What are the differences between std::hash and a custom hash function in C++11?

What are the differences between std::hash and a custom hash function in C++11?

In C++11, std::hash is a default hash function provided by the standard library, while a custom hash function is user-defined. std::hash is generally optimized for common types, but a custom hash function allows for specific behavior tailored to your data.

When should I use a custom hash function instead of std::hash?

You should consider using a custom hash function when you have complex data types or when you need to optimize for specific performance characteristics that std::hash does not address.

Can you give an example of a custom hash function?

Sure! Here's a simple example of a custom hash function for a struct:

struct MyStruct {
    int a;
    std::string b;
};

struct MyStructHash {
    std::size_t operator()(const MyStruct& s) const {
        return std::hash<int>()(s.a) ^ std::hash<std::string>()(s.b);
    }
};

This combines the hashes of the individual members.

What are the performance implications of using a custom hash function?

Using a custom hash function can improve performance if it's well-designed for your specific data, reducing collisions. However, a poorly designed hash function can lead to increased collisions and slower performance.