file >> out.rdbuf(); fails to compile, why?

[Siemel Naran]

My compiler (Borland C++) fails to compile this code:

ifstream file(filename.c _str());
ostringstream out;
file >> out.rdbuf();

with the error in the 3rd line above that:
[C++ Error] realmain.cpp(83 ): E2015 Ambiguity between
'_STL::basic_is tream<char,_STL ::char_traits<c har> >::operator >>(bool &)'
and '_STL::basic_is tream<char,_STL ::char_traits<c har> >::operator >>(void *
&)'
But it compiles the following without error.

ifstream file(filename.c _str());
ostringstream out;
streambuf * outbuf = out.rdbuf();
file >> outbuf;
Why the discrepancy?

[Buster]

Siemel Naran wrote:

My compiler (Borland C++) fails to compile this code:

ifstream file(filename.c _str());
ostringstream out;
file >> out.rdbuf();

with the error in the 3rd line above that:
[C++ Error] realmain.cpp(83 ): E2015 Ambiguity between
'_STL::basic_is tream<char,_STL ::char_traits<c har> >::operator >>(bool &)'
and '_STL::basic_is tream<char,_STL ::char_traits<c har> >::operator >>(void *
&)'
The 'std::istream:: operator >> (std::streambuf *)' overload is a better
match than either of those. I suspect it's missing.
But it compiles the following without error.

ifstream file(filename.c _str());
ostringstream out;
streambuf * outbuf = out.rdbuf();
file >> outbuf;
Did you run it? I imagine that ">>" is calling 'std::istream:: operator

(void * &)', and trying to read a raw pointer.

I tried to test this hypothesis using Borland C++ Builder 6.0 and found
that it can't read raw pointers either. So you're on to a loser. Here's
the test.

#include <istream>
#include <ostream>
#include <sstream>
#include <iostream>
#include <limits>

int main ()
{
void * p = reinterpret_cas t <void *> (0xdeadbeef);
void * q;

std::ostringstr eam out;
out << p;

std::istringstr eam in (out.str ());

if (in >> q) std::cout << std::hex << q << '\n';
else std::cout << "fail\n";

// pause to view results if running from the Borland IDE
std::cin.ignore (std::numeric_l imits <std::streamsiz e>::max (), '\n');
}

Outputs "0xdeadbeef " on g++ 3.3.3, "fail" on bcb 6.0.
Why the discrepancy?

Your code is pretty much correct. Complain to Borland, and/or upgrade to
the latest version.

--
Regards,
Buster

[David Harmon]

On Fri, 17 Dec 2004 10:29:18 GMT in comp.lang.c++, "Siemel Naran"
<Si*********@RE MOVE.att.net> wrote,

My compiler (Borland C++) fails to compile this code:

ifstream file(filename.c _str());
ostringstream out;
file >> out.rdbuf();

You wanted the incantation
out << file.rdbuf();

http://groups.google.com/gr*********...ing.google.com
http://groups.google.c om/gr************* ************@ID-79865.news.uni-berlin.de

[Old Wolf]

Buster wrote:

int main ()
{
void * p = reinterpret_cas t <void *> (0xdeadbeef);
void * q;

std::ostringstr eam out;
out << p;
Undefined behaviour -- using the value of an invalid pointer

std::istringstr eam in (out.str ());

if (in >> q) std::cout << std::hex << q << '\n';
else std::cout << "fail\n";

// pause to view results if running from the Borland IDE
std::cin.ignore (std::numeric_l imits <std::streamsiz e>::max (), '\n'); }

Outputs "0xdeadbeef " on g++ 3.3.3, "fail" on bcb 6.0.

Since you have invoked UB, "fail" is a conforming output.

[Buster]

Old Wolf wrote:

Buster wrote:

int main ()
{
void * p = reinterpret_cas t <void *> (0xdeadbeef);
void * q;

std::ostringstr eam out;
out << p;

Undefined behaviour -- using the value of an invalid pointer

Oh, don't be silly. Change it to:
"int x; void * p = reinterpret_cas t <void *> (& x);"
and it still won't work.

std::istringstr eam in (out.str ());

if (in >> q) std::cout << std::hex << q << '\n';
else std::cout << "fail\n";

// pause to view results if running from the Borland IDE
std::cin.ignore (std::numeric_l imits <std::streamsiz e>::max (),

'\n');

}

Outputs "0xdeadbeef " on g++ 3.3.3, "fail" on bcb 6.0.

Since you have invoked UB, "fail" is a conforming output.