c++ - Undefined behaviour of const

Question

I never thought I will be going to ask this question but I have no idea why this happens.

const int a = 3; 
int *ptr;
ptr = (int*)( &a );
printf( "A=%d
", &a );
*ptr = 5; 
printf( "A=%d
", ptr );
printf( "A=%d
", a );
printf( "A=%d
", *ptr );

Output

A=6945404
A=6945404
A=3
A=5

How can this happen? How can one memory location hold two different values? I searched around and all I find is undefined behavior is undefined. Well that does not make any sense. There must be an explanation.

Edit

I get it, Marks answer makes alot of sense but still I wonder that const was added into the language so that user does not change the value unintentionally. I get that old compilers allows you to do that but I tried this on VS 2012 and I got the same behavior. Then again as haccks said, one memory location can't hold two values it looks like it does, then where is the second value stored?

Answer 1

The optimizer can determine that a is a constant value, and replace any reference to it with the literal 3. That explains what you see, although there's no guarantee that's what's actually happening. You'd need to study the generated assembly output for that.