# std::acos, std::acosf, std::acosl

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< cpp‎ | numeric‎ | math

Defined in header `<cmath>` |  |
| (1) |
float acos ( float num ); double acos ( double num ); long double acos ( long double num ); |  | (until C++23)
/* floating-point-type */ acos ( /* floating-point-type */ num ); |  | (since C++23) (constexpr since C++26)
float acosf( float num ); | (2) | (since C++11) (constexpr since C++26)
long double acosl( long double num ); | (3) | (since C++11) (constexpr since C++26)
Additional overloads (since C++11) |  |
Defined in header `<cmath>` |  |
template< class Integer > double acos ( Integer num ); | (A) | (constexpr since C++26)

1-3) Computes the principal value of the arc cosine of num. The library provides overloads of `std::acos` for all cv-unqualified floating-point types as the type of the parameter. (since C++23)

A) Additional overloads are provided for all integer types, which are treated as double. | (since C++11)

### Parameters

num | - | floating-point or integer value

### Return value

If no errors occur, the arc cosine of num (arccos(num)) in the range [0, π], is returned.

If a domain error occurs, an implementation-defined value is returned (NaN where supported).

If a range error occurs due to underflow, the correct result (after rounding) is returned.

### Error handling

Errors are reported as specified in math_errhandling.

Domain error occurs if num is outside the range `[`-1.0`,` 1.0`]`.

If the implementation supports IEEE floating-point arithmetic (IEC 60559),

- If the argument is +1, the value `+0` is returned.
- If |num| > 1, a domain error occurs and NaN is returned.
- if the argument is NaN, NaN is returned.

### Notes

The additional overloads are not required to be provided exactly as (A). They only need to be sufficient to ensure that for their argument num of integer type, std::acos(num) has the same effect as std::acos(static_cast<double>(num)).

### Example

Run this code

    #include <cerrno>
    #include <cfenv>
    #include <cmath>
    #include <cstring>
    #include <iostream>

    // #pragma STDC FENV_ACCESS ON

    int main()
    {
        std::cout << "acos(-1) = " << std::acos(-1) << '\n'
                  << "acos(0.0) = " << std::acos(0.0) << '\n'
                  << "2*acos(0.0) = " << 2 * std::acos(0) << '\n'
                  << "acos(0.5) = " << std::acos(0.5) << '\n'
                  << "3*acos(0.5) = " << 3 * std::acos(0.5) << '\n'
                  << "acos(1) = " << std::acos(1) << '\n';

        // error handling
        errno = 0;
        std::feclearexcept(FE_ALL_EXCEPT);

        std::cout << "acos(1.1) = " << std::acos(1.1) << '\n';

        if (errno == EDOM)
            std::cout << "    errno == EDOM: " << std::strerror(errno) << '\n';
        if (std::fetestexcept(FE_INVALID))
            std::cout << "    FE_INVALID raised" << '\n';
    }

Output:

    acos(-1) = 3.14159
    acos(0.0) = 1.5708
    2*acos(0.0) = 3.14159
    acos(0.5) = 1.0472
    3*acos(0.5) = 3.14159
    acos(1) = 0
    acos(1.1) = nan
        errno == EDOM: Numerical argument out of domain
        FE_INVALID raised

### See also

asin asinf asinl (C++11)(C++11) | computes arc sine (\({\small\arcsin{x} }\)arcsin(x)) (function)
atan atanf atanl (C++11)(C++11) | computes arc tangent (\({\small\arctan{x} }\)arctan(x)) (function)
atan2 atan2f atan2l (C++11)(C++11) | arc tangent, using signs to determine quadrants (function)
cos cosf cosl (C++11)(C++11) | computes cosine (\({\small\cos{x} }\)cos(x)) (function)
acos(std::complex) (C++11) | computes arc cosine of a complex number (\({\small\arccos{z} }\)arccos(z)) (function template)
acos(std::valarray) | applies the function std::acos to each element of valarray (function template)

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