Assigning an int value to enum and vice versa in C++

Question

#include <iostream>    

typedef enum my_time {
  day,
  night
} my_time;

int main(){    
  // my_time t1 = 1; <-- will not compile
  int  t2 = night;
  return 0;  
} 

How is it expected that I can assign an enum value to an int but not the other way in C++?

Of course this is all doable in C.

Answer 1

Implicit conversions, or conversions in general, are not mutual. Just because a type A can be converted to a type B does not imply that B can be converted to A.

Old enums (unscoped enums) can be converted to integer but the other way is not possible (implicitly). Thats just how it is defined. See here for details: https://en.cppreference.com/w/cpp/language/enum

Consider that roughly speaking enums are just named constants and for a function

void foo(my_time x);

It is most likely an error to pass an arbitrary int. However, a

void bar(int x);

can use an enum for special values of x while others are still allowed:

 enum bar_parameter { NONE, ONE, MORE, EVEN_MORE, SOME_OTHER_NAME };
 bar(NONE);
 bar(SOME_OTHER_NAME);
 bar(42);

This has been "fixed" in C++11 with scoped enums that don't implicitly convert either way.

Answer 2

  You could cast to int. This expression makes an explicit conversion of the specified data type (int) and the given value (night).

 int t2 = static_cast<int>(night)