Why is this unordered_map not finding existing keys? (C++14) - c++

Question

I'm trying to use an unordered_map for a custom type. However, the map is storing duplicate entries, which have the same hash value and should evaluate as equal when using ==.
I've reduced my code to the following proof of concept, where I can see that the hash function runs correctly, but the equals operator is never called.
#include <unordered_map>
// Define a class with a single integer member.
class Example
{
public: int x;
public: Example(int x)
{
this->x = x;
}
// Overload == and compare the single member.
public: bool operator==(const Example &other) const
{
std::cout << "Comparing two objects\n";
return this->x == other.x;
}
};
// Define a hash function class
class ExampleHash
{
public: size_t operator()(const Example* key) const
{
// simply return the member variable as the hash value.
std::cout << "Returning hash value " << key->x << "\n";
return key->x;
}
};
int main()
{
// Create an empty map.
std::unordered_map<Example*, int, ExampleHash> m;
std::cout << "Inserting a new key\n";
// Insert an object with the value 1.
m[new Example(1)] = 1;
std::cout << "Existing hashes:\n";
ExampleHash fn;
for (auto const &item : m) {
size_t h = fn(item.first);
std::cout << " " << h << ", ";
}
std::cout << "\n";
std::cout << "Finding the key\n";
// Check if the object is in the map.
std::cout << ((m.find(new Example(1)) != m.end()) ? "Found" : "Not found") << "\n";
}
Output:
Inserting a new key
Returning hash value 1
Existing hashes:
Returning hash value 1
1,
Finding the key
Returning hash value 1
Not found
(Note the absence of the "Comparing two objects" line when calling unordered_map::find, despite the hash value clearly being in the map already.)

Answer 1

Pointers are not the objects they point to.
You are using pointers as keys. The objects equal operator will be ignored.