Mastering Compile-Time Programming in C++20

Unlocking the Power of Compile-Time Programming

Limiting Function Usage to Compile Time

Imagine having complete control over when and how your functions are executed. With C++20, you can do just that by using the consteval keyword. This allows you to declare immediate functions that can only produce constants and are limited to compile-time usage.

Take, for example, a simple sum function that adds three integers together. By declaring it with consteval, you ensure that it can only be called from within a constant expression:
cpp
consteval auto sum(int x, int y, int z) { return x + y + z; }

Attempting to call sum with non-constant expressions will result in a compilation error:
cpp
auto x = 10;
auto s = sum(x, 2, 3); // Error, expression is not const

Compile-Time Polymorphism with if constexpr

But what if you want to write template functions that can adapt to different types at compile time? That’s where the if constexpr statement comes in. This powerful tool allows you to evaluate different scopes within a function at compile time, enabling true compile-time polymorphism.

Consider a speak function that needs to differentiate between member functions based on the type of animal:
“`cpp
struct Bear { auto roar() const { std::cout << “roar\n”; } };
struct Duck { auto quack() const { std::cout << “quack\n”; } };

template
auto speak(const Animal& a) {
if constexpr (std::issamev) { a.roar(); }
else if constexpr (std::issamev) { a.quack(); }
}
“
Whenspeakis invoked withAnimal == Bear, the compiler generates a function that callsroar(). Similarly, whenspeakis invoked withAnimal == Duck, the compiler generates a function that callsquack()`.

The Benefits of Compile-Time Programming

By leveraging consteval and if constexpr, you can unlock the full potential of compile-time programming. This allows you to write more efficient, flexible, and maintainable code that takes advantage of the compiler’s capabilities.

In the world of compile-time programming, the possibilities are endless. You can create functions that adapt to different types, optimize performance-critical code, and even generate code at compile time. The future of C++ development has never been brighter.