C ++容器上的段故障循环

Question

Here's the code(find union of 2 sets) :

template <class dtype>
void printSet(dtype data)
{
    std::cout<<"\n";
    for(auto i = data.begin(); i != data.end(); i++)
    {
        std::cout<<*i<<" ,";
    }
}

int main(int argc, char const *argv[])
{
    int arr[] = {1,2,3,4,5};
    std::set<long long> a(arr, arr + 5), b(arr, arr + 5), c;
    std::vector<long long> v;
    b.insert(1000);
    printSet(a);
    printSet(b);
    set_union(a.begin(), a.end(), b.begin(), b.end(), v.begin());
    printSet(v);
    return 0;
}

And here's what I got:

1 ,2 ,3 ,4 ,5 ,
[1]    9444 segmentation fault  ./a.out

Where did I mess up ?

Answer 1

v is empty, so writing to v.begin() isn't possible. You should use std::back_inserter(v) instead.

Answer 2

set_union expects a valid output iterator, in the sense that it should be allowed to write at that iterator. v is empty, so v.begin() isn't a valid iterator.

Try declaring your vector as

std::vector<long long> v(42);

Answer 3

For clarity: if you flush std::cout before the set_union line, you should see that b was printed as well.

As to the error, I suggest adding this include:

#include <iterator>

and changing your set_union line to:

set_union(a.begin(), a.end(), b.begin(), b.end(), back_inserter(v));

The back_insert_iterator returned by back_inserter is an output iterator suitable for use here: it will call v.push_back for each item assigned to it by set_union .

---

A note on your platform & environment:

In general, you should make sure your core file size limit is non-zero (just make it unlimited unless you're writing massive programs), and learn to examine the core file with your debugger.