Re: [Boost-users] Need help with Errors in boost MPL tutorials

Subject: Re: [Boost-users] Need help with Errors in boost MPL tutorials
From: Jeffrey Lee Hellrung, Jr. (jeffrey.hellrung_at_[hidden])
Date: 2012-01-24 00:17:50

On Mon, Jan 23, 2012 at 6:03 PM, Linh Ha <lha_at_[hidden]> wrote:

> Dear all,
>
> I try to learn Meta-programing but i got stuck with lambda meta-function
> and place holder.
> I try to compile very simple program
>
> #include <boost/bind.hpp>
> #include <boost/mpl/lambda.hpp>
> #include <boost/type_traits.hpp>
>
> template <class F, class X>
> struct twice : F::template apply<typename F::template apply<X>::type > {
> };
>
> template <class X>
> struct two_pointers
> : twice<typename boost::mpl::lambda<boost::add_**pointer<_1> >::type, X>
> {};
> int main(int argc, char** argv) {
> return 0;
> }
>
> It is from the boost tutorial http://www.boost.org/doc/libs/**
> 1_48_0/libs/mpl/doc/tutorial/**the-lambda-metafunction.html<http://www.boost.org/doc/libs/1_48_0/libs/mpl/doc/tutorial/the-lambda-metafunction.html>. But it can not be complied. The error messages are
>
> main.cpp:21: error: type/value mismatch at argument 1 in template
> parameter list for ‘template<class T> struct boost::add_pointer’
> main.cpp:21: error: expected a type, got ‘<unnamed>::_1’
> main.cpp:21: error: template argument 1 is invalid
> main.cpp:21: error: template argument 3 is invalid
> main.cpp:21: error: template argument 1 is invalid
> main.cpp:22: error: expected ‘::’ before ‘{’ token
> main.cpp:22: error: expected class-name before ‘{’ token
>
> I use
>
> g++ (Ubuntu/Linaro 4.4.4-14ubuntu5) 4.4.5
>
> I have no idea what happened. Any inside would be appreciated. It is
> important I think since lambda is used all over place with MPL.
>
> Thanks,
> Linh Ha
>

Just on a first glance, looks like you're mistaking ::_1 (placeholder from
Boost.Bind) for boost::mpl::_1 (placeholder from Boost.MPL). They're
incompatible; try using the latter...?

- Jeff