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Michael
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 Posted: Sat Apr 17, 2004 7:20 pm Post subject: 'compile-time if statement' using a template function |
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Hi all
Is it possible to get the equivalent of a 'compile-time if statement' using
template functions? I'm guessing that template specialisation is how it
would be done, but I have no experience.
My first thought (which I don't believe works) is this:
class Base {
};
class Derived_1 : public Base {
};
class Derived_2 : public Base {
};
template <class T>
void do_it_if_D2( const Vec& v1, const Vec& v2, Vec& pt )
{
}
template <>
void do_it_if_D2<Derived_1>( const Vec& v1, const Vec& v2, Vec& pt )
{
}
template <>
void do_it_if_D2<Derived_2>( const Vec& v1, const Vec& v2, Vec& pt )
{
do_stuff_to_pt_using_v1_&_v2( v1, v2, pt );
}
....and then calling it like this.
do_it_if_D2<class_sent>( vec_a, vec_b, point );
Would be great if someone could point me in the right direction.
TIA
Michael
[excessively long sig deleted --mod]
[ See http://www.gotw.ca/resources/clcm.htm for info about ]
[ comp.lang.c++.moderated. First time posters: Do this! ]
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Carl Barron
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| Posted: Sun Apr 18, 2004 10:00 am Post subject: Re: 'compile-time if statement' using a template function |
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In article <BCA5DFC2.21777%mwh_acs (AT) hotmail (DOT) com>, Michael
<mwh_acs (AT) hotmail (DOT) com> wrote:
| Quote: | Hi all
Is it possible to get the equivalent of a 'compile-time if statement' using
template functions? I'm guessing that template specialisation is how it
would be done, but I have no experience.
|
Sure if the condition is a compile computable value.
Loki contains this
template <bool B,typename T,typename F> struct Select {typedef T
type;};
template <typename T,typename F>struct Select<false,T,F>
{
typedef F type;
};
typedef Select< bool_expr,true_type,false_type>::type result;
if bool_expr is true result is true_type else it is false_type.
| Quote: | My first thought (which I don't believe works) is this:
class Base {
};
class Derived_1 : public Base {
};
class Derived_2 : public Base {
};
template <class T
void do_it_if_D2( const Vec& v1, const Vec& v2, Vec& pt )
{
}
template
void do_it_if_D2
{
}
template
void do_it_if_D2<Derived_2>( const Vec& v1, const Vec& v2, Vec& pt )
{
do_stuff_to_pt_using_v1_&_v2( v1, v2, pt );
}
...and then calling it like this.
do_it_if_D2<class_sent>( vec_a, vec_b, point );
Would be great if someone could point me in the right direction.
the easiest way is to create some tags [empty classes] |
struct Derived1_tag{};
struct Derived2_tag{};
struct else_tag{};
and a conversion template compile time function:
template <class T>
struct switch_tag {typedef else_tag type;};
template <>
struct switch_tag<Derived1> {typedef Derived1_tag type;};
template <>
struct switch_tag<Derived2> {typedef Derived2_tag type;};
then provide the functions with an extra not used tag argument type,
that does the work for each case:
void do_it_if_helper(const Vec &v1,const Vec &v2,Vec &pt ,
Derived1_tag);
void do_it_if_helper(const Vec &v1,const Vec &v2,Vec &pt,
Derived2_tag);
void do_it_if_helper(const Vec &v1,const Vec &v2,Vec &pt, else_tag);
finally the template that is used to select the proper function at
compile time.
template <class T>
void do_it_if(const Vec &v1,const Vec &v2,Vec &pt)
{
return do_it_if_helper(v1,v2,pt,switch_tag<T>::type());
}
this approach is used by many of the algorithms in <algorithm> to
provide an optimzation iteatator categories via std::iterator_traits.
[ See http://www.gotw.ca/resources/clcm.htm for info about ]
[ comp.lang.c++.moderated. First time posters: Do this! ]
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