I have this simple line of code:
int x;
x automatically has the value of 1. I don't set it to anything but when I debug, it shows that x is 1.
Does an int have a default value of 1?!
No. int has an undefined default value. It just happens to be 1 in this case. It could just as easily be -18382 or 22 or 0xBAADF00D.
Always initialize your variables in C.
The initial value is undefined, and in this case will be whatever happened to be in that memory location before x started using it.
(Depending on the surrounding code, you might find that in your specific case it's always 1, but you can't be sure of that.)
No, on the contrary, x has no default value at all. What you're seeing is the garbage that the variable was placed upon when you created it.
Instance variables are initialized to 0 before your initializer runs..
ref:
来源：https://stackoverflow.com/questions/5980749/does-an-int-in-objective-c-have-a-default-value-of-1