Question

Say I have something like this in a C Code. I know you can use a #define instead, to make the compiler not compile it, but just out of curiosity I’m asking if the compiler will also figure this thing out.

I think this is even more important for Java Compiler as it does not support #define.

const int CONDITION = 0;
........
// Will the compiler compile this?
if ( CONDITION )
{

}
.......

Answer 1

in Java, the code inside the if won’t even be part of the compiled code. It must compile, but it won’t be written to the compiled bytecode. It actually depends on the compiler, but I don’t know of a compiler that doesn’t optimize it. the rules are defined in the JLS:

An optimizing compiler may realize that the statement x=3; will never
be executed and may choose to omit the code for that statement from
the generated class file, but the statement x=3; is not regarded as
“unreachable” in the technical sense specified here.

The rationale for this differing treatment is to allow programmers to
define “flag variables” such as:

static final boolean DEBUG = false;

and then write code such as:

if (DEBUG) { x=3; }

The idea is that it should be possible to change the value of DEBUG
from false to true or from true to false and then compile the code
correctly with no other changes to the program text.

Don’t know about C.