RFC: why do our std::ofsteam functions add ios_base::trunc ?

[Jonathan Wakely <jwakely at redhat dot com>]

While adding the new C++17 overloads taking filesystem paths to
<fstream> I noticed that ofstream uses out|trunc for the default
arguments instead of out as the stadnard says. The effect is the same,
but my preference would be to do what the standard says. Is there a
good reason for deviating from what the standard says?

Although the out|trunc default arguments have no difference in
behaviour from just using out, the comments saying 

 "Calls @c std::basic_filebuf::open(__s,__mode|out|trunc)"

are definitely wrong. That would not be a conforming behaviour.

Also, I think we can remove the comments saying to use c_str() as
fstreams accept std::string arguments since C++11.

Any comments?


diff --git a/libstdc++-v3/include/std/fstream b/libstdc++-v3/include/std/fstream
index 52830945fe2..db41a697085 100644
--- a/libstdc++-v3/include/std/fstream
+++ b/libstdc++-v3/include/std/fstream
@@ -487,9 +487,6 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
        *  @param  __mode  Open file in specified mode (see std::ios_base).
        *
        *  @c ios_base::in is automatically included in @a __mode.
-       *
-       *  Tip:  When using std::string to hold the filename, you must use
-       *  .c_str() before passing it to this constructor.
        */
       explicit
       basic_ifstream(const char* __s, ios_base::openmode __mode = ios_base::in)
@@ -587,9 +584,6 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
        *
        *  Calls @c std::basic_filebuf::open(s,__mode|in).  If that function
        *  fails, @c failbit is set in the stream's error state.
-       *
-       *  Tip:  When using std::string to hold the filename, you must use
-       *  .c_str() before passing it to this constructor.
        */
       void
       open(const char* __s, ios_base::openmode __mode = ios_base::in)
@@ -687,15 +681,11 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
        *  @param  __s  Null terminated string specifying the filename.
        *  @param  __mode  Open file in specified mode (see std::ios_base).
        *
-       *  @c ios_base::out | @c ios_base::trunc is automatically included in
-       *  @a __mode.
-       *
-       *  Tip:  When using std::string to hold the filename, you must use
-       *  .c_str() before passing it to this constructor.
+       *  @c ios_base::out is automatically included in @a __mode.
        */
       explicit
       basic_ofstream(const char* __s,
-		     ios_base::openmode __mode = ios_base::out|ios_base::trunc)
+		     ios_base::openmode __mode = ios_base::out)
       : __ostream_type(), _M_filebuf()
       {
 	this->init(&_M_filebuf);
@@ -708,12 +698,11 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
        *  @param  __s  std::string specifying the filename.
        *  @param  __mode  Open file in specified mode (see std::ios_base).
        *
-       *  @c ios_base::out | @c ios_base::trunc is automatically included in
-       *  @a __mode.
+       *  @c ios_base::out is automatically included in @a __mode.
        */
       explicit
       basic_ofstream(const std::string& __s,
-		     ios_base::openmode __mode = ios_base::out|ios_base::trunc)
+		     ios_base::openmode __mode = ios_base::out)
       : __ostream_type(), _M_filebuf()
       {
 	this->init(&_M_filebuf);
@@ -789,15 +778,11 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
        *  @param  __s  The name of the file.
        *  @param  __mode  The open mode flags.
        *
-       *  Calls @c std::basic_filebuf::open(__s,__mode|out|trunc).  If that
+       *  Calls @c std::basic_filebuf::open(__s,__mode|out).  If that
        *  function fails, @c failbit is set in the stream's error state.
-       *
-       *  Tip:  When using std::string to hold the filename, you must use
-       *  .c_str() before passing it to this constructor.
        */
       void
-      open(const char* __s,
-	   ios_base::openmode __mode = ios_base::out | ios_base::trunc)
+      open(const char* __s, ios_base::openmode __mode = ios_base::out)
       {
 	if (!_M_filebuf.open(__s, __mode | ios_base::out))
 	  this->setstate(ios_base::failbit);
@@ -813,12 +798,11 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
        *  @param  __s  The name of the file.
        *  @param  __mode  The open mode flags.
        *
-       *  Calls @c std::basic_filebuf::open(s,mode|out|trunc).  If that
+       *  Calls @c std::basic_filebuf::open(s,mode|out).  If that
        *  function fails, @c failbit is set in the stream's error state.
        */
       void
-      open(const std::string& __s,
-	   ios_base::openmode __mode = ios_base::out | ios_base::trunc)
+      open(const std::string& __s, ios_base::openmode __mode = ios_base::out)
       {
 	if (!_M_filebuf.open(__s, __mode | ios_base::out))
 	  this->setstate(ios_base::failbit);
@@ -894,9 +878,6 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
        *  @brief  Create an input/output file stream.
        *  @param  __s  Null terminated string specifying the filename.
        *  @param  __mode  Open file in specified mode (see std::ios_base).
-       *
-       *  Tip:  When using std::string to hold the filename, you must use
-       *  .c_str() before passing it to this constructor.
        */
       explicit
       basic_fstream(const char* __s,
@@ -993,9 +974,6 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
        *
        *  Calls @c std::basic_filebuf::open(__s,__mode).  If that
        *  function fails, @c failbit is set in the stream's error state.
-       *
-       *  Tip:  When using std::string to hold the filename, you must use
-       *  .c_str() before passing it to this constructor.
        */
       void
       open(const char* __s,