CODEWITHSUNDEEP
c++file-iostream
Robin Len'hog
Robin Len'hog

Reputation: 47

Keep a stream open from a function to another

Is there any way to keep a stream (to read or write in a file) open from a function to another in C++?

Upvotes: 0

Views: 5229

Answers (4)

Nikita
Nikita

Reputation: 6427

Since C++11 file stream got move constructor (6). You can use it to pass opened stream between functions. Consider the following code snippet:

#include <iostream>
#include <fstream>

bool open_stream(const std::wstring& filepath, std::ifstream& stream)
{
  std::ifstream innerStream;
  innerStream.open(filepath.c_str(), std::ios::in | std::ios::binary);
  if (innerStream.is_open())
  {
    stream = std::move(innerStream); // <-- Opened stream state moved to 'stream' variable 
    return true;
  }
  return false;
}  // <-- innerStream is destructed, but opened stream state is preserved as it was moved to 'stream' variable

Consider the next code to illustrate the usage of open_stream:

int main()
{
  std::ifstream outerStream;
  std::wcout << L"outerStream is opened: " << outerStream.is_open() << std::endl; // <-- outerStream is opened: 0

  if (!open_stream(L"c:\\temp\\test_file.txt", outerStream))
  {
    return 1;
  }

  std::wcout << L"outerStream is opened: " << outerStream.is_open() << std::endl; // <-- outerStream is opened: 1 

  // outerStream is opened and ready for reading here

  return 0;
}

Upvotes: 2

Caesar
Caesar

Reputation: 9841

Pass it by reference

void myFunction(ifstream &myStream)

Upvotes: 2

Razvi
Razvi

Reputation: 2818

Yes, you can either create the stream outside of the functions and pass it as a parameter to the methods:

void myFunction(ifstream &stream) {...}

Later close the stream when you are done with it: stream.close().

Or create the stream within the first function and return it to the calling method and then pass it to the second function.

Upvotes: 2

Florin Stingaciu
Florin Stingaciu

Reputation: 8285

Make it global or pass it as an argument but ensure that if you pass it as an argument you past it by reference not by value! If you pass it by value the compiler will NOT complain and weird things start happening.

Upvotes: 1

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