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kanze@gabi-soft.fr
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 Posted: Thu Jun 26, 2003 10:31 pm Post subject: Re: Single character of a constant string as template parame |
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"Yuriy Sorokin" <yura (AT) cad (DOT) ntu-kpi.kiev.ua> wrote
| Quote: | Look at the fragment below
const char ST[]="12345";
template<char CH> struct Digit
{
enum { result=CH-'0' };
};
int main(int argc, char* argv[])
{
Digit<'0'> d1;// Looks good
Digit<ST[0]> d2; //Error
return 0;
}
Is such code legal?
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No.
| Quote: | ST[0] is a constant expression,
|
Not according to the standard. There are two types of constant
expressions, integral constant expressions, and other constant
expressions. An integral constant expression "can involve only [...]
const variables or static data members of integral or enumeration types
initialized with constant expressions. [...] In particular, except in
sizeof expressions, funcitons, class objects, pointers, or references
shall not be used, [...]" Here, we have a const variable (ST), but it
is NOT of integral or enumeration type. And of course, in the
expression a[b], an array type decays immediately to a pointer, and
pointers are banned by the second sentence that I quoted.
The constraints on other constant expressions are even more restrictive.
| Quote: | but the compilers(Borland 5.6.1 and GCC 3.2.1) do not accept it.
Is there another way to pass a single characters of a constant string
as a template parameter?
|
Not that I know of.
--
James Kanze GABI Software mailto:kanze (AT) gabi-soft (DOT) fr
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Markus Werle
Guest
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| Posted: Mon Jun 30, 2003 5:58 pm Post subject: Re: Single character of a constant string as template parame |
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Yuriy Sorokin wrote:
| Quote: | Is there another way to pass a single characters of a constant
string as a template parameter?
|
If you use mpl::vector for the string representation, yes.
#include "boost/mpl/integral_c.hpp"
#include "boost/mpl/vector.hpp"
#include "boost/mpl/at.hpp"
#include "boost/mpl/int.hpp"
#include "boost/mpl/transform.hpp"
#include <iostream>
namespace mpl = boost::mpl;
// crying for template typedefs ...
template <char C>
struct Char : public mpl::integral_c<char, C>
{};
template<class> struct Digit;
template<char CH>
struct Digit<Char
{
enum { value=CH-'0' };
typedef mpl::integral_c<int, value> type;
};
template <class T>
struct DigitsOfString
{
typedef typename mpl::transform<T, Digit::type AllDigits;
template <int Pos> struct At
{
typedef typename mpl::at<AllDigits, mpl::int_::type type;
enum { value = type::value };
};
};
int main()
{
// maybe U should write a maccro similar to the TYPELIST macro
// in loki-lib (see books of Andrei Alexandrescou and sf.net)
typedef mpl::vector<
Char<'H'>,
Char<'e'>,
Char<'l'>,
Char<'l'>,
Char<'o'>,
Char<'!'> > Hello;
// do it for one element
std::cerr << (
Digit<
mpl::at::type
<< std::endl;
// do it for all elements:
// (result is mpl::vector )
typedef mpl::transform<Hello, Digit::type AllDigits;
std::cerr << mpl::at::type::value << 'n';
// delegate (my favourite!)
std::cerr << DigitsOfString
}
Markus
--
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Visit http://daixtrose.sourceforge.net/
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