我可以问一个 iostream 是否可写吗？

Question

In my code I am passed a reference to an iostream, that in this case is ultimately a file.
Is there any way to find out if, when it was opened, it was set to [in|out] not just [in]?

Answer 1

After looking at all the members of IOS, iostream, ostream, i did not see any way to retrieve the openmode flag used. I guess you'll have to try to write something to your reference and catch any error it throws. Or depending on how it is set, check the result of bad().

Answer 2

The openmode is passed down to std::streambuf , but there are no methods to retrieve it. The only way to know if the file is open for write operations is trying to write and checking for f.fail() , or optionally f.bad() (equivalent to f.rdstate() & std::fstream::badbit ).

Answer 3

...kind of

if ( dynamic_cast<ostream*>( MyStreamPtr ) == 0 )
{
    // Not for output...
}