如何为qsort写比较器函数？

Question

例如:

class classname{
public:
int N,M;
}; 

classname a> classname b如果aN> BN

Answer 1

class classname{
  public:
    int N,M;

    bool operator< (const classname& other) const { return N < other.N; }
};

...
std::vector<classname> arr;
...
std::sort(arr.begin(), arr.end());

或者你想使用C qsort？

static int compare_classname (const void* a, const void* b) {
   const classname* _a = reinterpret_cast<const classname*>(a);
   const classname* _b = reinterpret_cast<const classname*>(b);
   return _a->N < _b->N ? -1 : _a->N > _b->N ? 1 : 0;
}
...
classname* arr = new classname[n];
...
qsort(arr, n, sizeof(arr[0]), compare_classname);