Unlocking the Power of Compile-Time Hashing
The Quest for Efficient Hashing
Imagine a world where your code runs faster and more efficiently. A world where the compiler does the heavy lifting, freeing you to focus on the creative aspects of programming. This world is possible, thanks to the magic of compile-time hashing.
Crafting a Hash Function
To unlock this power, we need to create a hash function that can be evaluated at compile time. We can achieve this by rewriting our hash function to take a raw null-terminated char string as a parameter. This allows us to mark the function as constexpr, enabling the compiler to calculate the hash sum at compile time.
The Anatomy of a Compile-Time Hash Function
Here’s an example of what our constexpr hash function might look like:
constexpr auto hash_function(const char* str) -> size_t {
auto sum = size_t{0};
for (auto ptr = str; *ptr!= '\0'; ++ptr)
sum += *ptr;
return sum;
}
Putting it into Practice
Now, let’s see how this function works in practice. We can invoke it with a raw literal string known at compile time, like this:
auto hash = hash_function("abc");
The compiler will generate the following code, which is the sum of the ASCII values corresponding to ‘a’, ‘b’, and ‘c’ (97, 98, and 99):
auto hash = size_t{294};
Building a PrehashedString Class
With our hash function in place, we can now create a PrehashedString class that uses it. This class consists of a constructor that takes a raw string as a parameter and calculates the hash at construction time.
Important Notes
While our example hash function is simple to understand, it’s not suitable for real-world applications. A better approach would be to combine individual characters using boost::hash_combine(), as explained in Chapter 4, Data Structures. Additionally, constexpr functions will only evaluate at compile time if the compiler knows the string at compile time; otherwise, they’ll execute at runtime like any other expression.
The Future of Efficient Coding
By harnessing the power of compile-time hashing, we can write faster, more efficient code that takes advantage of the compiler’s capabilities. Whether you’re building high-performance applications or simply looking to optimize your code, this technique is sure to become an essential tool in your programming toolkit.