In C++ programming, hash literals are a convenient way to represent values using a compact and expressive syntax. While C++ already provides hash literals for string literals ("..."_hash) and integer literals (42_hash), you might often encounter scenarios where you need to define your own custom types and create hash literals for them.

This blog post will guide you through the process of creating custom hash literals in C++.

What are Hash Literals?

Before diving into custom hash literals, let’s briefly understand what hash literals are. Hash literals in C++ are special syntaxes that allow you to obtain the hash value of a literal at compile-time. They provide a concise and efficient way to compute hashes without needing to write extensive boilerplate code.

Creating Custom Hash Literals

To create custom hash literals, you need to define a user-defined literal operator and associate it with the hash functionality for your custom type. Here’s a step-by-step guide to help you get started:

Step 1: Define your custom type

First, define your custom type for which you want to create hash literals. Let’s assume you have a custom type called MyCustomType.

class MyCustomType {
  // Define your custom type here
};

Step 2: Implement the hash function for your custom type

Next, implement a hash function for your custom type. The hash function should take an instance of your custom type as input and return a hash value.

#include <functional>

namespace std {
  template<>
  struct hash<MyCustomType> {
    size_t operator()(const MyCustomType& obj) const {
      // Define your hash computation logic here
      // Return the computed hash value
    }
  };
}

Make sure to include the <functional> header, as it provides the necessary tools for defining hash functions.

Step 3: Define the user-defined literal operator

Now, define the user-defined literal operator for your custom type. This operator allows you to create hash literals using a specific syntax.

constexpr MyCustomType operator"" _hash(const char* str, size_t length) {
  // Create an instance of your custom type based on the provided string
  // Return the created instance
}

Step 4: Test your custom hash literals

Finally, you can test your custom hash literals by using them in your code.

#include <iostream>

int main() {
  MyCustomType obj = "example"_hash;

  // Use the obj and perform operations
  
  return 0;
}

Conclusion

Creating custom hash literals in C++ allows you to represent your own types using a concise and efficient syntax. By following the steps outlined in this blog post, you can define your custom type, implement the hash function, and create hash literals for your type.

Using hash literals can simplify code and improve readability by providing a compact representation for hash values.