std::trunc, std::truncf, std::truncl

float       trunc ( float num );
double      trunc ( double num );
long double trunc ( long double num );

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

float       truncf( float num );

long double truncl( long double num );

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

double

Parameters

Return value

If no errors occur, the nearest integer value not greater in magnitude than num (in other words, num rounded towards zero) is returned.

Return value

num

Error handling

Errors are reported as specified in math_errhandling.

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

• The current rounding mode has no effect.
• If num is ±∞, it is returned, unmodified
• If num is ±0, it is returned, unmodified
• If num is NaN, NaN is returned

Notes

FE_INEXACT may be (but isn't required to be) raised when truncating a non-integer finite value.

The largest representable floating-point values are exact integers in all standard floating-point formats, so this function never overflows on its own; however the result may overflow any integer type (including std::intmax_t), when stored in an integer variable.

The implicit conversion from floating-point to integral types also rounds towards zero, but is limited to the values that can be represented by the target type.

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::trunc(num) has the same effect as std::trunc(static_cast<double>(num)).

Example

#include <cmath>
#include <initializer_list>
#include <iostream>
 
int main()
{
    const auto data = std::initializer_list<double>
    {
        +2.7, -2.9, +0.7, -0.9, +0.0, 0.0, -INFINITY, +INFINITY, -NAN, +NAN
    };
 
    std::cout << std::showpos;
    for (double const x : data)
        std::cout << "trunc(" << x << ") == " << std::trunc(x) << '\n';
}

trunc(+2.7) == +2
trunc(-2.9) == -2
trunc(+0.7) == +0
trunc(-0.9) == -0
trunc(+0) == +0
trunc(+0) == +0
trunc(-inf) == -inf
trunc(+inf) == +inf
trunc(-nan) == -nan
trunc(+nan) == +nan

See also