On Fri, 21 Nov 2003, Peter Dimov wrote:

Allen Bierbaum wrote:

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Hello all.

I am having trouble with the boost lambda library. I am convinced that my troubles come from some little thing I am overlooking, but for the life of me I can't seem to find the problem. I have been looking at it for the entire afternoon with no solution in sight.

I have included some sample code below showing the basic idea of what I am trying to accomplish. Can anyone provide me with a working solution?

The summary is: - I have a vector of shared_ptr's to a class A - That class has a method that returns a value I would like to test for in a find_if call

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Can anyone tell me how I can call a method from a lamda method iterating over a sequence of shared_ptrs? And then how do I have this method also call another method?

The short answer is that you can't, at least with the current implementation.

There are three possible ways to call a member function with Lambda:

(bl::_1 ->* &Foo::bar )(foo_ptr); bl::bind(&Foo::bar, *bl::_1)(foo_ptr); bl::bind(&Foo::bar, bl::_1)(foo_ptr);

The first alternative doesn't work since shared_ptr does not have operator->*.

The second alternative doesn't work since Lambda doesn't support *_1 for shared_ptr.

In particular, Lambda's return type deduction templates are not aware of shared_ptr. The second example can be made to work by telling the library what the return type of calling operator* on a shared_ptr<Foo> is: bl::bind(&Foo::bar, bl::ret<&Foo>(*bl::_1))(foo_ptr);

The third alternative doesn't work since it is not supported by Lambda (probably by design), although it is supported by Bind, and may be supported by a future version of Lambda.

Yup, bind is special cased for shared_ptr's, lambda doesn't know anything about them. Best, Jaakko

Peter Dimov wrote: [...] Oh well, not truly offlist, sorry. ;-)

"Peter Dimov" writes:

Hi,

Jaakko Jarvi wrote:

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The second alternative doesn't work since Lambda doesn't support *_1 for shared_ptr.

In particular, Lambda's return type deduction templates are not aware of shared_ptr.

They do not need to be aware of shared_ptr. shared_ptr provides ::element_type, for consistency with std::auto_ptr. boost::shared_ptr also has ::reference, although tr1::shared_ptr might not. You just need to look for the individual typedef and not instantiate iterator_traits prematurely.

But what if iterator_traits is specialized? The typedef could say the wrong thing. -- Dave Abrahams Boost Consulting www.boost-consulting.com