Re: [Boost-users] Lambda and placeholders

Subject: Re: [Boost-users] Lambda and placeholders
From: Sebastian Redl (sebastian.redl_at_[hidden])
Date: 2012-03-30 15:05:44

On 30.03.2012, at 19:19, Sven Bauhan wrote:

> Hi,
> 
> I just tried to use boost::lambda to prevent from creating an extra Functor 
> class just for one call.
> This is the code I tried:
> 
> 	std::deque<LightningSlice>::const_iterator l
> 		= std::find_if( m_lightnings.begin(), m_lightnings.end(),
> 		boost::lambda::_1.timeslice().contains(time_) );
> 
> with 
> 	typedef std::deque<LightningSlice> LightningQueue;
> 	LightningQueue m_lightnings;  //!< The lightnings in timeslices
> 
> 	const boost::posix_time::time_period& LightningSlice::timeslice() const 
> {
> 		return this->m_period;
> 	}
> 
> Then I get the error:
> ../LightningIndex.cpp:38: error: 'const struct 
> boost::lambda::lambda_functor<boost::lambda::placeholder<1> >' has no member 
> named 'timeslice'
> 
> What did I wrong?

Calling member functions doesn't work that way with lambdas. You have to use boost::lambda::bind.

http://www.boost.org/doc/libs/1_49_0/doc/html/lambda/le_in_details.html#lambda.bind_expressions

So for your code:

namespace ll = boost::lambda;
... = std::find_if(..., ll::bind(&Timeslice::contains, ll::bind(&LightningSlice::timeslice, ll::_1), time_));

Yes, it's ugly. Unfortunately, you can't really improve on it.

Sebastian