Why can duplicate objective-C objects be declared in a loop, but not in sequence?

Question

I am just learning objective-C, and am wondering why the following code results in an error?

Person *p = [[Person alloc] init];
Person *p = [[Person alloc] init];

In this case, the second line gives an error stating the "p" pointer has already been defined.

This makes sense to me; however, if it is true that a pointer cannot be re-defined, then why does the following code work?

for(int i = 0; i<10; i+=1) {
    Person *p = [[Person alloc] init];
}

When I enter this in XCode it does not give any errors, and it compiles and runs just fine. In this case, I'd expect the loop to simply re-define the "p" pointer, similar to having them be declared sequentially, but this is not the case.

Does anyone have an explanation for why this is?

Answer 1

Person *p = [[Person alloc] init];
Person *p = [[Person alloc] init];

In the first case, two p in the same local scope, they are all local variables, so they have the same priority. We can't have two variable with the same name and the same priority.

Person *p = [[Person alloc] init];
{
    Person *p = [[Person alloc] init];
}

The above case will be ok because the second p has higher priority in its local scope than the first p.

for(int i = 0; i<10; i+=1) {
    Person *p = [[Person alloc] init];
}

In the second case, all the p are in different local scopes.

Answer 2

In a nutshell, it is because in the case of the for-loop the variable p is local to the loop itself, so at the end of each iteration p and any other variables local only to the loop's scope is deallocated.