Inserting into an unordered_set with custom hash function

Question

I have the following code to make an unordered_set<Interval>. This compiles fine.

struct Interval {
  unsigned int begin;
  unsigned int end;
  bool updated;   //true if concat.  initially false
  int patternIndex;  //pattern index. valid for single pattern
  int proteinIndex;   //protein index.  for retrieving the pattern
};

struct Hash {
  size_t operator()(const Interval &interval);
};

size_t Hash::operator()(const Interval &interval){
  string temp = to_string(interval.begin) + to_string(interval.end) + to_string(interval.proteinIndex);
  return hash<string>()(temp);
}

unordered_set<Interval, string, Hash> test;

However, I cannot compile when I try to insert using this code:

for(list<Interval>::iterator i = concat.begin(); i != concat.end(); ++i){
  test.insert((*i));
}

Moreover, I cannot determine what the problem is from the error messages, for example:

note: candidate is:
note: size_t Hash::operator()(const Interval&)
note:   candidate expects 1 argument, 2 provided  

I thought I only provided 1 argument...

What is the problem with my insertion code?

---

Here's the new instantiation code: unordered_set<Interval, Hash> test; However, I'm still receiving a slew of error messages, for example:

note: candidate is:
note: size_t Hash::operator()(const Interval&) <near match>
note:   no known conversion for implicit ‘this’ parameter from ‘const Hash*’ to ‘Hash*’

Answer 1

I think, Andy Prowl perfectly fixed the problems with your code. However, I would add the following member function to your Interval, which describes what makes two intervals identical:

std::string getID() const { return std::to_string(b) + " " + std::to_string(e) + " " + std::to_string(proteinIndex); }

Please note, that I also followed Andy Prowl's suggestion and renamed the members begin to b and end to e. Next, you can easily define the hash and comparison functions by using lambda expressions. As a result, you can define your unordered_set as follows:

auto hash = [](const Interval& i){ return std::hash<std::string>()(i.getID()); };
auto equal = [](const Interval& i1, const Interval& i2){ return i1.getID() == i2.getID(); };
std::unordered_set<Interval, decltype(hash), decltype(equal)> test(8, hash, equal);

Finally, for reasons of readability, I converted your for loop into a range-based for loop:

std::list<Interval> concat {{1, 2, false, 3, 4}, {2, 3, false, 4, 5}, {1, 2, true, 7, 4}};

for (auto const &i : concat)
    test.insert(i);

for (auto const &i : test)
    std::cout << i.b << ", " << i.e << ", " << i.updated << std::endl;

Output (I just printed first three members of each Interval):

2, 3, 0
1, 2, 0

As you can see, there are only two intervals printed. The third one ({1, 2, true, 7, 4}) was not inserted to test, because its b, e, and proteinIndexare equal to that of the first interval ({1, 2, false, 3, 4}).

Code on Ideone