Problem casting STL complex to fftw_complex

Question

The FFTW manual says that its fftw_complex type is bit compatible to std::complex class in STL. But that doesn\'t work for me:

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Answer 1

Re-write your code as follows:

#include 
#include 
int main()
{
   std::complex x(1,0);
   fftw_complex fx;
   memcpy( &fx, &x, sizeof( fftw_complex ) );
}

Every compiler I've used will optimise out the memcpy because it is copying a fixed, ie at compile time, amount of data.

This avoids pointer aliasing issues.

Edit: You can also avoid strict aliasing issues using a union as follows:

#include 
#include 
int main()
{
   union stdfftw
   {
       std::complex< double > stdc;
       fftw_complex           fftw;
   };
   std::complex x(1,0);
   stdfftw u;
   u.stdc = x;
   fftw_complex fx = u.fftw;
}

Though strictly this C99 rules (Not sure about C++) are broken as reading from a different member of a union to the one written too is undefined. It works on most compilers though. Personally I prefer my original method.