Writing stringstream contents into ofstream

Question

I'm currently using std::ofstream as follows:

std::ofstream outFile;
outFile.open(output_file);

Then I attempt to pass a std::stringstream object to outFile as follows:

GetHolesResults(..., std::ofstream &outFile){
  float x = 1234;
  std::stringstream ss;
  ss << x << std::endl;
  outFile << ss;
}

Now my outFile contains nothing but garbage: "0012E708" repeated all over.

In GetHolesResults I can write

outFile << "Foo" << std:endl;

and it will output correctly in outFile.

Any suggestion on what I'm doing wrong?

I suggest you rephrase the title to something in the line of: 'writting stringstream contents into ofstream' (or ostream for what matters) — David Rodríguez - dribeas, Nov 27 '08 at 22:09
Eric, you should mark Johannes' answer as accepted so that I can delete mine (which is clearly worse than his approach) — David Rodríguez - dribeas, Jul 19 '13 at 16:30
:) Someone upvoted my answer today, which might indicate that they chose a less-than-perfect solution when a better one was available. — David Rodríguez - dribeas, Jul 19 '13 at 16:39

score 117 · Accepted Answer · edited Dec 19 '20 at 07:42

117

You can do this, which doesn't need to create the string. It makes the output stream read out the contents of the stream on the right side (usable with any streams).

outFile << ss.rdbuf();

edited Dec 19 '20 at 07:42

Aykhan Hagverdili

28,141
6
41
93

answered Nov 28 '08 at 02:25

Johannes Schaub - litb

496,577
130
894
1,212

15

I'm curious: why this solution works with `std::stringstream` but not with `std::ostringstream`? In the second case I am getting an empty file. – Javi Feb 02 '15 at 11:29
12

@JaviV because you can [only write](http://en.cppreference.com/w/cpp/io/basic_ostream) to `std::oANYSTREAM` and only read from `std::iANYSTREAM`. There will be no sense for `output_stream` if it would have also `read` operations. If you need both just use `std::ANYSTREAM` and you are home :) – S.R Oct 25 '17 at 08:11
I'm backing here after upvoting years ago. Its a bit weird that the compiler does not warning about call rdbuf from ostringstream – Vinicius Almada Oct 04 '22 at 04:42

score 25 · Answer 2 · answered Mar 16 '15 at 06:00

25

If you are using std::ostringstream and wondering why nothing get written with ss.rdbuf() then use .str() function.

outFile << oStream.str();

answered Mar 16 '15 at 06:00

Digital_Reality

4,488
1
29
31

11

your answer led me to the following realization: to get a `char` array from `ss`, you can do `ss.str().c_str()`. (just posting here for the benefit of newbies who stumble on this in the future.) – abcd Oct 16 '15 at 16:57
2

@Digital_Reality, thanks! nothing was getting written when i used rdbuff. What is the reason for this? – Nasir Oct 28 '15 at 06:49

score 4 · Answer 3 · edited Dec 07 '12 at 09:46

4

When passing a stringstream rdbuf to a stream newlines are not translated. The input text can contain \n so find replace won't work. The old code wrote to an fstream and switching it to a stringstream losses the endl translation.

edited Dec 07 '12 at 09:46

SchmitzIT

9,227
9
65
92

answered Dec 07 '12 at 09:26

2ndshot

49
1

6

This should be a comment not an answer because you haven't told how to write a std::stringstream to a std::ofstream – Alex Bitek Jun 12 '13 at 06:13

score 2 · Answer 4 · answered Dec 14 '18 at 01:08

I'd rather write ss.str(); instead of ss.rdbuf(); (and use a stringstream).

If you use ss.rdbuf() the format-flags of outFile will be reset rendering your code non-reusable. I.e., the caller of GetHolesResults(..., std::ofstream &outFile) might want to write something like this to display the result in a table:

outFile << std::setw(12) << GetHolesResults ...

...and wonder why the width is ignored.

Writing stringstream contents into ofstream

4 Answers4

Linked