Defined in header <cmath>
float       trunc( float arg );
(1) (since C++11)
double      trunc( double arg );
(2) (since C++11)
long double trunc( long double arg );
(3) (since C++11)
double      trunc( Integral arg );
(4) (since C++11)
1-3) Computes the nearest integer not greater in magnitude than arg.
4) A set of overloads or a function template accepting an argument of any integral type. Equivalent to 2) (the argument is cast to double).


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

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


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.


#include <cmath>
#include <iostream>
int main()
    std::cout << std::fixed
              << "trunc(+2.7) = " << std::trunc(+2.7) << '\n'
              << "trunc(-2.9) = " << std::trunc(-2.9) << '\n'
              << "trunc(-0.0) = " << std::trunc(-0.0) << '\n'
              << "trunc(-Inf) = " << std::trunc(-INFINITY) << '\n';

Possible output:

trunc(+2.7) = 2.000000
trunc(-2.9) = -2.000000
trunc(-0.0) = -0.000000
trunc(-Inf) = -inf

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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