Re: [Boost-users] Can not compile a program that uses typeof package by gcc?

20 Oct 2008


      On Mon, Oct 20, 2008 at 11:14 AM, Steven Watanabe <watanabesj@gmail.com> wrote:
...
AMDG
Peng Yu wrote:
...
template <typename T1, typename T2>
   struct multiply_traits;
template <typename T1, typename T2>
   struct multiply_traits<X<T1>, T2> {
     typedef X<T1> result_type;
   };
The most reliable way to define multiply_traits in general is
template<class T>
T make();
template<class T1, class T2>
struct multiply_traits {
  BOOST_TYPEOF_NESTED_TYPEDEF_TPL(nested, make<T1>() * make<T2>());
  typedef typename nested::type type;
};
...
Y<BOOST_TYPEOF(T1() * T2())> operator*(const Y<T1> &y, const T2 &t)
You need to use BOOST_TYPEOF_TPL.
http://www.boost.org/doc/libs/1_36_0/doc/html/typeof/refe.html#typeof.typo.
Some compilers can handle this (I just tried it on msvc 9.0), but I strongly
advise using multiply_traits as defined above for portability.
I changed the code according to you message (see below this message).
I got the following error, would you please help me figure out what
was wrong?

main.cc: In instantiation of 'A::multiply_traits<A::X<int>, int>::nested':
main.cc:32:   instantiated from 'A::multiply_traits<A::X<int>, int>'
main.cc:65:   instantiated from here
main.cc:31: error: no match for 'operator*' in 'A::make [with T =
A::X<int>]() * A::make [with T = int]()'
main.cc: In function 'int main()':
main.cc:65: error: no match for 'operator*' in 'x * 3'
main.cc:66: error: no match for 'operator*' in 'A::X<int>() * 0'
main.cc:66: error: no match for 'operator*' in 'y * 5'
make: *** [main-g.o] Error 1


Thanks,
Peng

#include <boost/typeof/typeof.hpp>
#include <iostream>

namespace A {

  template <typename T>
    class X {
      public:
        X() { }
        X(T t) : _t(t) { }
        const T &the_t() const { return _t; }
      private:
        T _t;
    };

  template <typename T1, typename T2>
    struct multiply_traits;

#if 0
  template <typename T1, typename T2>
    struct multiply_traits<X<T1>, T2> {
      typedef X<T1> result_type;
    };
#endif

  template<class T>
    T make();

  template<class T1, class T2>
    struct multiply_traits {
      BOOST_TYPEOF_NESTED_TYPEDEF_TPL(nested, make<T1>() * make<T2>())
      typedef typename nested::type type;
    };

  template <typename T1, typename T2>
    typename multiply_traits<X<T1>, T2>::result_type operator*(const
X<T1> &x, const T2 &t) {
      return X<T1>(x.the_t() * t);
    }

}

namespace B {

  template <typename T>
    class Y {
      public:
        Y(T t) : _t(t) { }
        const T &the_t() const { return _t; }
      private:
        T _t;
    };

  template <typename T1, typename T2>
    //Y<typename multiply_traits<T1, T2>::result_type> operator*(const
Y<T1> &y, const T2 &t) {
    //Y<T1> operator*(const Y<T1> &y, const T2 &t) {
    Y<BOOST_TYPEOF_TPL(T1() * T2())> operator*(const Y<T1> &y, const T2 &t) {
      return Y<T1>(y.the_t() * t);
    }
}

int main () {
  A::X<int> x(2);
  B::Y<A::X<int> > y(x);

  std::cout << (x * 3).the_t() << std::endl;
  std::cout << (y * 5).the_t().the_t() << std::endl;
}