Use a heap to merge multiple files.

heap_merge.cpp

typedef pair<int, int> Elem;

bool compare(const Elem& lhs, const Elem& rhs)
{
    return lhs.first < rhs.first;    
}

int main()
{
    vector<ifstream*> files;
    vector<Elem > heap;
    
    for (int i = 0; i < files.size(); ++i)
    {
        int num = 0;
        if ((*files[i]) >> num)
        {
            heap.push_back(make_pair(num, i));
        }
    }
    make_heap(heap.begin(), heap.end(), compare);
    
    ofstream os("out.txt");
    while (!heap.empty())
    {
        const Elem top = heap.front();
        os << top.first << endl;
        pop_heap(heap.begin(), heap.end());
        heap.pop_back();
        int num = 0;
        if (*files[top.second] >> num)
        {
            heap.push_back(make_pair(num, top.second));
            push_heap(heap.begin(), heap.end());
        }
    }
    
    for (int i = 0; i < files.size(); ++i)
    {
        delete files[i];
    }
}