C++ floating point to integer type conversions

Question

What you are looking for is 'type casting'. typecasting (putting the type you know you want in brackets) tells the compiler you know what you are doing and are cool with it. The old way that is inherited from C is as follows.

float var_a = 9.99;
int   var_b = (int)var_a;

If you had only tried to write

int var_b = var_a;

You would have got a warning that you can't implicitly (automatically) convert a float to an int, as you lose the decimal.

This is referred to as the old way as C++ offers a superior alternative, 'static cast'; this provides a much safer way of converting from one type to another. The equivalent method would be (and the way you should do it)

float var_x = 9.99;
int   var_y = static_cast<int>(var_x);

This method may look a bit more long winded, but it provides much better handling for situations such as accidentally requesting a 'static cast' on a type that cannot be converted. For more information on the why you should be using static cast, see this question.

Normal way is to:

float f = 3.4;
int n = static_cast<int>(f);

Size of some float types may exceed the size of int. This example shows a safe conversion of any float type to int using the int safeFloatToInt(const FloatType &num); function:

#include <iostream>
#include <limits>
using namespace std;

template <class FloatType>
int safeFloatToInt(const FloatType &num) {
   //check if float fits into integer
   if ( numeric_limits<int>::digits < numeric_limits<FloatType>::digits) {
      // check if float is smaller than max int
      if( (num < static_cast<FloatType>( numeric_limits<int>::max())) &&
          (num > static_cast<FloatType>( numeric_limits<int>::min())) ) {
         return static_cast<int>(num); //safe to cast
      } else {
        cerr << "Unsafe conversion of value:" << num << endl;
        //NaN is not defined for int return the largest int value
        return numeric_limits<int>::max();
      }
   } else {
      //It is safe to cast
      return static_cast<int>(num);
   }
}
int main(){
   double a=2251799813685240.0;
   float b=43.0;
   double c=23333.0;
   //unsafe cast
   cout << safeFloatToInt(a) << endl;
   cout << safeFloatToInt(b) << endl;
   cout << safeFloatToInt(c) << endl;
   return 0;
}

Result:

Unsafe conversion of value:2.2518e+15
2147483647
43
23333

For most cases (long for floats, long long for double and long double):

long a{ std::lround(1.5f) }; //2l
long long b{ std::llround(std::floor(1.5)) }; //1ll