What does this mean?

[Protoman]

What does this do?

int& fn(int& arg){arg++;return arg;}
//later
int y;
fn(y)=5;

Why can you do it? What do you use it for? Thanks!!!

 

[Dan Cernat]

See below

What does this do?

int& fn(int& arg){arg++;return arg;}
//later
int y;
fn(y)=5; <-- use of uninitialised variable y

according to the body of fn, you may get an overflow. otherwise the
statement above will initialise y with 5

Why can you do it?

Why not?
What do you use it for?
I would say if you want to change the value of a static variable inside fn
between calls to fn
example (untested)

int& fn(int arg)
{
static int persist = 2;

// so some work here

return persist;
}

int main()
{
fn(1); // persist == 2 after this call
fn(2) = 7; // persist will be 7 after this
fn(3); // persist is still 7

return 0;
}

Thanks!!!
Dan

 

[Rolf Magnus]

according to the body of fn, you may get an overflow. otherwise the
statement above will initialise y with 5

Actually, the behavior will be undefined, because the value of an
uninitialized variable is used (within the function). That happens before
the assignment.