How to initialize const member variable in a class?

Question

The const variable specifies whether a variable is modifiable or not. The constant value assigned will be used each time the variable is referenced. The value assigned cannot be modified during program execution.

Bjarne Stroustrup's explanation sums it up briefly:

A class is typically declared in a header file and a header file is typically included into many translation units. However, to avoid complicated linker rules, C++ requires that every object has a unique definition. That rule would be broken if C++ allowed in-class definition of entities that needed to be stored in memory as objects.

A const variable has to be declared within the class, but it cannot be defined in it. We need to define the const variable outside the class.

T1() : t( 100 ){}

Here the assignment t = 100 happens in initializer list, much before the class initilization occurs.

Well, you could make it static:

static const int t = 100;

or you could use a member initializer:

T1() : t(100)
{
    // Other constructor stuff here
}

There are couple of ways to initialize the const members inside the class..

Definition of const member in general, needs initialization of the variable too..

1) Inside the class , if you want to initialize the const the syntax is like this

static const int a = 10; //at declaration

2) Second way can be

class A
{
  static const int a; //declaration
};

const int A::a = 10; //defining the static member outside the class

3) Well if you don't want to initialize at declaration, then the other way is to through constructor, the variable needs to be initialized in the initialization list(not in the body of the constructor). It has to be like this

class A
{
  const int b;
  A(int c) : b(c) {} //const member initialized in initialization list
};

If you don't want to make the const data member in class static, You can initialize the const data member using the constructor of the class. For example:

class Example{
      const int x;
    public:
      Example(int n);
};

Example::Example(int n):x(n){
}

if there are multiple const data members in class you can use the following syntax to initialize the members:

Example::Example(int n, int z):x(n),someOtherConstVariable(z){}

1. You can upgrade your compiler to support C++11 and your code would work perfectly.

2. Use initialization list in constructor.

   T1() : t( 100 )
   {
   }
   

Another solution is

class T1
{
    enum
    {
        t = 100
    };

    public:
    T1();
};

So t is initialised to 100 and it cannot be changed and it is private.

If a member is a Array it will be a little bit complex than the normal is:

class C
{
    static const int ARRAY[10];
 public:
    C() {}
};
const unsigned int C::ARRAY[10] = {0,1,2,3,4,5,6,7,8,9};

or

int* a = new int[N];
// fill a

class C {
  const std::vector<int> v;
public:
  C():v(a, a+N) {}
};