switch case with variables

Why the following code doesn't give errors when I try to compile with gcc, and....

Code:

---

#include <stdio.h>
int x = 2;
const int y = 2;

switch(x)
{
    case(y)
          {
              break;
          }
          default:
          break;
}

---

this one does?

Code:

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#include <stdio.h>
#include "remote_conf.hxx"


int x = 2;
auto_ptr<Remote> aXML (Remote_("../config/remote_conf.xml"));

const int y =(const int)( aXML->y() );

switch(x)
{
    case(y)
          {
              break;
          }
          default:
          break;
}

---

error: ‘y’ cannot appear in a constant-expression

> const int y = 2;
This one is a const known at compile time.

> const int y =(const int)( aXML->y() );
This is a run-time initialisation. The 'const' comes in later, if you then try to assign something to y.

case labels need to be known at compile time.

One is a compile-time constant (the compiler knows the value of x), the other is a run-time constant. A switch case can only use compile-time constants. To use other kinds of values, just use if...else.

Quote:

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Originally Posted by Salem

> const int y = 2;
This one is a const known at compile time.

> const int y =(const int)( aXML->y() );
This is a run-time initialisation. The 'const' comes in later, if you then try to assign something to y.

case labels need to be known at compile time.

---

How can I make a switch case with run-time variables then?

You could use an if-else chain. Certain associative containers might also be suitable depending on what exactly you are trying to do.