Question

Is it possible to compute the square root of an integer with a metafunction with the following signature :

template<unsigned int N> inline double sqrt();

(or maybe using the constexpr keyword, I don’t know what is the best).
With that, sqrt<2>() would be replaced by 1.414... at compile-time.

What would be the best implementation for a such function ?

Answer 1

This may not be what you are looking for, but I wanted to make sure you realized that typically with optimization the compiler will calculate the result at compile time anyway. For example, if you have this code:

void g()
{
  f(sqrt(42));
}

With g++ 4.6.3 with optimization -O2, the resulting assembly code is:

   9 0000 83EC1C                subl    $28, %esp
  11 0003 DD050000              fldl    .LC0
  12 0009 DD1C24                fstpl   (%esp)
  13 000c E8FCFFFF              call    _Z1fd
  14 0011 83C41C                addl    $28, %esp
  16 0014 C3                    ret
  73                    .LC0:
  74 0000 6412264A              .long   1244009060
  75 0004 47EC1940              .long   1075440711

The sqrt function is never actually called, and the value is just stored as part of the program.

Therefore to create a function that technically meets your requirements, you simply would need:

template<unsigned int N> inline double meta_sqrt() { return sqrt(N); }