Re: [Boost-users] Lambda, how difficult should it be?

10 Nov 2006

      Library based lambdas cannot be made perfect, unfortunately.
BLL works in most cases the natural way, and then when it doesn't,  
things can be confusing.
One of the biggest issues is that _1.foo() cannot be made to work.

It is hard to give precise advice on your email, but judging from the  
discussion on scopes
you may have some confusion on how placeholders work.

A placeholder is "in scope" for the entire lambda expression and will  
get the same value everywhere.
That is, all occurrences of _1 are replaced by the same argument (of  
type foo* in your example).

Here's a lambda function and a call to it which could be what you are  
after (I am not exactly sure what
you wanted to do). As you can see, casts are special too, as they  
cannot be overloaded.

Best, Jaakko Järvi

#include 
#include 
#include 
#include 
#include <vector>
using namespace boost::lambda;

struct foo
{
    int a;
    void* b;
};

int main () {

   std::vector v;
   v.push_back(new foo);

   if_then_else(
	        bind(&foo::a, _1) == 0,
                 bind(&foo::a, _1) = 1,
                 (*(ll_reinterpret_cast(bind(&foo::b, *_1)))) = 42
               )(v[0]);
}

On Nov 9, 2006, at 5:51 AM, Martin Young wrote:
...
Hi,
I've just started trying to use the Boost lambda library and I'm  
finding
that I can't get anything much to work how I'd expect.  I've read  
the docs
and I'm none the wiser. I'm running on Cygwin using GCC 3.4.4 if  
that makes
any difference.  My problem is pretty nebulous - I can't get the
non-workingness nailed down a specific issue - whenever I try  
something
different I get several screenfulls of different error messages.
So, previously I had a alot of code in the form below which  
performs linear
searches of a vector. Clearly this is not the Right Thing,  
especially as I
will want to change the vectors to different containers at a later  
point.
typedef struct
{
   int a;
   void* b;
} foo;
vector* v = buildAndPopulateVector();
for( int i=0, iL=v->size(); ia == 0 )
   {
       thisFoo->a = 1;
       *(int*)(thisFoo->b) = 42;
   }
}
Instead, I'd like to use the for_each algorithm to iterate over the  
contents
of the vector like this:
foreach( v->begin(), b->end(), ### )
Where the ### would normally be a function pointer (or relative  
thereof).
I'd like to use lambda so I can put my code inline.  At first I  
thought I
could do something like:
foreach( v->being(), b->end(),
   if_then( _1->a == 0,
   (
       _1->a = 1,
       *(int*)(_1->b) = 42
   )
)
I quickly discovered that the placeholders don't work like that and  
a pile
of bind and/or ->* operators are required to extract any interesting
information. I think I've got this bit sorted now though.
What I'm getting stuck on now is constructing a working version of the
if/then construct.  The condition expression seems okay...
if_then( bind(&foo::a,_1)==0,
but I just can't get the "body" part to compile.  I have to enclose  
it in
round brackets so it appears as one expression to the compiler, but  
then
it's not a lambda expression so I need to wrap it in something to  
make it a
lambda. I should then pass _1 in as the single parameter of my new  
nested
lambda because the previous _1 won't be in scope anymore - is this  
right?
Is there another (better) way to include multiple expressions in an  
if_then
construct without making a nested lambda and thus losing the  
bindings from
the original lambda?
What should I wrap my expression in to make it a unary lambda into  
which I
can pass _1 from the main lambda?  It's not getting automatically  
picked up
as a lambda expression because the use of _1 is in the middle of  
one of the
comma separated sub exprssions. Can I even pass _1 into a nested  
lambda like
that or does it need massaging first?  I've found dealing with the
placeholders to be very painful so far...
Are there any examples of this kind of thing anywhere? All the  
examples I've
found are incomplete or for very trivial cases, or have not been  
answered.
Even the "Beyond the std lib/boost" book has only very simple  
examples and
esoteric gotchas. It seems to me this ought to be a very simple  
thing - am I
going about it in the wrong way?
I've had a good Google and a read through this list's archives and  
there
seems to be very little discussion about the lambda library. Is it  
moribund
or am I the only person who can't make it work?
Sorry for all the rather vague questions and thanks for any pointers.
-Martin.
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