[boost-users][lambda] crash in std::for_each

Igor R

3 Feb 2009 3 Feb '09

9:45 a.m.

The following code crashes in for_each when trying to output the 1st pair. What's wrong with it? Thanks! namespace bl = boost::lambda; std::map temp; temp["a"] = "1"; temp["b"] = "2"; const std::map attrs(temp); typedef std::pair string_pair; std::stringstream stream; std::for_each(attrs.begin(), attrs.end(), stream << bl::bind(&string_pair::first, bl::_1) << bl::bind(&string_pair::second, bl::_1));

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Robert Jones

3 Feb 3 Feb

12:29 p.m.

On Tue, Feb 3, 2009 at 9:45 AM, Igor R wrote:

...

The following code crashes in for_each when trying to output the 1st pair. What's wrong with it?

Thanks!

namespace bl = boost::lambda; std::map temp; temp["a"] = "1"; temp["b"] = "2"; const std::map attrs(temp); typedef std::pair string_pair; std::stringstream stream; std::for_each(attrs.begin(), attrs.end(), stream << bl::bind(&string_pair::first, bl::_1) << bl::bind(&string_pair::second, bl::_1));

_______________________________________________ Boost-users mailing list Boost-users@lists.boost.org http://lists.boost.org/mailman/listinfo.cgi/boost-users

What's the problem? This works for me - full source follows. #include "boost/lambda/lambda.hpp" #include "boost/lambda/bind.hpp" #include <algorithm> #include <map> #include <iostream> #include <sstream> int main( ) { namespace bl = boost::lambda; std::map temp; temp["a"] = "1"; temp["b"] = "2"; const std::map attrs(temp); typedef std::pair string_pair; std::stringstream stream; std::for_each(attrs.begin(), attrs.end(), stream << bl::bind(&string_pair::first, bl::_1) << bl::bind(&string_pair::second, bl::_1)); } Using Visual Studio 2005 & Boost 1.36 Rob. --

Robert Jones

12:40 p.m.

Oh no it doesn't ! Please ignore my last post. On Tue, Feb 3, 2009 at 12:29 PM, Robert Jones wrote:

...

On Tue, Feb 3, 2009 at 9:45 AM, Igor R wrote:

...
The following code crashes in for_each when trying to output the 1st pair. What's wrong with it?

Thanks!

namespace bl = boost::lambda; std::map temp; temp["a"] = "1"; temp["b"] = "2"; const std::map attrs(temp); typedef std::pair string_pair; std::stringstream stream; std::for_each(attrs.begin(), attrs.end(), stream << bl::bind(&string_pair::first, bl::_1) << bl::bind(&string_pair::second, bl::_1));

_______________________________________________ Boost-users mailing list Boost-users@lists.boost.org http://lists.boost.org/mailman/listinfo.cgi/boost-users

What's the problem? This works for me - full source follows.

#include "boost/lambda/lambda.hpp" #include "boost/lambda/bind.hpp" #include <algorithm> #include <map> #include <iostream> #include <sstream>

int main( ) { namespace bl = boost::lambda; std::map temp; temp["a"] = "1"; temp["b"] = "2"; const std::map attrs(temp); typedef std::pair string_pair; std::stringstream stream; std::for_each(attrs.begin(), attrs.end(), stream << bl::bind(&string_pair::first, bl::_1) << bl::bind(&string_pair::second, bl::_1)); }

Using Visual Studio 2005 & Boost 1.36

Rob.

--

-- ACCU - Professionalism in programming - http://www.accu.org

Stuart Dootson

2:22 p.m.

On Tue, Feb 3, 2009 at 9:45 AM, Igor R wrote:

...

The following code crashes in for_each when trying to output the 1st pair. What's wrong with it?

Thanks!

namespace bl = boost::lambda; std::map temp; temp["a"] = "1"; temp["b"] = "2"; const std::map attrs(temp); typedef std::pair string_pair; std::stringstream stream; std::for_each(attrs.begin(), attrs.end(), stream << bl::bind(&string_pair::first, bl::_1) << bl::bind(&string_pair::second, bl::_1));

Try changing the string_pair typedef to: std::pair string_pair; Not 100% sure *why* this makes it run rather than crash (with VC++ 2008, Boost 1.37.0), but it makes some sort of sense, as the key in a map is immutable. Stuart Dootson

Steven Watanabe

5 p.m.

AMDG Stuart Dootson wrote:

...

Try changing the string_pair typedef to:

std::pair string_pair;

Not 100% sure *why* this makes it run rather than crash (with VC++ 2008, Boost 1.37.0), but it makes some sort of sense, as the key in a map is immutable.

The reason is that std::pairs can be converted to each other. If the type doesn't match exactly: 1) It will be implicitly converted to the correct type. 2) Then you get a reference to first. 3) Then the temporary created in step 1 is destroyed 4) Then you stream first (which has just been destroyed) In Christ, Steven Watanabe

Igor R

5:10 p.m.

...

The reason is that std::pairs can be converted to each other. If the type doesn't match exactly: 1) It will be implicitly converted to the correct type. 2) Then you get a reference to first. 3) Then the temporary created in step 1 is destroyed 4) Then you stream first (which has just been destroyed)

Thank you, now I see.

Stuart Dootson

7:24 p.m.

On Tue, Feb 3, 2009 at 5:00 PM, Steven Watanabe wrote:

...

AMDG

Stuart Dootson wrote:

...
Try changing the string_pair typedef to:

std::pair string_pair;

Not 100% sure *why* this makes it run rather than crash (with VC++ 2008, Boost 1.37.0), but it makes some sort of sense, as the key in a map is immutable.

The reason is that std::pairs can be converted to each other. If the type doesn't match exactly: 1) It will be implicitly converted to the correct type. 2) Then you get a reference to first. 3) Then the temporary created in step 1 is destroyed 4) Then you stream first (which has just been destroyed)

In Christ, Steven Watanabe

Thanks Steven - it's always good to get some educashun! :-) Stuart D

Robert Jones

2:36 p.m.

On Tue, Feb 3, 2009 at 9:45 AM, Igor R wrote:

...

The following code crashes in for_each when trying to output the 1st pair. What's wrong with it?

Thanks!

namespace bl = boost::lambda; std::map temp; temp["a"] = "1"; temp["b"] = "2"; const std::map attrs(temp); typedef std::pair string_pair; std::stringstream stream; std::for_each(attrs.begin(), attrs.end(), stream << bl::bind(&string_pair::first, bl::_1) << bl::bind(&string_pair::second, bl::_1));

Or essentially the same solution, but tidied up

#include "boost/lambda/lambda.hpp" #include "boost/lambda/bind.hpp" #include <algorithm> #include <map> #include <iostream> #include <sstream> int main( ) { namespace bl = boost::lambda; typedef std::map map_type; map_type temp; temp["a"] = "1"; temp["b"] = "2"; const map_type attrs(temp); typedef map_type :: value_type string_pair; std::stringstream stream; std::for_each(attrs.begin(), attrs.end(), stream << bl::bind(&string_pair::first, bl::_1) << bl::bind(&string_pair::second, bl::_1)); } Cheers, Rob.

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Igor R
Robert Jones
Steven Watanabe
Stuart Dootson