[c.math.abs]

[Note 1:

The headers <cstdlib> and <cmath> declare the functions described in this subclause.

— end note]

constexpr int abs(int j);
constexpr long int abs(long int j);
constexpr long int abs(long long int j);

Effects: These functions have the semantics specified in the C standard library for the functions abs, labs, and llabs, respectively.

Remarks: If abs is called with an argument of type X for which is_unsigned_v<X> is true and if X cannot be converted to int by integral promotion, the program is ill-formed.

[Note 2:

Allowing arguments that can be promoted to int provides compatibility with C.

— end note]

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constexpr floating-point-type abs(floating-point-type x);

Returns: The absolute value of x.

See also: ISO/IEC 9899:2024, 7.12.8.3, 7.24.7.1