How to use boost::optional

Question

I am trying to use boost::optional as below.

#include <iostream>
#include <string>

#include <boost/optional.hpp>

struct myClass
{
   int myInt;
   void setInt(int input) { myInt = input; }
   int  getInt(){return myInt; }
};

boost::optional<myClass> func(const std::string &str)
{
   boost::optional<myClass> value;
   if(str.length() > 5)
   {
      // If greater than 5 length string. Set value to 10
      value.get().setInt(10);
   }
   else if (str.length() < 5)
   {
      // Else set it to 0
      value.get().setInt(0);
   }
   else
   {
      // If it is 5 set the value to 5
      value.get().setInt(5);
   }

   return value;
}


int main()
{
   boost::optional<myClass> v1 = func("3124");
   boost::optional<myClass> v2 = func("helloWorld");
   boost::optional<myClass> v3 = func("hello");

   if (v1)
       std::cout << "v1 is valid" << std::endl;
   else
       std::cout << "v1 is not valid" << std::endl;

   if (v2)
       std::cout << "v2 is valid" << std::endl;
   else
      std::cout << "v3 is not valid" << std::endl;

   if (v3)
      std::cout << "v3 is valid" << std::endl;
   else
      std::cout << "v3 is not valid" << std::endl;

  return 0;
 }

I get following error

prog.exe: /usr/local/boost-1.55.0/include/boost/optional/optional.hpp:631: boost::optional::reference_type boost::optional::get() [with T = myClass; boost::optional::reference_type = myClass&]: Assertion `this->is_initialized()' failed.

Presumably, the optional variable is not initialized properly. How to do it the correct way?

EDIT:: Got some very good answers, just couple of more questions 1. Is it a good idea to use make_optional at the end of 'func' function and return it? Also 2. I was thinking of assigning boost::none to emphasize that I have no value to assign and that's why boost::none. But not sure if that is valid?

Answer 1

A default-constructed boost::optional is empty - it does not contain a value, so you can't call get() on it. You have to initialise it with a valid value:

boost::optional<myClass> value = myClass();

Alternatively, you can use an in-place factory to avoid copy initialisation (but the copy will most likely be elided anyway); however, I have no experience with that, so I can't provide an example.

---

As a side note, you can use -> in place of get(), like this:

value->setInt(10);

But that's just a matter of stylistic preference, both are equally valid.