Stringstream object and text file data access issues!

[geeloso]

Hey guys! Please I need your help. I have two puzzling issues I am dealing with.

Issue 1: I am using a stringstream object in a block of my program that needs to be visited repeatedly depending on a user's selection from a menu. I want the contents of this stringstream object to be cleared any time control gets to this part of the program. I have tried the clear and flush functions to no avail. Any ideas?

Issue 2: I am reading data from a source text file that would be regularly changed during the course of program run. After the program run is over, I am supposed to save the results(which is basically the source text file AND all updates) in a destination file. This destination file would then serve as the source file when next the program is run. In other words, I want a scenario where my results overwrite the original contents of the source file; implying that my source and destination files are now one, pretty much. How can I do this?

[keskiverto]

1. The documentation of clear() and flush() makes it clear that they are not the functions that you are looking for. Try:
http://www.cplusplus.com/reference/sstream/stringstream/str/

2. How about:
- read file to memory
- modify memory
- (over)write memory to file

[geeloso]

Thx keskiverto, I have resolved the first issue. I am still having problems with the second one though. Your second suggestion is the trivial solution that first pops up in one's mind which I have tried. I opened the same file name as my source and destination in function main(); read the data into the program; however, I observed I could not access the 'read' contents of the file during program execution. It defies logic for this to happen; I hope it's not some oversight of mine somewhere else in the program. Is this the same approach you would have adopted?

[keskiverto]

Close file after reading. Open for write only at end.

Foo data;

{
  istream f("file");
  f >> data;
}

data += 42;

{
  ostream f("file", overwrite-mode-flag);
  f << data;
}

[geeloso]

It's still not working! What happens is I always find a blank file after the program has run. In essence, I cannot see any output showing the results. Consequently, the next run of the program has no source data to load!!

What is that

overwrite-mode-flag

stuff in your post? I'm guessing it's some file read/write mode; right?

[keskiverto]

openmode. See the documentation for std::ofstream.

Mine writes fine:

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

int main()
{
  std::string data;
  {
    std::ifstream f("trunc.txt");
    f >> data;
  }
  std::cout << data << '\n';
  data = "world\n";
  {
    std::ofstream g("trunc.txt", std::ios::trunc | std::ios::out);
    g << data;
  }
  return 0;
}

$ g++ -W -Wall -o trunc trunc.cpp
$ echo "Hello" > trunc.txt
$ cat trunc.txt
Hello
$ ./trunc
Hello
$ cat trunc.txt
world
$

[geeloso]

I can see that your data is roughly a simple data type(a string); mine is a structured data type (a multi-data-member object) based off of a linked-list class and several other classes. I overloaded the stream insertion operator for two of the classes that are derived classes from the linked-list base class. However, the function tasked with printing to the file is a member function in one of the derived classes. Should this difference in data formats have any bearing, at all, on this read-and-write-same-file scheme I am seeking?

[geeloso]

@keskiverto:
Thx for your replies. I have finally resolved the issue; it is now working.