[LWG #596] std::filebuf::open(..., ios::app) unexpectedly succeeds

• Key: STDCXX-319
• Type: Bug
• Status: Open
• Priority: Trivial
• Assignee: Martin Sebor
• Reporter: Martin Sebor
• Component/s: 27. Input/Output
• Affects Version/s: 4.1.2, 4.1.3
• Fix Version/s: 4.3.0

Description

According to filebuf.members, p2, filebuf::open(const char*, openmode mode) is not some combination of the flags listed in Table 113 the call is supposed to fail. Since ios::app only appears in conjunction with ios::out (and ios::binary|ios::out) the call in the program below should fail.

$ cat t.cpp && make t && ./t
#include <cassert>
#include <fstream>

int main ()
{
std::filebuf fb;

const std::filebuf* const pfb = fb.open ("dummy", std::ios::app);

assert (0 == pfb);
}
gcc -c -I/build/sebor/dev/stdlib/include/ansi -D_RWSTDDEBUG -D_RWSTD_USE_CONFIG -I/build/sebor/dev/stdlib/include -I/build/sebor/gcc-4.1.0-11s/include -I/build/sebor/dev/stdlib/../rwtest -I/build/sebor/dev/stdlib/../rwtest/include -I/build/sebor/dev/stdlib/tests/include -pedantic -nostdinc++ -g -W -Wall -Wcast-qual -Winline -Wshadow -Wwrite-strings -Wno-long-long t.cpp
gcc t.o -o t -L/build/sebor/gcc-4.1.0-11s/rwtest -lrwtest11s -L/build/sebor/gcc-4.1.0-11s/lib -lstd11s -lsupc++ -lm
Assertion failed: 0 == pfb, file t.cpp, line 10
Abort (core dumped)

Comments

[Martin Sebor]

This is actually by design and documented (for ifstream and ofstream) in the User's Guide. Classic Iostream behavior is the same as is C stdio. Quoting from http://incubator.apache.org/stdcxx/doc/stdlibug/index.html:

30.3.1.1 The in and out Open Modes

Input (and output) file streams always have the in (or out) open mode flag set implicitly. An output file stream, for instance, knows that it is in output mode and you need not set the output mode explicitly. Instead of writing:

std::ofstream Str("out.txt", std::ios_base::out | std::ios_base::app);

you can simply say:

std::ofstream Str("out.txt", std::ios_base::app);

Bidirectional file streams, on the other hand, do not have the flag set implicitly. This is because a bidirectional stream does not have to be in both input and output mode in all cases. You might want to open a bidirectional stream for reading only or writing only. Bidirectional file streams therefore have no implicit input or output mode. You must always set a bidirectional file stream's open mode explicitly.

[Martin Sebor]

So the issue should be addressed in the standard – see my post to the C++ reflector below:

-------- Original Message --------
Subject: another missing openmode (issue 596)
Date: Sun, 27 May 2007 17:06:54 -0600
From: Martin Sebor <[e-mail]>
Reply-To: [e-mail]
Organization: Rogue Wave Software
To: undisclosed-recipients:;

To: C++ libraries mailing list
Message c++std-lib-18744

http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-active.html#596

...besides "a+" and "a+b" the C++ table is also missing a row
for a lone app bit which in at least two current implementation
as well as in Classic Iostreams corresponds to the C stdio "a"
mode and has been traditionally documented as implying ios::out.
Which means the table should also have a row for in|app meaning
the same thing as "a+" already proposed in the issue.

Howard, unless this is somehow implied somewhere else in the text
and I missed it, can you please add my comments to the issue?

Thanks
Martin

[Martin Sebor]

Added LWG #596 tag to the Summary to indicate a dependency on a Library Working Group issue, lowered Priority to Trivial (since it's by design), and deferred until after the C++ committee has had a chance to review the issue.

[Martin Sebor]

LWG issue 596 is still in Review status. Deferred until the issue has been voted into the Working Paper.