It seems to me that eval_if doesn't use lazy evaluation either. The error message quite obviously refers to an attempt of the compiler to evaluate the expression on the 'false' branch, whereas the condition (is_arithmetic<int>) is true to my best knowledge. Thx, Agoston --------------------------------------------------------------------- #include <iostream> #include <boost/mpl/eval_if.hpp> #include <boost/type_traits.hpp> #include <boost/mpl/identity.hpp> #include <vector> using namespace std; using namespace boost; using namespace mpl; template<typename T> struct value_type { typedef typename eval_if< is_arithmetic<T>, identity<T>, identity<typename T::value_type> >::type // ERROR type; }; int _tmain(int argc, _TCHAR* argv[]) { cout << typeid(value_type<int>::type).name() << endl; // ERROR cout << typeid(value_type<double>::type).name() << endl; cout << typeid(value_type< vector<int> >::type).name() << endl; cout << typeid(value_type< vector<double> >::type).name() << endl; return 0; } --------------------------------------------------------------------- Error: error C2825: 'T::value_type': cannot form a qualified name c:\guszti\prog\vcpp\Boost\xEvalIf\xEvalIf.cpp(27) : see reference to class template instantiation 'value_type<T>' being compiled with [ T=int ] error C2143: syntax error : missing ',' before 'value_type' error C2275: 'value_type' : illegal use of this type as an expression "Robert Ramey" <ramey@rrsd.com> az alábbiakat írta a következõ hírüzenetben: cqaid5$sp1$1@sea.gmane.org...
Look in the mpl documentation under eval_if
Robert Ramey
Agoston Bejo wrote:
Hi! I'm looking for some "lazy" version of boost::mpl::if_. I'm not very much into MPL, so I
don't really know either if such a function exists in boost or whether it can be easily
constructed. Platform: VC++7.1
Thx in advance, Agoston
This example shows you how it may cause problems that both "branches" of if_ get evaluated:
//------------------------------------------------------------ #include <iostream> #include <vector> #include <boost/mpl/if.hpp> #include <boost/type_traits.hpp>
using namespace std; using namespace boost; using namespace mpl;
template<typename T> struct t_type { typedef typename if_< is_arithmetic<T>, T, typename T::value_type>::type // ERROR! type; };
int _tmain(int argc, _TCHAR* argv[]) { cout << typeid(t_type<int>::type).name() << endl; // 'int' cout << typeid(t_type<vector<double> >::type).name() << endl; // 'double' return 0; }
//------------------------------------------------------------ Error message:
error C2825: 'T::value_type': cannot form a qualified name see reference to class template instantiation 't_type<T>' being compiled with [ T=int ]