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

The straightforward solution is to use static_cast (as the other answer has already posted it):

periods mp;
if (argc == 2)
{
   std::string min_prd(argv[1]); //the index should be 1
   mp = static_cast(atoi(min_prd.c_str()));
}

But then atoi should not be used to convert the c-string into int, because atoi doesn't check for error in the input string, hence it is unsafe.

C++11 provides safer conversion functions, so you could use std::stoi as:

try
{
   periods mp;
   if (argc == 2)
   {
      //std::stoi could throw exception on error in input
      mp = static_cast(std::stoi(argv[1]));
   }
   //use mp here
}
catch(std::exception const & e)
{
   std::cout << "exception caught with message : " << e.what() << std::endl;
}

Now that is a better solution.

---

However, there is an alternative solution which you could use as:

period mp;
if (argc == 2)
{
   mp = to_period(argv[1]); //how should we implement it?
   if (mp == period_end)
   {
       std::cout << "command line input error" << std::endl;
       return 0;
   }
}

Now the question is, how should we implement to_period function?

Note that this solution assumes that the command line argument for enum value one would be its string representation, i.e it would be "one" instead of 1 which is integral representation.

I would implement this solution as:

• First create a header file called period_items.h as:

  //period_items.h
  E(one)
  E(five)
  E(ten)
  E(fifteen)
  E(thirty)
  

• then create another header file called period.h as:

  //period.h
  #include 
  
  enum period 
  {
     #define E(item)  item,
       #include "period_items.h"
     #undef E
     period_end
  };
  
  period to_period(std::string const & name)
  {
     #define E(item)  if(name == #item) return item;
       #include "period_items.h"
     #undef E
     return period_end;
  }
  

Now you can simply include period.h and use to_period function. :-)

Note that in the alternative solution, I've used singular form rather than plural form, means I've used period rather than periods. I feel period is appropriate.

You could also add this function to period.h as:

std::string to_string(period value)
{
    #define E(item)  if(value == item) return #item;
        #include "period_items.h"
    #undef E
    return "";
}

Now, you could write this:

#include "period.h"

period v = to_period(argv[1)); //string to period
std::string s = to_string(v);  //period to string

Hope that helps.