Re: bool value as template argument

From:

=?ISO-8859-1?Q?Daniel_Kr=FCgler?= <daniel.kruegler@googlemail.com>

Newsgroups:

comp.lang.c++.moderated

Date:

Thu, 28 Jan 2010 12:04:42 CST

Message-ID:

<de8e8666-bfb2-451a-ada7-46fa01be378e@r19g2000yqb.googlegroups.com>

On 28 Jan., 14:53, Rune Allnor <all...@tele.ntnu.no> wrote:

Hi all.

I have an application where i need to traverse a data container
in either the ascending or descending direction.

The naive C-style case would go something like

void traverse(std::vector<T>,bool ascending)
{
    size_t step = -1;
    if (ascending)
    {
        step = 1;
    }

}

I need to have one ascending and one descending tracker,
so I want to have a function object for each direction, and
specify the direction through a template argument:

template<bool ascending>
class tracker{

private:
    size_t step_;

}

template<bool ascending>
tracker<bool>::tracker()
{
     if (ascending) step_ = 1;
    else step_ = -1;

}

The problem is that the compiler doesn't like this way
of doing things. It complaiths that it can't find specializations
for the classes tracker<0> and tracker<1>.

We need to see your usage code to understand your problem.
Your above example is so buggy that I don't know where to
start ;-), therefore I invert the situation and assert that the
following program should be well-formed (but not useful):

template<bool ascending>
class tracker {
public:
     tracker();
private:
     int step_;
};

template<bool ascending>
tracker<ascending>::tracker()
{
     if (ascending) step_ = 1;
     else step_ = -1;
}

int main() {
   tracker<true> tr1;
   tracker<false> tr2;
}

Is it possible to achieve what I want by testing the template
argument in one function, as indicated above? Or do I need to
write full-blown specializations for both classes?

This should work.

HTH & Greetings from Bremen,

Daniel Kr?gler

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