I am developing a project in plain C++ using VS2010, I need to calculate the arc cosine of a real value (can be out of -1;1 range, so need to deal with complex numbers) and compare it with a double to see whether or not a threshold is satisfied

To make things clear, I need to convert this code (which works in matlab)
   % dotprods is a vector of real values
   tempvar=acos(dotprods); % inverse cosine, some of these values are complex
   [vals,indx] = sort(tempvar);  % Take inverse cosine and sort results
   % value compare
   if (vals(1) < threshold * secondvalue)
      % condition is TRUE
      % condition is FALSE

Unfortunately the acos function does not work with complex numbers.

I tried to arrange things this way:

for(int j...) // cycle values of the vector
     // dotprods is a ublas double vector with real values

     complex<double> c(0,1);	// i
     complex<double> one(1,0);    // 1
     complex<double> _acos = -c*log(dotprods[j]+c*pow(one-pow(dotprods[j],2),1/2));

     // here there is a cycle to sort values 

     if( abs(_acos) < threshold * secondvalue) // threshold and secondvalue are double
           // TRUE
          // FALSE

The problem is that the formula I wrote (should be equal to arccosine function theorically) is not working. Results are wrong.

Can I use something like "cacos" function in a VS2010 C++ program?

I need a help please