Trunc

Defined in header <cmath>.

Description

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 num  (since C++23).

⏳Additional Overloads are provided for all integer types, which are treated as double.

Declarations

C++23

// 1)
constexpr /* floating-point-type */ trunc( /* floating-point-type */ num );
// 2)
constexpr float truncf( float num );
// 3)
constexpr long double truncl( long double num );

Additional Overloads

// 4)
template< class Integer >
constexpr double trunc ( Integer num );

C++11

// 1)
float trunc ( float num );
// 2)
double trunc ( double num );
//3)
long double trunc ( long double num );
// 4)
float erfcf( float num );
// 5)
long double erfcl( long double num );

Additional Overloads

// 6)
template< class Integer >
double trunc ( Integer num );

Parameters

​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.

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 Additional Overloads. 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)).

Examples

#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 Result

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