acos

template<class T>
    complex<T> acos(complex<T>& x);

The acos function computes the complex arc cosine of x, with branch cuts outside the interval [-1, +1] along the real axis.

From IBM® XL C/C++ for AIX®, V12.1, you can use C99 library facilities in your C++ code. This new TR1 template function acos produces the same results as the existing C99 functions, as indicated in the following table.
TR1 template function Corresponding C99 function
template <> complex<float> 
acos(complex<float>& x);
float complex cacosf(float complex z);
template <> complex<double> 
acos(complex<double>& x);
double complex cacos(double complex z);
template <> complex<long double> 
acos(complex<long double>& x);
long double complex 
cacosl(long double complex z);
Note: To enable the TR1 headers, you must define the macro __IBMCPP_TR1__ as 1.