std::arg(std::complex)
From cppreference.com
                    
                                        
                    
                    
                                                            
                    | Defined in header <complex>
  | ||
| template< class T >  T arg( const complex<T>& z ); | ||
Returns the phase angle of the complex number z, i.e. atan2(std::imag(z), std::real(z)).
[edit] Parameters
| z | - | complex value | 
[edit] Return value
the phase angle of z
[edit] See also
| returns the squared magnitude (function template) | |
| returns the complex conjugate (function template) | |
| constructs a complex number from magnitude and phase angle (function template) | |