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J Lindrud
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 Posted: Sun Aug 29, 2004 10:46 pm Post subject: Lambda expression question |
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Hi all,
I have the following container
std::vector< boost::function funcs;
, containing function objects that are to be called with a
std::ostream argument. In order to iterate through the container and
make all the function calls, I can write:
for (unsigned int i=0; i
{
funcs[i]( std::cout );
}
However, I would rather use the more abstract form
std::for_each(
funcs.begin(),
funcs.end(),
???
);
where the ??? represents a boost::lambda expression, but I simply
can't figure out what the lambda expression should look like. Would
have liked to write _1( boost::ref(std::cout)), but that's a no-go.
Any suggestions?
Thanks!
Jarl.
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Carl Barron
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| Posted: Mon Aug 30, 2004 10:55 am Post subject: Re: Lambda expression question |
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J Lindrud <jlindrud (AT) hotmail (DOT) com> wrote:
| Quote: | Hi all,
I have the following container
std::vector< boost::function funcs;
, containing function objects that are to be called with a
std::ostream argument. In order to iterate through the container and
make all the function calls, I can write:
for (unsigned int i=0; i<funcs.size(); i++)
{
funcs[i]( std::cout );
}
However, I would rather use the more abstract form
std::for_each(
funcs.begin(),
funcs.end(),
???
);
|
Use a functor at a namespace scope.
typedef boost::function
class DoIt
{
std::ostream &os;
public:
DoIt(std::ostream &a) s(a){}
void operator () (Function &f) {f(os);}
};
int main()
{
// ...
std::for_wach(funcs.begin(),funcs.end(),DoIt(std::cout));
}
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Maxim Yegorushkin
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| Posted: Mon Aug 30, 2004 11:45 pm Post subject: Re: Lambda expression question |
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Lindrud <jlindrud (AT) hotmail (DOT) com> wrote:
| Quote: | I have the following container
std::vector< boost::function funcs;
, containing function objects that are to be called with a
std::ostream argument. In order to iterate through the container and
make all the function calls, I can write:
for (unsigned int i=0; i<funcs.size(); i++)
{
funcs[i]( std::cout );
}
However, I would rather use the more abstract form
std::for_each(
funcs.begin(),
funcs.end(),
???
);
where the ??? represents a boost::lambda expression, but I simply
can't figure out what the lambda expression should look like. Would
have liked to write _1( boost::ref(std::cout)), but that's a no-go.
Any suggestions?
|
Unless there is a newer syntax for binding operator() in boost::lambda,
you may use something like this:
int main()
{
typedef boost::function
std::vector<fun_t> funcs;
std::for_each(
funcs.begin()
, funcs.end()
, boost::lambda::bind(&fun_t::operator(), boost::lambda::_1, boost::lambda::var(std::cout))
);
}
--
Maxim Yegorushkin
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J Lindrud
Guest
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| Posted: Tue Aug 31, 2004 10:11 am Post subject: Re: Lambda expression question |
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[email]cbarron3 (AT) ix (DOT) netcom.com[/email] (Carl Barron) wrote in message news:<1gjarbs.oy6it4iyw66tN%cbarron3 (AT) ix (DOT) netcom.com>...
| Quote: |
Use a functor at a namespace scope.
|
Yeah, I know about functors, but if I have to write a whole new class
every time I want to use std::for_each, I'll probably just stick to
the plain old for loop.
Using lambda expressions makes std::for_each look a whole lot nicer, I
just didn't see how to make it work in this particular case.
/Jarl
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David Abrahams
Guest
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| Posted: Tue Aug 31, 2004 7:52 pm Post subject: Re: Lambda expression question |
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[email]jlindrud (AT) hotmail (DOT) com[/email] (J Lindrud) writes:
| Quote: | Hi all,
I have the following container
std::vector< boost::function funcs;
, containing function objects that are to be called with a
std::ostream argument. In order to iterate through the container and
make all the function calls, I can write:
for (unsigned int i=0; i
{
funcs[i]( std::cout );
}
However, I would rather use the more abstract form
std::for_each(
funcs.begin(),
funcs.end(),
???
);
where the ??? represents a boost::lambda expression, but I simply
can't figure out what the lambda expression should look like. Would
have liked to write _1( boost::ref(std::cout)), but that's a no-go.
Any suggestions?
|
I'm not sure what the answer is, but you'll surely have better luck
posting on one of the appropriate Boost mailing lists
http://www.boost.org/more/mailing_lists.htm
I've asked the lambda library authors to try to follow up on this one.
HTH,
--
Dave Abrahams
Boost Consulting
http://www.boost-consulting.com
[ See http://www.gotw.ca/resources/clcm.htm for info about ]
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Terje Sletteb?
Guest
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| Posted: Wed Sep 01, 2004 9:58 am Post subject: Re: Lambda expression question |
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[email]jlindrud (AT) hotmail (DOT) com[/email] (J Lindrud) wrote in message news:<ff70dbdb.0408290540.25af91e8 (AT) posting (DOT) google.com>...
| Quote: | Hi all,
I have the following container
std::vector< boost::function funcs;
, containing function objects that are to be called with a
std::ostream argument. In order to iterate through the container and
make all the function calls, I can write:
for (unsigned int i=0; i<funcs.size(); i++)
{
funcs[i]( std::cout );
}
However, I would rather use the more abstract form
std::for_each(
funcs.begin(),
funcs.end(),
???
