c++ enum can compare to integer but not assign from integer?

Question

#include <iostream>

enum mode { MODE0=0, MODE1, NUM_MODES};

int main(int args, char ** argv) {
  int i = 1;
  std::cout << (i == MODE0 ? "true" : "false") << "\n";
  std::cout << (i == MODE1 ? "true" : "false") << "\n";
  mode test;
  test = i; // error
}

Why is it that the comparison of i to enum values works fine, but I get compilation error when assigning mode test variable to an integer value?

enum.cc:10:8: error: invalid conversion from 'int' to 'mode' [-fpermissive]

My question is specifically about why comparison works and assignment doesn't (not how to fix my code) and it has received a couple of good explanations below.

Answer 1

MODE0, MODE1 and NUM_MODES are guaranteed to be convertible to int (the underlying type of the enum) but the reverse is not true. Not all int can be converted to mode. For example, what is the matching mode for the int 42? Simply put, only the implicit conversion from enum to int is defined, the opposite implicit conversion is not defined.

If you want to convert from int to mode you can preform a static_cast to signal that you are taking the responsibility for the value being converted. Try

test = static_cast<mode>(i);

You can use strongly typed enumerations by adding the class keyword to your enum to prevent any implicit casts and to limit the scope of the enum value names. The definition would look like enum class mode { MODE0 = 0, MODE1, NUM_MODES };. In this case, you must quality the enum value names, for example, you would need to use mode::MODE0 instead of MODE0. This has the advantage that it avoids name collisions.