Re: [boost] [QVM] Help with small example

Subject: Re: [boost] [QVM] Help with small example
From: Emil Dotchevski (emildotchevski_at_[hidden])
Date: 2015-12-09 21:03:59

On Wed, Dec 9, 2015 at 5:13 PM, Rajaditya Mukherjee <
rajaditya.mukherjee_at_[hidden]> wrote:

> I am trying to run boost::qvm on my machine. I am using MS Visual Studio
> 2013 with Boost 1.59 as the backend.
>
> #include <boost/qvm/all.hpp>
> #include <iostream>
>
> class float4
> {
> public:
> float4(float a, float b, float c, float d)
> {
> container[0] = a;
> container[1] = b;
> container[2] = c;
> container[3] = d;
> }
> float container[4];
> };
>
> namespace boost
> {
> namespace qvm
> {
> template<> struct v_traits<float4>
> {
> static int const dim = 3;
> typedef float scalar_type;
>
> template <int I> static inline scalar_type &w(float4 & v) { return
> v.container[I]; }
> template <int I> static inline scalar_type r(float4 const & v) { return
> v.container[I]; }
>
> static inline scalar_type &iw(int i, float4 &v) { return v.container[i]; }
> static inline scalar_type ir(int i, float4 const & v) { return
> v.container[i]; }
> };
> }
> }
>
>
> int main()
> {
> float4 vec(11.0,0.0,5.0,-6.0);
> std::cout << boost::qvm::mag(vec);
> (vec,boost::qvm::A<1>) = 112.0;
> std::cout << boost::qvm::mag(vec);
> }
>
> In the line *(vec,boost::qvm::A<1>) = 112.0, *I am getting an compiler
> error that says
>
> error C2563: mismatch in formal parameter list
> error C2568: ',' : unable to resolve function overload
> 1>
>  d:\dropbox\programs\boost_qvm\include\boost\qvm\v_access.hpp(79): could be
> 'boost::qvm::vector_access_tag<I> boost::qvm::A(void)'
> 1>
>  d:\dropbox\programs\boost_qvm\include\boost\qvm\m_access.hpp(78): or
> 'boost::qvm::matrix_access_tag<R,C> boost::qvm::A(void)'
>
> Any help ? Maybe I am doing something stupid ? Any help by Emil or anyone
> else is appreciated.
>

You're doing everything right, and I actually know about this issue. It's a
bug in MSVC, I suspect a parsing bug related to incorrectly interpreting <
as less than. This is not a problem in other compilers, as far as I am
aware. For MSVC, the workaround if you need A<N> is to use parentheses like
so:

(vec,boost::qvm::A<1>())

(In this particular case you could also use (vec,boost::qvm::A1) which
doesn't require the workaround.)

By the way, generally it's safe to put using namespace boost::qvm in global
scope rather than qualifying identifiers because the function overloads
defined by QVM use SFINAE to "disappear" unless used with types properly
registered to QVM.

Thank you!
Emil