<endl;;

            i++;

        }

        cout<<"total column is: "<<i<<endl;

    }

 

    infile.clear (); //For the portability and reusability of the code, it is best to bring this sentence to clear the flag. Some systems may have problems if they are not cleared.

    infile.close();

    outfile.clear (); //For the portability and reusability of the code, it is best to bring this sentence to clear the flag. Some systems may have problems if they are not cleaned up.

    outfile.close();

 

    return 0;

}