Have you ever encountered a situation where you have a template code that needs to be decompressed at compile-time? This can happen when you have a compressed representation of a template code and you want to generate the actual code at compile-time. In such cases, recursive templates can be a powerful tool.

What are Recursive Templates?

Recursive templates in C++ refer to the technique of using templates that call themselves during compilation. This allows for the generation of code based on a recursive algorithm. In the context of template code decompression, recursive templates can be used to expand the compressed template code into the actual code.

Implementing Recursive Template Decompression

To illustrate the concept, let’s consider a simple example where we have a compressed representation of a template function that adds two integers:

template <int K>
struct CompressedAdd {
    static const int value = K + 1;
};

In this example, the template parameter K represents the compressed integer value. Our goal is to decompress this code at compile-time to generate the actual addition function Add.

Here’s how we can use recursive templates to achieve this:

// Base template
template <int K>
struct DecompressedAdd {
    static const int value = K;
};

// Recursive template
template <int K>
struct DecompressedAddHelper {
    static const int value = DecompressedAdd<K - 1>::value + 1;
};

// Specialization for termination
template <>
struct DecompressedAdd<0> {
    static const int value = 0;
};

// Usage
int main() {
    constexpr int result = DecompressedAdd<5>::value;
    std::cout << result << std::endl; // Output: 5
    return 0;
}

In this code, we have two template structs: DecompressedAdd and DecompressedAddHelper. DecompressedAdd is the base template that represents the decompressed code. The DecompressedAddHelper template calls itself recursively until it reaches the termination condition (DecompressedAdd<0>). Each recursive call increments the value by 1 until the termination condition is reached.

By providing the desired compressed value as the template parameter, we can access the decompressed value using the value member variable.

Conclusion

Recursive templates are a powerful technique in C++ that allow for the generation of code at compile-time. In the context of template code decompression, recursive templates can be used to expand compressed template code into the actual code. By leveraging recursive calls and termination conditions, we can achieve a recursive template that decompresses the template code. This technique can be handy in scenarios where compressed template code needs to be expanded at compile-time.

#C++ #TemplateCodeDecompression