How to write to middle of a file in C++?

问题

I think this should be quite simple, but my googling didn\'t help so far... I need to write to an existing file in C++, but not necessarily at the end of the file.

I know that when I just want to append text to my file, I can pass the flag ios:app when calling open on my stream object. However, this only let\'s me write to the very end of the file, but not into its middle.

I made a short program to illustrate the issue:

#include <iostream>
#include <fstream>

using namespace std;

int main () {

  string path = \"../test.csv\";

  fstream file;
  file.open(path); // ios::in and ios::out by default

  const int rows = 100;
  for (int i = 0; i < rows; i++) {
    file << i << \"\\n\";
  }  

  string line;
  while (getline(file, line)) {
    cout << \"line: \" << line << endl; // here I would like to append more text to certain rows
  }


  file.close();

}

回答1:

You cannot insert in the middle of the file. You have to copy the old file to a new file and insert whatever you want in the middle during copying to the new file.

Otherwise, if you intend to overwrite data/lines in the existing file, that is possible by using std::ostream::seekp() to identify the position within the file.

回答2:

You could write to the end and swap lines until it ends up in the right position. Here's what I had to do. Here's the test.txt file before:

Here's a sample of my program

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

using namespace std;

fstream& goToLine(fstream& file, int line){
    int charInLine = 10;  //number of characters in each line + 2
                          //this file has 8 characters per line

    int pos = (line-1)*charInLine;

    file.seekg(pos);
    file.seekp(pos);

    return file;
}

fstream& swapLines(fstream& file, int firstLine, int secondLine){
    string firstStr, secondStr;

    goToLine(file,firstLine);
    getline(file,firstStr);
    goToLine(file,secondLine);
    getline(file,secondStr);

    goToLine(file,firstLine);
    file.write(secondStr.c_str(),8);    //Make sure there are 8 chars per line
    goToLine(file,secondLine);
    file.write(firstStr.c_str(),8);

    return file;
}

int main(){
    fstream file;
    int numLines = 5; //number of lines in the file

    //open file once to write to the end
    file.open("test.txt",ios::app); 
    if(file.is_open()){
        file<<"someText\n"; //Write your line to the end of the file.
        file.close();
    }

    //open file again without the ios::app flag
    file.open("test.txt"); 
    if(file.is_open()){
        for(int i=numLines+1;i>3;i--){ //Move someText\n to line 3
            swapLines(file,i-1,i);
        }
        file.close();
    }

    return 0;
}

Here's the test.txt file after:

I hope this helps!

来源：https://stackoverflow.com/questions/30642827/how-to-write-to-middle-of-a-file-in-c

标签

c++

fstream

ifstream

ofstream