What is the purpose of using std::ios_base::trunc flag with std::ios_base::out

Question

What is the purpose of using std::ios_base::trunc flag with std::ios_base::out? I saw this in many examples.

I thought that it's guaranteed by the standard that std::ios_base::out also truncates the file (and all of the STL implementations which I know do this). Am I wrong and should explicitly notify that I want to truncate the file?

Answer 1

Yes, std::ios_base::out is equivalent to "w" in fopen.

The point of std::ios_base::trunc is when std::ios_base::in and std::ios_base::out are used the same time.

• in | out is equivalent to "r+"
• in | out | trunc is equivalent to "w+"
• binary | in | out is equivalent to "rb+"
• binary | in | out | trunc is equivalent to "wb+"

Maybe a table would be more obvious:

binary  in  out  trunc | stdio equivalent
-----------------------+-----------------
         +   +         |      "r+"
         +   +     +   |      "w+"
  +      +   +         |      "r+b"
  +      +   +     +   |      "w+b"

Answer 2

To replace a file's content rather than extending the file, it must be opened in

std::ios_base::out | std::ios_base::trunc

For output file streams the open mode out is equivalent to out|trunc, that is, trunc flag can be omitted.

For bidirectional file streams, however, trunc must always be explicitly specified.

To extend an output file, flag std::ios_base::ate | std::ios_base::app is used.

Here, the file content is retained because the trunc flag is not set, and the initial file position is at the file's end.

However, additionally trunc flag can be set, and the file content is discarded and the output is done at the end of an empty file.

Answer 3

It is redundant - in other words, it makes no difference if you have it or not.

Obviously, in some combinations, such as std::ios_base::out | std::ios_base::in it would NOT be redundant.