Why constant data member of a class need to be initialized at the constructor?

Question

I want to know why constant data member of a class need to be initialized at the constructor and why not somewhere else? What is the impact of doing so and not doing so?

I also see that only static constant integral data can be initialized inside the class other than that non of the data members can be initialized inside the class.

for eg:- Suppose below is my class declaration

class A{
     int a; // This we can initialize at the constructor or we can set this member by calling "vSet" member function
     const int b;
     static const int c = 10; //This works fine
public:
     A();
     ~A();
     void vSet(int a);
     int iAdd();
     void vDisplay();    
};

And the constructor is defined as mentioned below:-

Edited Section: As previous constructor definition example was wrong

 A::A():a(1),b(9){}

Please correct me if I am wrong. Thanks in advance.

Answer 1

A::A(){
      a = 1;
      b = 9; // Why we need to initialize this only at the constructor. 
   }

Is not initialization but it is Assignment.
a and b are already constructed and you assign them values in this case. The const qualifier demands that the variable not be changed after its initialization, allowing this assignment would break that contract.

This is Initialization using Member Initialization list.

A::A():a(1),b(9)
{}

You might want to have a look at this answer of mine to know the difference:

What is the difference between Initializing and Assignment inside constructor?

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As for another question, regarding only static constant integral data can be initialized inside the class, please read this answer of mine which explains in greater detail:

Why I can't initialize non-const static member or static array in class?