template question

[Valeriu Catina]

Hi,

given the next code:


#define ENUM_CAST(x) int(x)

template<class T,int N>
class Array
{
public:
enum { rank = N };

protected:

// members ...

template<bool>
struct Select{ };

public:

template<class T_expr>
Array& evaluate(T_expr expr, Select<true>& )
{
// ... code
return *this;
}

template<class T_expr>
Array& evaluate(T_expr expr, Select<false>& )
{
// ... code
return *this;
} template<class T_expr>
Array& evaluateExpr(T_ expr expr)
{
return
evaluate(expr,S elect<ENUM_CAST (rank)==ENUM_CA ST(T_expr::rank )>());
}

public:
// ctors, dtor, function members ...
};

template<class T,int N>
class SomeExpr
{
public:
enum { rank = N };
};

void testFunc()
{
Array<double,2> A;
SomeExpr<double ,2> x;
SomeExpr<double ,1> y;

A.evaluateExpr( x);
A.evaluateExpr( y);

}

Intel c++ for linux and g++ 3.3.1 do not compile the code (the error
messages are below). In the meantime Intel c++ for Windows compiles it.
If I change the evaluate member functions like below

template<class T_expr>
Array& evaluate(T_expr expr, Select<true> )
{
// ... code
return *this;
}

template<class T_expr>
Array& evaluate(T_expr expr, Select<false> )
{
// ... code
return *this;
}
the compilation succedes.

Which compiler is right ?



Here is the output of the compiler (g++ 3.3.1):

stest.cpp: In member function `Array<T, N>& Array<T,
N>::evaluateExp r(T_expr) [with T_expr = SomeExpr<double , 2>, T = double,
int N = 2]':
stest.cpp:57: instantiated from here
stest.cpp:36: error: no matching function for call to `Array<double,
2>::evaluate(So meExpr<double, 2>&, Array<double, 2>::Select<true >)'
stest.cpp:21: error: candidates are: Array<T, N>& Array<T,
N>::evaluate(T_ expr, Array<T, N>::Select<true >&) [with T_expr =
SomeExpr<double , 2>, T = double, int N = 2]
stest.cpp:28: error: Array<T, N>& Array<T,
N>::evaluate(T_ expr, Array<T, N>::Select<fals e>&) [with T_expr =
SomeExpr<double , 2>, T = double, int N = 2]
stest.cpp: In member function `Array<T, N>& Array<T,
N>::evaluateExp r(T_expr) [with T_expr = SomeExpr<double , 1>, T = double,
int N = 2]':
stest.cpp:58: instantiated from here
stest.cpp:36: error: no matching function for call to `Array<double,
2>::evaluate(So meExpr<double, 1>&, Array<double, 2>::Select<fals e>)'
stest.cpp:21: error: candidates are: Array<T, N>& Array<T,
N>::evaluate(T_ expr, Array<T, N>::Select<true >&) [with T_expr =
SomeExpr<double , 1>, T = double, int N = 2]
stest.cpp:28: error: Array<T, N>& Array<T,
N>::evaluate(T_ expr, Array<T, N>::Select<fals e>&) [with T_expr =
SomeExpr<double , 1>, T = double, int N = 2]


and Intel (icc) 7.1 for linux:

stest.cpp(37): error: no instance of overloaded function "Array<T,
N>::evaluate [with T=double, N=2]" matches the argument list
argument types are: (SomeExpr<doubl e, 2>, Array<double,
2>::Select<true >)
evaluate(expr,S elect<ENUM_CAST (rank)==ENUM_CA ST(T_expr::rank )>());
^
detected during instantiation of "Array<T, N> &Array<T,
N>::evaluateExp r(T_expr) [with T=double, N=2, T_expr=SomeExpr <double, 2>]"

stest.cpp(37): error: no instance of overloaded function "Array<T,
N>::evaluate [with T=double, N=2]" matches the argument list
argument types are: (SomeExpr<doubl e, 1>, Array<double,
2>::Select<fals e>)
evaluate(expr,S elect<ENUM_CAST (rank)==ENUM_CA ST(T_expr::rank )>());
^
detected during instantiation of "Array<T, N> &Array<T,
N>::evaluateExp r(T_expr) [with T=double, N=2, T_expr=SomeExpr <double, 1>]"

compilation aborted for stest.cpp (code 2)

[Gianni Mariani]

Re: template question

Valeriu Catina wrote:
....[color=blue]
> Array& evaluateExpr(T_ expr expr)
> {
> return
> evaluate(expr,S elect<ENUM_CAST (rank)==ENUM_CA ST(T_expr::rank )>());[/color]

try :

evaluate<T>(exp r,Select<ENUM_C AST(rank)==ENUM _CAST(T_expr::r ank)>());

The problem is obviously in overload resolution and the standard is very
wordy about how and how not to resolve. I have yet to grok all of that
but I think that since your implicitly doing a conversion (from Select<>
to a Select<>&), it is not able to find a corresponding template to
resolve for.

[Rob Williscroft]

Re: template question

Valeriu Catina wrote in news:bmmmf7$dvm $1@f40-3.zfn.uni-bremen.de:
[color=blue]
> Hi,
>
> given the next code:
>
>
> #define ENUM_CAST(x) int(x)
>
> template<class T,int N>
> class Array
> {
> public:
> enum { rank = N };
>
> protected:
>
> // members ...
>
> template<bool>
> struct Select{ };
>
> public:
>
> template<class T_expr>
> Array& evaluate(T_expr expr, Select<true>& )[/color]

Make this:

Array& evaluate(T_expr expr, Select<true> const & )

or as you've found out yourself:

Array& evaluate(T_expr expr, Select<true> )

[snip]
[color=blue]
> Array& evaluateExpr(T_ expr expr)
> {
> return[/color]

Here you try to "bind" a temporary ( the Select<...>() ) to a
non-const reference, this is illegal in Standard C++, the compiler
that excepts it is probably operating in a msvc v6.0 compatibility
mode.
[color=blue]
> Which compiler is right ?
>[/color]

The one('s) that gave you the error.

HTH

Rob.
--

http://www.victim-prime.dsl.pipex.com/