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<base href="http://llvm.org/bugs/" />
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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW --- - Non conformance caused by _Stream&& operator<<(_Stream&& __os, const _Tp& __x)"
href="http://llvm.org/bugs/show_bug.cgi?id=20596">20596</a>
</td>
</tr>
<tr>
<th>Summary</th>
<td>Non conformance caused by _Stream&& operator<<(_Stream&& __os, const _Tp& __x)
</td>
</tr>
<tr>
<th>Product</th>
<td>libc++
</td>
</tr>
<tr>
<th>Version</th>
<td>unspecified
</td>
</tr>
<tr>
<th>Hardware</th>
<td>Macintosh
</td>
</tr>
<tr>
<th>OS</th>
<td>MacOS X
</td>
</tr>
<tr>
<th>Status</th>
<td>NEW
</td>
</tr>
<tr>
<th>Severity</th>
<td>normal
</td>
</tr>
<tr>
<th>Priority</th>
<td>P
</td>
</tr>
<tr>
<th>Component</th>
<td>All Bugs
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedclangbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>gpakosz@pempek.net
</td>
</tr>
<tr>
<th>CC</th>
<td>llvmbugs@cs.uiuc.edu, mclow.lists@gmail.com
</td>
</tr>
<tr>
<th>Classification</th>
<td>Unclassified
</td>
</tr></table>
<p>
<div>
<pre>The following snippets compiles successfully with clang++ on MacOS X 10.9.4
$ clang --version
Apple LLVM version 5.1 (clang-503.0.40) (based on LLVM 3.4svn)
Target: x86_64-apple-darwin13.3.0
Thread model: posix
---
#include <string>
#include <iostream>
#include <sstream>
void print(const std::string& s)
{
std::cout << s << std::endl;
}
int main()
{
int v = 1;
print((std::ostringstream() << "value: " << v).str());
return 0;
}
The type of ostringstream() << "value: " is rvalue ostringstream.
While it should be lvalue ostream.
According to the addition of the following operator in C++11,
template< class CharT, class Traits, class T >
basic_ostream< CharT, Traits >& operator<<( basic_ostream<CharT,Traits>&& os,
const T& value );
The code sample above should be written the following way:
print(static_cast<std::ostringstream&>(std::ostringstream() << "value: " <<
v).str());
But then, clang++ with libc++ returns the following error message:
$ clang++ --std=c++11 -g main.cpp
main.cpp:13:9: error: non-const lvalue reference to type
'basic_ostringstream<[3 * ...]>' cannot bind to a temporary of type
'basic_ostringstream<[3 * ...]>'
print(static_cast<std::ostringstream&>(std::ostringstream() << "value: " <<
v).str());
---
In the end, the most reliable way to achieve that one-liner is to write:
print(static_cast<std::ostringstream&>(std::ostringstream().flush() << "value:
" << v).str());
Now it compiles successfully and works as expected with G++, clang++ (libstdc++
or libc++) and also MSVC++ 2012. Either in C++2003 or C++11. Note that in the
case of MSVC++ 2012, I went for flush() instead of seekp(0) because seekp(0) on
a newly constructed ostringstream sets the failbit...</pre>
</div>
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