converting from int to enum

Question

I have declared the following enum :

  enum periods {one, five, ten, fifteen, thirty};

and now I want to pass it as a commandline argument

Answer 1

I have declared the following enum :

enum periods {one, five, ten, fifteen, thirty};

and now I want to pass it as a commandline argument in my main function.

periods mp = atoi(argv[2]);   // simplified for answer...

So, there are several issues:

• you need to cast the int returned by atoi to the enum type... static_cast(...)
• you should realise that an argv[2] of "0" will be mapped to the enumeration identifier "one", "1" will map to "five" etc...
  • if you actually want "1" to map to "one", "5" to "five" etc., the easiest way is to change your enum: enum periods { one = 1, five = 5, ten = 10, fifteen = 15, thirty = 30 };, but your example's obviously a little contrived so it's impossible to guess what will work best for your real needs
• there's no validation

You're better off creating a function:

periods to_periods(const std::string& s)
{
    if (s == "one") return one;
    if (s == "five") return five;
    if (s == "ten") return ten;
    if (s == "fifteen") return fifteen;
    if (s == "thirty") return thirty;
    throw std::runtime_error("invalid conversion from text to periods");
}

(When there are more cases, it's not uncommon to use a std::map or sorted std::vector to track these associations, and it allows reusable algorithms and the same data to support conversions from enum numeric value to textual identifier.)