What the point of the std::ios_base::ate (other than std::ios_base::app, for example) and std::ios_base::trunc (other than std::ios_base::out, for example)?
And should i preferly write std::ios_base::smth instead of std::ios::smth?
1 Answer
std::ios_base::ate position the cursor at the end of the text whereas std::ios_base_app appends text (with a write operation) at the end, though you can still read from the beginning :)
std::ios_base::trunc truncates the file so it is emptied, whereas std::ios_base::out just specify you want to write to the stream.
I currently can't quote the standard (on my tablet and Acrobat Reader won't let met copy) but from paragraph 27.4.2.1.4 from ISO 14882:1998 the information you can see on the link is almost exact: http://cplusplus.com/reference/iostream/ios_base/openmode/
To sum up:
std::ios_base::app = append Append at the end of the stream by "seek[ing] to end before each write"
std::ios_base::ate = At The End Open and seek immediately at the end after opening
std::ios_base::binary = binary Perform operation in binary as opposed to text
std::ios_base::in = input Open in read mode
std::ios_base::out = output Open in write mode
std::ios_base::trunc = truncate Truncate the stream on opening.
These values are just flags, so you can open a stream in read/write binary at the end with :
std::ios_base::in | std::ios_base::out | std::ios_base::ate | std::ios_base::binary Concerning the way of using those values, it is as you wish. They are declared as public static fields in std::ios_base class (see 27.4.2) thus it is possible to access them using std::ios::ate or even something like cout.binary !
The points where you must take attention is that std::ios_base::ate does NOT imply std::ios_base::app nor does std::ios_base::out implies std::ios_base::trunc. Each field has a different meaning, and a different case of use, though most of them can't be used alone :)
6 Comments
FrozenHeart
But std::ios_base::out also clear the data in the file, no? Can you give me an example where std::ios_base::out don't clear the file?
Jaffa
From here : cplusplus.com/reference/iostream/ios_base/openmode you get the definition of each value. Moreover, this answer @veer's comment about std::ios_base::in and std::ios::in
FrozenHeart
And what do you mean by "you can still read from the beginning"? Can you give me an example, please?
Jaffa
I mean that ios::ate positions the cursor at the end of the stream, so you can still move it around if you like (and if the stream isn't empty and permit it)
FrozenHeart
Why i can't do this with std::ios_base::app? I can use seekp also
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std::ios_base::inandstd::ios::inshould be identical, it's up to you.