cannot appear in a constant-expression

Question

In the following c++ program:

static const int row = (dynamic_cast<int>(log(BHR_LEN*G_PHT_COUNT)/log(2)));
static const int pht_bits = ((32*1024)/(G_PHT_COUNT * G_PHT_COUNT * BHR_LEN));
unsigned char tab[pht_bits][1<<row];

I get the error message double log(double)’ cannot appear in a constant-expression. why am I getting this problem since i have put an integer cast in front? How should i fix this?