问题:

I have the following code in C++

#include <iostream>
#include <set>

using namespace std;

int main()
{
    set<int> a;
    int n;

    for(int i=0;i<3;i++){
    cin>>n;
    a.insert(n);
    }

    cout << *a.end();
    return 0;
}

Why is it always printing "3" instead of the greatest element in the set? Replacing cout << *a.end(); with cout << *--a.end(); works fine.

回答1:

Why is it always printing "3" instead of the greatest element in the set?

Because a.end() is one past the end of your vector. It doesn't hold any valid data, it's a marker for the end of your vector. As in:

for(auto i = a.begin(); i != a.end(); ++i) 
    // do stuff with i

EDIT: (thanks to Nathan) dereferencing a.end() yields the dreaded undefined behaviour. Anything could have happened: get a 3, get the last element in the set, could even have canceled X-mas!!!

回答2:

*a.end()

Undefined behaviour. In C++ iterator terminology, "end" does not mean the same thing as "last element".

Replacing cout << *a.end(); with cout << *--a.end(); works fine.

Consider a reverse iterator: *a.rbegin().

Keep in mind that both begin() and rbegin() only return dereferencable iterators if the container is not empty. For empty containers, begin() == end() and rbegin() == rend().