Re: C++0x lambda and capturing computed values

From:
Sohail Somani <sohail@taggedtype.net>
Newsgroups:
comp.lang.c++
Date:
Thu, 07 May 2009 19:34:19 GMT
Message-ID:
<%IGMl.25992$Db2.14803@edtnps83>
SG wrote:

On 7 Mai, 19:27, Sohail Somani <soh...@taggedtype.net> wrote:

The undesirable alternative is:

for(...)
{
  value_type & t = *it;
  functions.push_back([t](){whatever(t);});
}


I had to look into the draft again to check whether it actually does
what you seem to think it does due to decltype(t) being a reference.
But yes, it captures the referred-to object by value. So, inside the
lambda's body decltype(t) will not be a reference and it'll be
equivalent to what your first version with boost::bind does.


I had to double-check as well :-)

Just to clarify, your understanding comes from [expr.prim.lambda]/7 ?

It would be ideal if I could do something like:

functions.push-back([t=*it](){whatever(t);});

Any thoughts or suggestions?


Interesting idea. But it'll hardly save you typing. IMHO,

  auto&& t = *it;
  functions.push_back([=]{whatever(t);});

is an okay-solution which doesn't justify a more complicated grammar
in the capture clause.


Well the actual case is something like:

for(iterator it1, it2, it3 ...)
{
  .. some stuff here ..
  functions.push_back(bind(whatever,*it1,*it2,*it3));
}

To me, the alternative is quite verbose.

For comparison:

for(iterator it1, it2, it3 ...)
{
  auto&& t1=*it1;
  auto&& t2=*it2;
  auto&& t3=*it3;
  functions.push_back([=](){whatever(t);});
}

or

for(iterator it1, it2, it3 ...)
{
  functions.push_back([t1=*it1,t2=*it2,t3=*it3]()
                      {whatever(t1,t2,t3);});
}

Less typing even if it is for 6*3 characters is better if you ask me.
Seems that bind is not obsoleted by lambda (I didn't expect it to be
anyway.)

--
Sohail Somani
http://uint32t.blogspot.com

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