Defined in header <complex>  

template< class T > complex<T> acos( const complex<T>& z );  (since C++11) 
Computes complex arc cosine of a complex value z
. Branch cuts exist outside the interval [−1 ; +1] along the real axis.
z    complex value 
If no errors occur, complex arc cosine of z
is returned, in the range [0 ; ∞) along the real axis and in the range [−iπ ; iπ] along the imaginary axis.
Errors are reported consistent with math_errhandling
.
If the implementation supports IEEE floatingpoint arithmetic,
std::acos(std::conj(z)) == std::conj(std::acos(z))
z
is (±0,+0)
, the result is (π/2,0)
z
is (±0,NaN)
, the result is (π/2,NaN)
z
is (x,+∞)
(for any finite x), the result is (π/2,∞)
z
is (x,NaN)
(for any nonzero finite x), the result is (NaN,NaN)
and FE_INVALID
may be raised. z
is (∞,y)
(for any positive finite y), the result is (π,∞)
z
is (+∞,y)
(for any positive finite y), the result is (+0,∞)
z
is (∞,+∞)
, the result is (3π/4,∞)
z
is (+∞,+∞)
, the result is (π/4,∞)
z
is (±∞,NaN)
, the result is (NaN,±∞)
(the sign of the imaginary part is unspecified) z
is (NaN,y)
(for any finite y), the result is (NaN,NaN)
and FE_INVALID
may be raised z
is (NaN,+∞)
, the result is (NaN,∞)
z
is (NaN,NaN)
, the result is (NaN,NaN)
Inverse cosine (or arc cosine) is a multivalued function and requires a branch cut on the complex plane. The branch cut is conventionally placed at the line segments (∞,1) and (1,∞) of the real axis. The mathematical definition of the principal value of arc cosine is acos z =
1 
2 
For any z, acos(z) = π  acos(z).
#include <iostream> #include <cmath> #include <complex> int main() { std::cout << std::fixed; std::complex<double> z1(2, 0); std::cout << "acos" << z1 << " = " << std::acos(z1) << '\n'; std::complex<double> z2(2, 0.0); std::cout << "acos" << z2 << " (the other side of the cut) = " << std::acos(z2) << '\n'; // for any z, acos(z) = pi  acos(z) const double pi = std::acos(1); std::complex<double> z3 = pi  std::acos(z2); std::cout << "cos(pi  acos" << z2 << ") = " << std::cos(z3) << '\n'; }
Output:
acos(2.000000,0.000000) = (3.141593,1.316958) acos(2.000000,0.000000) (the other side of the cut) = (3.141593,1.316958) cos(pi  acos(2.000000,0.000000)) = (2.000000,0.000000)
(C++11)  computes arc sine of a complex number (arcsin(z)) (function template) 
(C++11)  computes arc tangent of a complex number (arctan(z)) (function template) 
computes cosine of a complex number (cos(z)) (function template) 

(C++11)(C++11)  computes arc cosine (arccos(x)) (function) 
applies the function std::acos to each element of valarray (function template) 
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