Comma operator with undeclared variable - why does it compile?

Question

Why does this code not throw a compilation error for y being undeclared?

int x = 10, y;
printf("%d", y);

There's no expression like int y;. In my case, the console print out is 32764, which seems to just be uninitialized memory. Looking at the assembly code for the first line, it's the same whether the , y is there or not, even if y is used in the print statement.

Expected to see

error: use of undeclared identifier 'y'    printf("%d", y);

Answer 1

This:

int x = 10, y;

Is not an instance of the comma operator. The , is part of the grammar of a declaration which allows multiple variables to be declared on the same line. Specifically, it declares x and initializes it to 10 and declares y which is uninitialized. It's equivalent to:

int x = 10;
int y;

Had you instead done this:

int x = (10, y);

Then you would have an instance of the comma operator and an error for an undeclared identifier.