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

Defined in header <cmath>
float       trunc ( float num );
double      trunc ( double num );
long double trunc ( long double num );
(until C++23)
constexpr /* floating-point-type */
            trunc ( /* floating-point-type */ num );
(since C++23)
float       truncf( float num );
(2) (since C++11)
(constexpr since C++23)
long double truncl( long double num );
(3) (since C++11)
(constexpr since C++23)
Additional overloads (since C++11)
Defined in header <cmath>
template< class Integer >
double      trunc ( Integer num );
(A) (constexpr since C++23)
1-3) Computes the nearest integer not greater in magnitude than num. The library provides overloads of std::trunc for all cv-unqualified floating-point types as the type of the parameter. (since C++23)
(since C++11)


num - floating-point or integer value

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

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


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)).


#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';

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

nearest integer not greater than the given value
nearest integer not less than the given value
nearest integer, rounding away from zero in halfway cases
C documentation for trunc

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