[Boost-users] Re: Looking for lazy if_

24 Dec 2004


      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 
#include 
#include 
#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"  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 
#include 
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::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
        ]