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From "Martin Sebor (JIRA)" <j...@apache.org>
Subject [jira] Commented: (STDCXX-319) std::filebuf::open(..., ios::app) unexpectedly succeeds
Date Fri, 01 Jun 2007 21:42:15 GMT

    [ https://issues.apache.org/jira/browse/STDCXX-319?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#action_12500868
] 

Martin Sebor commented on STDCXX-319:
-------------------------------------

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.

> std::filebuf::open(..., ios::app) unexpectedly succeeds
> -------------------------------------------------------
>
>                 Key: STDCXX-319
>                 URL: https://issues.apache.org/jira/browse/STDCXX-319
>             Project: C++ Standard Library
>          Issue Type: Bug
>          Components: 27. Input/Output
>    Affects Versions: 4.1.3, 4.1.2
>         Environment: all
>            Reporter: Martin Sebor
>            Assignee: Martin Sebor
>
> 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)

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