Expand code n times?

Hi!

I'm using a template class mp_image which has a template argument that specifies the number of channels in the image. "Normal" image has 3 channels, either RGB, Lab, HSL, HSV or any other. A gray scale image has one channel.

Now, for a simple example, I have a member function called SetColor which takes a color (a pointer to an array containing one value for each channel), and puts that color to the whole picture. Currently, I have an iterator i going through each layer one at a time. But if the number of channels would have been known when I wrote the code, I would simply have implemented the code that many times, one time for each channel, without making a loop. Is there some way to perform this (maybe using some template trick) when the number of channels isn't known at the time of coding but at the time of compiling?

I don't know any trick to do this, but you should not do it anyway. Your compiler should be smart enough to know when it's better to unroll a loop and your template parameter is a compile time constant so it should be pretty easy.

So if I, for example, write

Code:

---

int i;
for (i = 0; i < 100; i++) printf("&#37;d\n", i);

---

Will the compiler do some form of unroll (not a complete unroll), so that there's not only one printf statement in each loop, but maybe two or more (cause it would run a little bit faster, wouldn't it)? Of course I'm not using 100 channels in my images. ^^

If the compiler thinks it's worth unrolling the loop, it will do so itself.

Quote:

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Originally Posted by TriKri

So if I, for example, write

Code:

---

int i;
for (i = 0; i < 100; i++) printf("%d\n", i);

---

Will the compiler do some form of unroll (not a complete unroll), so that there's not only one printf statement in each loop, but maybe two or more (cause it would run a little bit faster, wouldn't it)? Of course I'm not using 100 channels in my images. ^^

---

Yes, if the compiler think it's right, it will do something like:

Code:

---

for(i = 0; i < 100; i+=2)
{
  printf("%d\n", i);
  printf("%d\n", i+1);
}

---

Thus halfing the overhead of the loop in comparison to the processing within the loop. Probably not worth it for printf, as the overhead of 100 loops would dwarf in comparison with the code inside printf - but the compiler may not know that... ;-)

--
Mats

>> I'm using a template class mp_image which has a template argument that specifies the number of channels in the image

You could use member template specialization based on the "number-of-channels" template argument. Member template specialization (MTS) inside a template class is a bit tricky because: 1) MTS's must be implemented at "namespace scope" 2) The containing template must also be specialized when implementing MTS's.

Code:

---

template<size_t NC>
struct Image
{
    char image[NC];

    void SetColor(char *c)
    {
        return SetColorT<NC>(c);
    }

    template<size_t NC2>
    void SetColorT(char *c)
    // comment out this code to get a compile/link error if a value
    // of NC is used that hasn't been specialized for SetColorT<>
    {
        for (size_t n = 0; n < NC2; ++n)
            image[n] = c[n];
    }
};

template<>
template<>
void Image<2>::SetColorT<2>(char *c)
{
    image[0] = c[0];
    image[1] = c[1];
}

template<>
template<>
void Image<4>::SetColorT<4>(char *c)
{
    image[0] = c[0];
    image[1] = c[1];
    image[2] = c[2];
    image[3] = c[3];
}

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Now the compiler knows exactly what to do when NC is 2 or 4.

gg