[bind] How to dereference _1 in a boost::bind expression

Peter Barker

2 Oct 2008 2 Oct '08

10:12 a.m.

Hello, Here's an example of my problem: #include <algorithm> #include <vector> #include class TheClass { }; void accept(TheClass& tc) { } int main() { std::vector cont; // Compile error - how to dereference _1 ??? std::for_each(cont.begin(),cont.end(),boost::bind(&accept,_1)); } How can I dereference _1 so I can use the accept function? Regards, Pete

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Robert Jones

2 Oct 2 Oct

10:39 a.m.

On Thu, Oct 2, 2008 at 11:12 AM, Peter Barker wrote:

...

Hello,

Here's an example of my problem:

#include <algorithm> #include <vector>

#include

class TheClass { };

void accept(TheClass& tc) { }

int main() { std::vector cont;

// Compile error - how to dereference _1 ??? std::for_each(cont.begin(),cont.end(),boost::bind(&accept,_1)); }

How can I dereference _1 so I can use the accept function?

#include <algorithm> #include <vector> #include #include class TheClass { }; void accept(TheClass& tc) { } int main() { using namespace boost; using namespace boost::lambda; std::vector cont; // Compile error - how to dereference _1 ??? std::for_each(cont.begin(),cont.end(),bind(accept,*_1)); } - Rob.

Franz Alt

10:51 a.m.

New subject: [bind] How to dereference _1 in a boost::bind expression

Robert Jones schrieb:

...

On Thu, Oct 2, 2008 at 11:12 AM, Peter Barker wrote:

...
Hello,

Here's an example of my problem:

#include <algorithm> #include <vector>

#include

class TheClass { };

void accept(TheClass& tc) { }

int main() { std::vector cont;

// Compile error - how to dereference _1 ??? std::for_each(cont.begin(),cont.end(),boost::bind(&accept,_1)); }

How can I dereference _1 so I can use the accept function?

#include <algorithm> #include <vector>

#include #include

class TheClass { };

void accept(TheClass& tc) { }

int main() { using namespace boost; using namespace boost::lambda;

std::vector cont;

// Compile error - how to dereference _1 ??? std::for_each(cont.begin(),cont.end(),bind(accept,*_1)); }

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

Hi Rob, with std::for_each(cont.begin(),cont.end(),bind(&accept,*_1)); you can dereference _1. Best regards Franz

Peter Barker

1:36 p.m.

On Thu, Oct 2, 2008 at 11:51 AM, Franz Alt wrote:

...

Robert Jones schrieb:

...
On Thu, Oct 2, 2008 at 11:12 AM, Peter Barker wrote:

...
Hello,

Here's an example of my problem:

#include <algorithm> #include <vector>

#include

class TheClass { };

void accept(TheClass& tc) { }

int main() { std::vector cont;

// Compile error - how to dereference _1 ??? std::for_each(cont.begin(),cont.end(),boost::bind(&accept,_1)); }

How can I dereference _1 so I can use the accept function?

#include <algorithm> #include <vector>

#include #include

class TheClass { };

void accept(TheClass& tc) { }

int main() { using namespace boost; using namespace boost::lambda;

std::vector cont;

// Compile error - how to dereference _1 ??? std::for_each(cont.begin(),cont.end(),bind(accept,*_1)); }

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

Hi Rob,

with

std::for_each(cont.begin(),cont.end(),bind(&accept,*_1));

you can dereference _1.

Best regards

Franz

Franz, I think that gave an illegal indirection compiler error. Regards, Pete

Peter Barker

11:48 a.m.

On Thu, Oct 2, 2008 at 11:39 AM, Robert Jones wrote:

...

On Thu, Oct 2, 2008 at 11:12 AM, Peter Barker wrote:

...
Hello,

Here's an example of my problem:

#include <algorithm> #include <vector>

#include

class TheClass { };

void accept(TheClass& tc) { }

int main() { std::vector cont;

// Compile error - how to dereference _1 ??? std::for_each(cont.begin(),cont.end(),boost::bind(&accept,_1)); }

How can I dereference _1 so I can use the accept function?

#include <algorithm> #include <vector>

#include #include

class TheClass { };

void accept(TheClass& tc) { }

int main() { using namespace boost; using namespace boost::lambda;

std::vector cont;

// Compile error - how to dereference _1 ??? std::for_each(cont.begin(),cont.end(),bind(accept,*_1)); }

Rob, Thanks for the suggestion - my sample program was actually too stripped down. TheClass is a base with pure virtual methods and your suggestion gives me the "cannot instantiate abstract class" compiler error. Here's a new sample program: #include <algorithm> #include <vector> #include #include class TheClass { public: virtual ~TheClass() {} virtual void pvm() = 0; }; class DerivedClass : public TheClass { void pvm() {} }; void accept(TheClass& tc) { } int main() { std::vector cont; using namespace boost::lambda; using boost::lambda::_1; // Compile error - cannot instantiate abstract class - how to dereference _1 ??? std::for_each(cont.begin(),cont.end(),bind(&accept,*_1)); }

Robert Jones

12:53 p.m.

I think this may be a real limitation. See, http://lists.boost.org/Archives/boost/2008/07/139932.php It's not quite what you're trying to do, but close. Perhaps someone more knowledgeable than I will show us the way! - Rob.

Sven Gaerner

10:45 a.m.

New subject: [bind] How to dereference _1 in a boost::bind expression

On Thu, 2 Oct 2008 11:12:54 +0100 "Peter Barker" wrote:

...

