Defined in header <complex>  

template< class T > complex<T> polar( const T& r, const T& theta = T()); 
Returns a complex number with magnitude r
and phase angle theta
.
The behavior is undefined if r
is negative or NaN, or if theta
is infinite.
r    magnitude 
theta    phase angle 
a complex number determined by r
and theta
.
The following behaviorchanging defect reports were applied retroactively to previously published C++ standards.
DR  Applied to  Behavior as published  Correct behavior 

LWG 2459  C++98  behavior unclear for some inputs  made undefined 
LWG 2870  C++98  default value of parameter theta not dependent  made dependent 
returns the magnitude of a complex number (function template) 

returns the phase angle (function template) 
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