Why do const variables have to be initialized right away?

Question

This is a general programming question. I'm learning about C++ and I've learned that any const variables, ie: const int i, or int *const ptr, have to be initialized right away.

This is also the underlying reason that references to addresses must be initialized right away, because the addresses are const.

But I can't find the reason why this must be done / why this rule is imposed.

Can anyone explain this for me please?

Answer 1

Because there is no way you can initialize it, or assigned with a value, later on.

const int size; //no initialization (error)

size = 100; //error - you cannot assign a const variable.

Now if a variable which is neither having any meaningful value, nor are you allowed to make it to have value later on because it is a const variable, then what is the point of such a variable? It is completely useless.

However, this is true for only built-in and POD types:

struct A{}; //POD type
struct B{ B(){} }; //Non POD type because it has user-defined constructor!

const int i; //error - built-in type
const A a;   //error - POD type
const B b;   //ok -    Non POD type

//likewise
const std::string s; //ok - std::string is a non-POD
const std::vector<std::string> v; //ok - std::vector is a non-POD

Actually a NON-POD type cannot remain uninitialized, because the default constructor will be called, and the object would get initialized.

---

Now consider this struct,

struct C
{
   const int i;
   C() {}
};

C is definitely a non-POD type, because it has user-defined constructor. Also note that in the constructor, it doesn't initialize i which is int, declared as const. Because of this uninitialized const i, the following would give error:

const C c; //error - 

One might think the error is because of const in the above declaration of variable c. But that is short-sightedness and is not true. Even if you remove const, it would give error:

C c; //error - same error

The error is because of C::i which is declared const but has not been initialized.

Demo : http://ideone.com/NJT8L

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This analysis also demonstrates that built-in types do not get initialized automatically even if they're members of non-POD types. This is true of non-POD class types as well.

And the syntax to default initialization for built-in types (and POD types) is this:

struct C
{
    const int i;
    C() : i() {} //note the syntax - it is called member-initialization list
};

Now this is allowed :

C x; //ok
const C y; //ok

Demo : http://ideone.com/84vD9

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As for what makes a struct/class POD, see this topic:

• Can't C++ POD type have any constructor?

Answer 2

Because if you could assign to them later, they wouldn't be "const".