CODEWITHSUNDEEP
c++filefstream
luigi
luigi

Reputation: 160

Fstream management questions

I'm a bit puzzled on how to manage file stream with fstream. I'll order my main questions below:

1) First of all, why calling the constructors fstream file(file_name); or fstream file; file.open(file_name); without any flag like ios::in or ios::out doesn't create any new file?

2) Let's say i want to use the same file for input and output. I can do something like fstream file and then call file.open(file_name, ios::out) or file.open(file_name, ios::in) when I need to use it for output or input operation separately. But what if I need to do input and output at the same time? For example, I need to read from a file and the replace a line or a character whenever I find a specific word or number while reading. Is calling file.open(file_name) for an already created file (and without specifying any flag) possible? Does it allow me to do read/write operation with the same stream?

Upvotes: 1

Views: 193

Answers (2)

Thomas Matthews
Thomas Matthews

Reputation: 57688

Here are my answers:

1) First of all, why calling the constructors fstream file(file_name); or fstream file; file.open(file_name); without any flag like ios::in or ios::out doesn't create any new file?

There is no need to create a file until a write operation occurs. This is delayed processing.

There is no need to create a file when the default may be to open the file with read permission. To my knowledge, opening a file for read-only never creates a file.

Read up on the default mode for those cases.

2) Let's say i want to use the same file for input and output. I can do something like fstream file and then call file.open(file_name, ios::out) or file.open(file_name, ios::in) when I need to use it for output or input operation separately. But what if I need to do input and output at the same time?

If you need to perform reading and writing to the same file, open it with both ios::in and ios::out modes: ios::out | ios::in. This is informing the stream that you want to write and read using the same stream (file). This has been allowed at least since the C language was invented.

BTW, you should clarify "at the same time". Most everything is sequential on a computer, except when multiple processors (cores) are involved. Most files are sequential, meaning only one source can read or write to the file. Although there have been a few exceptions.

Many files have been treated as arrays. You can read and write the same section many times. This is called random access versus devices that are sequential access.

Upvotes: 1

Waheed Brown
Waheed Brown

Reputation: 61

This is very doable using ifstream and ofstream:

#include <fstream>
using namespace std;

int main() {
  ifstream source("source-file.txt");
  ofstream destination("source-file.txt");
  int x; // 0                                                                                                                   
  int y = 1;
  source >> x; // Reads one int from source-file.txt                                                                            
  source.close(); // Always close file when finished                                                                            
  destination << y; // Write to source-file.txt                                                                                 
  return 0;
} // destination.close() called by destructor

Think of a stream like a straw. You can't drink and blow into the straw at the same time. Take a sip, stop, blow back out, repeat.

Upvotes: 1

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