What does ... mean

int decr(int a)
{
a--;
if(a<0)
a=2;
return a;
}

int incr(int a)
{
a = (a+1)%3;
return a;
}



int main(void)
{
int n;
oper incr_decr;







ok there is a snippet of code. My question is how does
oper incr_decr work and what happens in

incr_decr = n%2 ? decr : incr;

if you need more of the code i can supply it.

I have no clue about the first code (think I need more code)



When you see the c? a:b operator this is what happens:

If c is true, a is returned. If c is false, b is returned. That is what I htink at least.

#include<iostream.h>


class cell {
int disc;
cell *next;
cell (int d)
{
disc = d;
next = this;
}
cell (int d, cell *n)
{
disc = d;
next = n;
}
friend class list;
};

class list{
cell *rear;
public:
int empty() { return rear == rear->next; }


void pop(int &i)
{
i = 0;
if( empty() ) return;
cell *front = rear->next;
rear->next = front->next;
i = front->disc;
delete front;
return;
}

void push(int d)
{
rear->next = new cell(d,rear->next);
}



list() { rear = new cell(0); }

~list()
{
int i,j,l;
long k;
while(!empty()) pop(i);
}
};


list pegs[3];

typedef int (*oper)(int);

int decr(int a)
{
a--;
if(a<0)
a=2;
return a;
}

int incr(int a)
{
a = (a+1)%3;
return a;
}



int main(void)
{
int n;
oper incr_decr;
int curr=0;
int p1 = 1, p2 = 2;
int d0,d1,d2;
cout << "Enter Number Of Discs:";
cin >> n;
for(int i = n; i > 0; i--)
pegs[0].push(i);
incr_decr = n%2 ? decr : incr; //conditional statement
while(1)
{
//rest of code

OK, first, look at this line:

oper incr_decr;

This declares a variable called incr_decr to be of type "oper". This is not a native type, so it's user defined. We see this is defined here:

typedef int (*oper)(int);

so a variable of type "oper" is a pointer to a function which takes an integer and returns an integer.

What it is doing is determining, by the statement:

n%2

if it should increment or decrement. It assigns incr_decr to point to either incr() or decr(), and presumably it will call the function (using the pointer) later on.