Hello,

Here's an example of my problem:

#include <algorithm> #include <vector>

#include

class TheClass { };

void accept(TheClass& tc) { }

int main() { std::vector cont;

// Compile error - how to dereference _1 ??? std::for_each(cont.begin(),cont.end(),boost::bind(&accept,_1)); }

How can I dereference _1 so I can use the accept function?

Regards,

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

You don't have to dereference _1. Your accept function must look like this: void accept(TheClass *arg) { } Your std::vector contains pointers to TheClass so std::for_each() passes objects of that type to your functor, in this case a pointer to types of TheClass. Sven

Peter Barker

1:36 p.m.

On Thu, Oct 2, 2008 at 11:45 AM, Sven Gaerner wrote:

...

On Thu, 2 Oct 2008 11:12:54 +0100 "Peter Barker" wrote:

...
Hello,

Here's an example of my problem:

#include <algorithm> #include <vector>

#include

class TheClass { };

void accept(TheClass& tc) { }

int main() { std::vector cont;

// Compile error - how to dereference _1 ??? std::for_each(cont.begin(),cont.end(),boost::bind(&accept,_1)); }

How can I dereference _1 so I can use the accept function?

Regards,

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

You don't have to dereference _1. Your accept function must look like this: void accept(TheClass *arg) { } Your std::vector contains pointers to TheClass so std::for_each() passes objects of that type to your functor, in this case a pointer to types of TheClass.

Sven, I was trying to avoid changing the signature of accept as it has other call sites and I would need to update them. It's what I did in the end though. Thanks, Pete

Sven Gaerner

2:12 p.m.

New subject: [bind] How to dereference _1 in a boost::bind expression

On Thu, 2 Oct 2008 14:36:01 +0100 "Peter Barker" wrote:

...

On Thu, Oct 2, 2008 at 11:45 AM, Sven Gaerner wrote:

...
On Thu, 2 Oct 2008 11:12:54 +0100 "Peter Barker" wrote:

...
Hello,

Here's an example of my problem:

#include <algorithm> #include <vector>

#include

class TheClass { };

void accept(TheClass& tc) { }

int main() { std::vector cont;

// Compile error - how to dereference _1 ??? std::for_each(cont.begin(),cont.end(),boost::bind(&accept,_1)); }

How can I dereference _1 so I can use the accept function?

Regards,

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

You don't have to dereference _1. Your accept function must look like this: void accept(TheClass *arg) { } Your std::vector contains pointers to TheClass so std::for_each() passes objects of that type to your functor, in this case a pointer to types of TheClass.

Sven,

I was trying to avoid changing the signature of accept as it has other call sites and I would need to update them. It's what I did in the end though. Thanks,

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

Peter, I didn't think about avoiding to change the signature. Sorry for that. What about using a specific delegate functor? Somethink like that: struct Delegate { void operator()(TheClass *arg) { accept(*arg); } }; And then pass this functor to std::for_each(); std::for_each(cont.begin(),cont.end(), Delegate()); That wouldn't break the signature of the accept function, but it depends on that particular implementation. Sven

Peter Dimov

2:25 p.m.

New subject: [bind] How to dereference _1 in a boost::bindexpression

Peter Barker: ...

...

void accept(TheClass& tc) { }

int main() { std::vector cont;

// Compile error - how to dereference _1 ??? std::for_each(cont.begin(),cont.end(),boost::bind(&accept,_1)); }

How can I dereference _1 so I can use the accept function?

With boost::bind, there's no easy way to dereference _1. You'd need template<class T> T& deref( T* p ) { return *p; } std::for_each( cont.begin(), cont.end(), boost::bind( accept, boost::bind( deref<TheClass>, _1 ) ) );

Peter Barker

3:05 p.m.

New subject: [bind] How to dereference _1 in a boost::bindexpression

On Thu, Oct 2, 2008 at 3:25 PM, Peter Dimov wrote:

...

Peter Barker: ...

...
void accept(TheClass& tc) { }

int main() { std::vector cont;

// Compile error - how to dereference _1 ??? std::for_each(cont.begin(),cont.end(),boost::bind(&accept,_1)); }

How can I dereference _1 so I can use the accept function?

With boost::bind, there's no easy way to dereference _1. You'd need

template<class T> T& deref( T* p ) { return *p; }

std::for_each( cont.begin(), cont.end(), boost::bind( accept, boost::bind( deref<TheClass>, _1 ) ) ); Peter,

It's nice to hear the answer from the horses mouth (so to speak)! Regards, Pete

Jeff Flinn

3:16 p.m.

New subject: [bind] How to dereference _1 in a boost::bindexpression

Peter Barker wrote:

...

On Thu, Oct 2, 2008 at 3:25 PM, Peter Dimov wrote:

...
Peter Barker: ...

...
void accept(TheClass& tc) { }

int main() { std::vector cont;

// Compile error - how to dereference _1 ??? std::for_each(cont.begin(),cont.end(),boost::bind(&accept,_1)); }

How can I dereference _1 so I can use the accept function? With boost::bind, there's no easy way to dereference _1. You'd need

template<class T> T& deref( T* p ) { return *p; }

std::for_each( cont.begin(), cont.end(), boost::bind( accept, boost::bind( deref<TheClass>, _1 ) ) ); Peter,

It's nice to hear the answer from the horses mouth (so to speak)!

IIRC, indirect_iterator takes this approach. The following should work as well: std::for_each( make_indirect_iterator(cont.begin()) , make_indirect_iterator(cont.end()) , boost::bind( accept, _1)); Jeff Flinn

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participants (6)

Franz Alt
Jeff Flinn
Peter Barker
Peter Dimov
Robert Jones
Sven Gaerner