On 11/12/2011 7:18 PM, Christopher Schmidt wrote:

Gennadiy Rozental writes:

...

Hi,

Now I am trying to perform transform which mutates:

[snip]

...

Both gcc and msvc complain like this:

test.cpp:29:46: error: passing ‘const B’ as ‘this’ argument of ‘C B::make_c()’ discards qualifiers.

What am I missing now?

Gennadiy

fusion::transform only accepts cref-qualified sequences. That is why the resulting transform view will be store a reference to z1 const and thus the arguments passed to make_c are const-qualified.

https://svn.boost.org/trac/boost/ticket/3446

Construct the transform view by hand and your example should work fine.

Again, as I said in the link you gave, I am willing to accept a patch if it includes docs + tests. The changes are simple enough. Just don't give me a whole change-everything-in-one-fell-swoop patch. I will not have enough time to review them all. Incremental changes such as this will be good. Regards, -- Joel de Guzman http://www.boostpro.com http://boost-spirit.com

Gennadiy Rozental writes:

Christopher Schmidt writes:

...

Construct the transform view by hand and your example should work fine.

I am rather a newbie fusion wise. Do you mind showing me an example?

Gennadiy

auto z2=fusion::as_list(fusion::transform_view(z1,make_c())); Christopher

On Sat, Nov 12, 2011 at 12:20 PM, Ovanes Markarian wrote:

Hi!

please see my answer below.

On Sat, Nov 12, 2011 at 12:03 PM, Gennadiy Rozental wrote:

...

Hi,

Now I am trying to perform transform which mutates:

int main() { B b1, b2; auto z1 = fusion::make_list( b1, b2 ); !!! create a list of copied Bs

auto z2 = fusion::as_list( fusion::transform( z1, make_c() ) ); !!! copies Bs again

Actually transformation does not copy Bs. They are only copied at list construction , but transform calls the function: inline typename result_of::as_list<Sequence const>::type as_list(Sequence const& seq) { return result_of::as_list<Sequence const>::call(seq); } Which deals with a const sequence... Regards, Ovanes

Ovanes Markarian writes:

You need to make a list of references to Bs:     B b1, b2;    auto z1 = fusion::make_list( ref(b1), ref(b2) );    auto z2 = fusion::as_list( fusion::transform( z1, make_c() ) );

This one indeed works, but what I am really after is something like this: auto z1 = fusion::make_list( B(), B() ); auto z2 = fusion::as_list( fusion::transform( z1, make_c() ) ); I know first line will move constructed instances into list z1. Next I need to mutate them and adding boost::ref around B() is impossible. In reality my use case somewhat more complicated. Something like this: #include #include #include #include namespace fusion=boost::fusion; struct C {}; struct B { C make_c() { return C(); } }; class A { public: B make_b() const { return B(); } }; //____________________________________________________________________________// struct make_b { template<typename Sig> struct result; template <typename T> struct result { typedef B type; }; template <typename T> struct result { typedef B type; }; template <typename T> B operator()(T const& t) const { return t.make_b(); } }; struct make_c { template<typename Sig> struct result; template <typename T> struct result { typedef C type; }; template <typename T> struct result { typedef C type; }; template <typename T> C operator()(T& t) const { return t.make_c(); } }; template<typename T> void do_something( T const& t ) { } int main() { auto z1 = fusion::as_list( fusion::transform( fusion::make_list( A(), A() ), make_b() ) ); size_t size = 1; // I calculate size here, irrelevant to the question while( size-- > 0 ) do_something( fusion::as_list( fusion::transform( z1, make_c() ) ) ); return 0; } Same problem. Gennadiy