NAME¶

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

Synopsis¶

Defined in header <cmath>
float trunc ( float arg ); (1) (since C++11)
(constexpr since C++23)
float truncf( float arg ); (2) (since C++11)
(constexpr since C++23)
double trunc ( double arg ); (3) (since C++11)
(constexpr since C++23)
long double trunc ( long double arg ); (4) (since C++11)
(constexpr since C++23)
long double truncl( long double arg ); (5) (since C++11)
(constexpr since C++23)
double trunc ( IntegralType arg ); (6) (since C++11)
(constexpr since C++23)

1-5) Computes the nearest integer not greater in magnitude than arg.
6) A set of overloads or a function template accepting an argument of any integral
type. Equivalent to (3) (the argument is cast to double).

Parameters¶

arg - floating point value

Return value¶

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

Return value¶

math-trunc.svg
Argument

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 arg is ±∞, it is returned, unmodified
* If arg is ±0, it is returned, unmodified
* If arg 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.

Example¶

// Run this code

#include <cmath>
#include <iostream>
#include <initializer_list>

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';
}
}

Possible output:¶

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¶

floor
floorf nearest integer not greater than the given value
floorl (function)
(C++11)
(C++11)
ceil
ceilf nearest integer not less than the given value
ceill (function)
(C++11)
(C++11)
round
roundf
roundl
lround
lroundf
lroundl
llround
llroundf
llroundl nearest integer, rounding away from zero in halfway cases
(C++11) (function)
(C++11)
(C++11)
(C++11)
(C++11)
(C++11)
(C++11)
(C++11)
(C++11)