MSVC const enum type

Question

const enum Alpha{
    X=9,
    Y=5,
    Z=2
}p;
int main(){
    enum Alpha a,b;
    a= X;
    b= Z;

    p = X;
    p = Y;

    printf("%d",a+b-p); 
    return 0; 
}

Why is p = X and p = Y allowed in MSVC compiler? This code outputs 6. Shouldn't a const value be assigned at initialization and never again?

Answer 1

That is a bug in the compiler itself. End of the story.

In fact, your little code shows two bugs in the compiler. The first bug is here itself:

const enum Alpha{
    X=9,
    Y=5,
    Z=2
}p;    //declaration of p is ill-formed!

The declaration of p is ill-formed, and thus the compiler should reject this code, because p is declared const but left uninitialized. A const scalar (and pod) type must be initialized in order to be well-formed:

const Alpha q;      //ill-formed (same case is with p in your code)
const Alpha r = X;  //well-formed

For detailed and extensive explanation, see this:

• Why do const variables have to be initialized right away?

Answer 2

Looks like a bug, indeed.

First of all, global const objects must be initialized when defined, and default-initialization is not an option for enumeration types. According to Paragraph 8.5/6 of the C++11 Standard:

To default-initialize an object of type T means:

— if T is a (possibly cv-qualified) class type (Clause 9), the default constructor for T is called (and the initialization is ill-formed if T has no accessible default constructor);

— if T is an array type, each element is default-initialized;

— otherwise, no initialization is performed.

If a program calls for the default initialization of an object of a const-qualified type T, T shall be a class type with a user-provided default constructor.

Secondly, a const object cannot be assigned after initialization.