Using struct as a type in a C++ template?

Question

I have a function like so

template <typename T>
double vectortest(int n, bool prealloc) {
std::vector<T> tester;
    std::unordered_set<T> seq = generatenums<T>(n);
}

Where generatenums is another templatized function

template <typename T>
std::unordered_set<T> generatenums(int n) {
  std::unordered_set<T> ret;
}

Please note: I have truncated the entire contents of these functions, leaving only what I think is relevant to my question.

I also have a struct

typedef struct Filler
{
    int value;
    int padding[2500];
};

And I want to be able to invoke my functions like so

vectortest<Filler>(5, true);

But this generates a lot of errors, and leaves me wondering why I can't use struct as a type for C++ templates, and if there's a way around this?

Answer 1

unordered_set on structs needs custom hash function.

similarlyuset on structs needs comparaor <operator to be defined.

#include <iostream>
#include <vector>
#include <unordered_set>


// class for hash function
template <typename T>
struct MyHashFunction {
    // id is returned as hash function
    size_t operator()(const T& f) const
    {
        return f.id;
    }
};

template <typename T>
double vectortest(int n, bool prealloc) {
std::vector<T> tester;
    std::unordered_set<T,MyHashFunction<T>> seq = generatenums<T>(n);
    return 73.0;
}

template <typename T>
std::unordered_set<T,MyHashFunction<T>> generatenums(int n) {
  std::unordered_set<T,MyHashFunction<T>> ret;

  return ret;
}

struct Filler
{
    int id;
    int value;
    int padding[2500];
};



int main(){
  
    
    vectortest<Filler>(5, true);
    return 0;
}