How to initialize const member , that are not static

Question

I know how to initialize const member in the initializer list, but that requires to know the value to be assigned already when calling the constructor. From what I understand, in java it's possible to initialize a final member in the constructor body, but I haven't seen an equivalent in c++ ( Java's final vs. C++'s const )

But what to do when the initialization is relatively complex? The best I could come up, is to have an initialization function that returns directly an instance. Is there something more concise?

Here is an example (https://ideone.com/TXxIHo)

class Multiplier
{
   const int mFactor1;
   const int mFactor2;
   static void initializationLogic (int & a, int & b, const int c )
   {
       a = c * 5;
       b = a * 2;
    }

  public:
   Multiplier (const int & value1, const int & value2)
     : mFactor1(value1), mFactor2(value2)
     {};
   /*
   //this constructor doesn't initialize the const members
   Multiplier (const int & value)
     {
        initializationLogic(mFactor1,mFactor2, value);
     };
    */
   //this initializes the const members, but it's not a constructor
   static Multiplier getMultiplierInstance (const int & value)
   {
       int f1, f2;
       initializationLogic(f1,f2, value);
       Multiplier obj(f1,f2);
       return obj;
   }

Simple: const int mFactor1 = 123;. See Constructors and member initializer lists. Drawing parallels between C++ and Java can prove to be counter-productive. — Ron
@Ron but then mFactor1 would be the same for all the instances, that's not what I want here. I want Factor1 to be assigned at the constructor (with any value, depending on the constructor argument), and not change anymore — lib
In C++ all objects are initialized before the constructor's body is entered. After that the best you can do is assign and a constant cannot be assigned. The initializer list is your only chance, but you can have a helper function. — user4581301
const members are limiting and rarely necessary. It's generally preferred to just use non-const members, even to represent constants, as it preserves assignability and movability for the class. The same can be said about reference members. — François Andrieux

user7860670 user7860670 · Accepted Answer · 2018-02-23T19:28:28

You can delegate object construction to a dedicated constructor accepting an instance of helper class preparing all the necessary parameters:

 private: class
 Init_Helper final
 {
    private: int m_value1;
    private: int m_value2;

    public: explicit Init_Helper(const int c)
    {
        m_value1 = c * 5;
        m_value2 = m_value1 * 2;
        // more init logic goes here...
    }

    public: int const & Get_Value1(void) const
    {
        return m_value1;
    }

    public: int const & Get_Value2(void) const
    {
        return m_value2;
    }
 };

 public: explicit  Multiplier(const int c)
 :  Multiplier{Init_Helper{c}}
 {}

 private: explicit Multiplier(Init_Helper && helper)
 :  mFactor1{helper.Get_Value1()}, mFactor2{helper.Get_Value2()}
 {}

How to initialize const member , that are not static

1 Answers