29 Numerics library [numerics]

29.7 Mathematical functions for floating-point types [c.math]

29.7.2 Absolute values [c.math.abs]

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[Note 1: 
The headers <cstdlib> and <cmath> declare the functions described in this subclause.
— end note]
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constexpr int abs(int j); constexpr long int abs(long int j); constexpr long long int abs(long long int j);
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Effects: These functions have the semantics specified in the C standard library for the functions abs, labs, and llabs, respectively.
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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);
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Returns: The absolute value of x.
See also: ISO/IEC 9899:2018, 7.12.7.2, 7.22.6.1