The usage of hash

[deritha]

The original code:

struct NUM
{
NUM(int a, int b) :a(a), b(b), val(a+b*sqrt(2)){}
bool operator<(const NUM& n) const{ return val > n.val; }
bool operator==(const NUM& n) const{ return a == n.a && b == n.b; }
int a, b;
double val;
};

struct HashNUM
{
size_t operator()(const NUM& n) const
{
return hash<int>()(n.a) ^ (hash<int>()(n.b)<<1);
}
};

The main code:
unordered_set<NUM, HashNUM> hash;
hash.emplace(0, 0);
NUM c(s.a + 1, s.b);
if (hash.emplace(c).second)
{
min_heap.emplace(c);
}

What's the detail usage of hash.emplace(c).second? I can't output it to get more detail information.

Thanks.