C++ metaprogramming with templates versus inlining

问题

Is it worth to write code like the following to copy array elements:

#include <iostream>
using namespace std;


template<int START, int N> 
struct Repeat { 
  static void copy (int * x, int * y) {
   x[START+N-1] = y[START+N-1];
   Repeat<START, N-1>::copy(x,y);
  }
};

template<int START> 
struct Repeat<START, 0> { 
  static void copy (int * x, int * y) {
   x[START] = y[START];
  }
};



int main () {


   int a[10];
   int b[10];

             // initialize
   for (int i=0; i<=9; i++) {
     b[i] = 113 + i;
     a[i] = 0;
   }

            // do the copy (starting at 2, 4 elements)
  Repeat<2,4>::copy(a,b);

             // show
   for (int i=0; i<=9; i++) {
   cout << a[i] << endl;
   }

} // ()

or is it better to use a inlined function?

A first drawback is that you can't use variables in the template.

回答1:

That's not better. First of all, it's not really compile time, since you make function calls here. If you are lucky, the compiler will inline these and end up with a loop you could have written yourself with much less amount of code (or just by using std::copy).

回答2:

General rule: Use templates for things known at compile time, use inlining for things known at run time. If you don't know the size of your array at compile time, then don't be using templates for it.

回答3:

You shouldn't do this. Templates were invented for different purpose, not for calculations, although you can do it. First you can't use variables, second templates will produce vast of unused structures at compilation, and third is: use for (int i = start; i <= end; i++) b[i] = a[i];

回答4:

That's better because you control and enforce the loop unrolling by yourself.

A loop can be unrolled by the compiler depending on optimizing options...

The fact that copying with copy is almost the best is not a good general answer because the loop unrolling can be done whatever is the computation done inside...

来源：https://stackoverflow.com/questions/2380143/c-metaprogramming-with-templates-versus-inlining

标签

c++

templates

metaprogramming