);
where the ??? represents a boost::lambda expression, but I simply
can't figure out what the lambda expression should look like. Would
have liked to write _1( boost::ref(std::cout)), but that's a no-go.
Any suggestions?
|
You can get there using Lambda's bind. Here's a complete, working
example for the above (tested on g++ 3.2):
--- Start ---
#include
#include <vector>
#include <algorithm>
#include <boost/function.hpp>
#include <boost/ref.hpp>
#include <boost/lambda/lambda.hpp>
#include <boost/lambda/bind.hpp>
using boost::lambda::_1;
void f(std::ostream &out)
{
out << "f()n";
}
typedef boost::function
int main()
{
std::vector<element_type> funcs;
funcs.push_back(element_type(f));
funcs.push_back(element_type(f));
funcs.push_back(element_type(f));
std::for_each(
funcs.begin(),
funcs.end(),
boost::lambda::bind<void>(_1,boost::ref(std::cout)));
}
--- End ---
This prints:
f()
f()
f()
Boost.Lambda needs an expression that can be turned into a function
object. Doing _1(...) will attempt to call the function call operator
on the _1 object, rather than on what _1 will be a placeholder for in
the expression.
Also, bind needs to be told the return type, void, as it can't deduce
it from the boost::function object (unlike a function). Try the
following substitutions in the above code, to see this:
typedef void(*element_type)(std::ostream &);
funcs.push_back(f);
funcs.push_back(f);
funcs.push_back(f);
boost::lambda::bind(_1,boost::ref(std::cout))); // No <void>
boost::function may, however, be adapted to work with Lambda's bind,
using something like this:
template<class Base,class R>
struct bind_adapter : Base
{
template<class P1>
bind_adapter(const P1 &p1) : Base(p1) {};
template<class Args>
struct sig { typedef R type; };
};
typedef bind_adapter<boost::function
element_type;
boost::lambda::bind(_1,boost::ref(std::cout))); // No <void> here,
either
Alternatively, if boost::function had a "sig" member template like the
above, it could be used as is. Perhaps a useful addition to
boost::function?
However, converging on the standard proposal result_of may be better
in the long run.
By the way, the above can also be done with Boost.Bind, with something
extra:
#include <boost/bind.hpp>
#include <boost/bind/apply.hpp>
boost::bind(boost::apply<void>(),_1,boost::ref(std::cout)));
Regards,
Terje
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tom_usenet
Guest
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| Posted: Wed Sep 01, 2004 9:24 pm Post subject: Re: Lambda expression question |
|
|
On 29 Aug 2004 18:46:27 -0400, [email]jlindrud (AT) hotmail (DOT) com[/email] (J Lindrud) wrote:
| Quote: | Hi all,
I have the following container
std::vector< boost::function funcs;
, containing function objects that are to be called with a
std::ostream argument. In order to iterate through the container and
make all the function calls, I can write:
for (unsigned int i=0; i<funcs.size(); i++)
{
funcs[i]( std::cout );
}
However, I would rather use the more abstract form
std::for_each(
funcs.begin(),
funcs.end(),
???
|
boost::lambda::bind
| Quote: | );
where the ??? represents a boost::lambda expression, but I simply
can't figure out what the lambda expression should look like. Would
have liked to write _1( boost::ref(std::cout)), but that's a no-go.
Any suggestions?
|
It seems that operator() hasn't been overloaded to support direct
calls; I imagine there were technical problems with supporting the
"obvious" syntax, which is why bind was supplied.
Tom
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David Abrahams
Guest
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| Posted: Thu Sep 02, 2004 11:54 am Post subject: Re: Lambda expression question |
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tom_usenet <tom_usenet (AT) hotmail (DOT) com> writes:
| Quote: | On 29 Aug 2004 18:46:27 -0400, [email]jlindrud (AT) hotmail (DOT) com[/email] (J Lindrud) wrote:
Hi all,
I have the following container
std::vector< boost::function funcs;
, containing function objects that are to be called with a
std::ostream argument. In order to iterate through the container and
make all the function calls, I can write:
for (unsigned int i=0; i<funcs.size(); i++)
{
funcs[i]( std::cout );
}
However, I would rather use the more abstract form
std::for_each(
funcs.begin(),
funcs.end(),
???
boost::lambda::bind
);
where the ??? represents a boost::lambda expression, but I simply
can't figure out what the lambda expression should look like. Would
have liked to write _1( boost::ref(std::cout)), but that's a no-go.
Any suggestions?
It seems that operator() hasn't been overloaded to support direct
calls; I imagine there were technical problems with supporting the
"obvious" syntax,
|
Well, yeah. _1 is a lambda expression that returns its first
argument:
_1(44) == 44
If you look at http://tinyurl.com/5jpr4#sect:placeholders you'll see:
int i, j, k;
_1(i, j, k) // returns i, discards j and k
| Quote: | which is why bind was supplied.
|
I don't think so; bind has other uses. Check the docs:
--
Dave Abrahams
Boost Consulting
http://www.boost-consulting.com
[ See http://www.gotw.ca/resources/clcm.htm for info about ]
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