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Showing content from https://en.cppreference.com/w/cpp/language/../numeric/math/fabs.html below:

std::abs(float), std::fabs, std::fabsf, std::fabsl - cppreference.com

(1) float       abs( float num );

double      abs( double num );

long double abs( long double num ); (until C++23)

constexpr /* floating-point-type */
            abs( /* floating-point-type */ num );

(since C++23) (2) float       fabs ( float num );

double      fabs ( double num );

long double fabs ( long double num ); (until C++23)

constexpr /* floating-point-type */
            fabs ( /* floating-point-type */ num );

(since C++23)

float       fabsf( float num );

(3) (since C++11)
(constexpr since C++23)

long double fabsl( long double num );

(4) (since C++11)
(constexpr since C++23)

template< class Integer >
double      fabs ( Integer num );

(A) (since C++11)
(constexpr since C++23)

1-4) Computes the absolute value of the floating-point value num. The library provides overloads of std::abs and std::fabs for all cv-unqualified floating-point types as the type of the parameter num.(since C++23)

A) Additional overloads are provided for all integer types, which are treated as double.

(since C++11)

For integral arguments, the integral overloads of std::abs are likely better matches. If std::abs is called with an unsigned integral argument that cannot be converted to int by integral promotion, the program is ill-formed.

[edit] Parameters num - floating-point or integer value [edit] Return value

If successful, returns the absolute value of arg (|arg|). The value returned is exact and does not depend on any rounding modes.

[edit] Error handling

This function is not subject to any of the error conditions specified in math_errhandling.

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

[edit] 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::fabs(num) has the same effect as std::fabs(static_cast<double>(num)).

[edit] Example 
#include <cmath>
#include <iostream>
 
int main()
{
    std::cout << "abs(+3.0) = " << std::abs(+3.0) << '\n'
              << "abs(-3.0) = " << std::abs(-3.0) << '\n';
 
    // special values
    std::cout << "abs(-0.0) = " << std::abs(-0.0) << '\n'
              << "abs(-Inf) = " << std::abs(-INFINITY) << '\n'
              << "abs(-NaN) = " << std::abs(-NAN) << '\n';
}

Possible output:

abs(+3.0) = 3
abs(-3.0) = 3
abs(-0.0) = 0
abs(-Inf) = inf
abs(-NaN) = nan
[edit] Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR Applied to Behavior as published Correct behavior LWG 2192 C++98 overloads of std::abs were
inconsistently declared in two headers declared these overloads
in both headers LWG 2735 C++11 overloads of std::abs for integer types
returning double was erroneously required removed the requirement [edit] See also

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