Re: Return type of ternary ?: and Sutter's GOTW 48 revisited
aleethorp@hotmail.com wrote:
I was revisiting Sutter's More Exceptional C++ and got stuck on the
very first item! This used to compile (and still does with GCC
3.4.4) but fails under GCC 4.1.3, 4.2.3 (probably all of the GCC
4.x.y) and also fails under the VC++ 9.0 C++ compiler.
#include <fstream>
#include <iostream>
int main( int argc, char* argv[] )
{
using namespace std;
(argc > 2
? ofstream(argv[2], ios::out | ios::binary)
: cout) // <-- line 10
<<
(argc > 1
? ifstream(argv[1], ios::in | ios::binary)
: cin)
.rdbuf();
}
The G++ error is :
[lots]
The VC++ error is similar but also picks up a second problem:
C:\>cl echo.cc
Microsoft (R) 32-bit C/C++ Optimizing Compiler Version
15.00.21022.08 for 80x86 Copyright (C) Microsoft Corporation. All
rights reserved.
echo.cc
C:\Program Files\Microsoft Visual Studio
9.0\VC\INCLUDE\xlocale(342) : warning C4530: C++ exception handler
used, but unwind semantics are not enabled.
Specify /EHsc
It says right here that you are missing a compiler setting!
C:\Program Files\Microsoft Visual Studio
9.0\VC\INCLUDE\ostream(584) : error C2248:
'std::basic_ios<_Elem,_Traits>::basic_ios' : cannot access private
member declared in class 'std::basic_ios<_Elem,_Traits>'
with
[
_Elem=char,
_Traits=std::char_traits<char>
]
C:\Program Files\Microsoft Visual Studio 9.0\VC\INCLUDE
\ios(151)
[snip]
VC++ 9 also complains about a problem with the compiler synthesized
basic_istream copy constructor (which GCC doesn't pick up until you
correct the first problem).
So here's the question: What is "?:" actually doing and why?
If the compiler error message is quite clear: its synthesizing a
copy constructor (which presumably then results in an access check
- or is it the other way around - a required copy constructor
access check that causes the copy constructor to be synthesized).
But why does it need to do this? cout is not a temporary.
No, but the ofstream is! :-)
It needs to have a copy constructor to be passed to the operator<<,
but that one is disabled by making the base class copy constructor
private. This is a change that the later compilers have obviously
picked up.
Bo Persson
--
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