vector<int> v;
cout<<v.size()-1;

The following will give you 18446744073709551615. This is because vector.size() returns a size_t type value, which is an alias for unsigned long int. In plain terms:

unsigned long int a=0;
cout<<a-1;

The above code will give the same result — 18446744073709551615.

Implications

for(int i=0;i<vector.size()-1;i++){
    ...
}

In the above for loop, if the vector has 0 elements, then the loop will be equivalent to:

for(int i=0;i<18446744073709551615;i++){
    ...
}

Which, I believe is not what you will be expecting. So the correct way to use for loops is the following:

for(int i=0;(i+1)<vector.size();i++){
    ...
}

---Edit---

As pointed out by Errichto and satyaki3794, we can just typecast vector.size() to an int, and use it normally. Like this:

for(int i=0;i<(int)vector.size()-1;i++){
    ...
}