[Solved] X, Y, XY why does this work in gcc? [closed]

Question

It’s undefined behavior to use the value of an uninitialized (auto) variable that could have been declared with the register keyword (see 6.3.2.1p2 in the C standard).

int XY; could have been declared with register (you’re not taking its address anywhere) and it’s still unitialized at the right hand side of int XY = XY;, so the behavior is undefined.

If you did int XY = *&XY; the behavior would no longer be undefined, but XY would get an indeterminate value.

Accepted Answer

It’s undefined behavior to use the value of an uninitialized (auto) variable that could have been declared with the register keyword (see 6.3.2.1p2 in the C standard).

int XY; could have been declared with register (you’re not taking its address anywhere) and it’s still unitialized at the right hand side of int XY = XY;, so the behavior is undefined.

If you did int XY = *&XY; the behavior would no longer be undefined, but XY would get an indeterminate value.