Compile Time Constants using Templates

Yes, variable templates can be used to create compile-time constants. By defining a variable template with a constexpr value, you can create variables that are evaluated at compile-time and have a constant value. Here's an example of using a variable template to create compile-time constants:

1template <typename T>
2constexpr T pi = T(3.14159265358979323846);
3
4template <int N>
5constexpr int factorial = N * factorial<N - 1>;
6
7template <>
8constexpr int factorial<0> = 1;
9
10int main() {
11  constexpr double pi_double = pi<double>;
12  constexpr float pi_float = pi<float>;
13
14  constexpr int fact_5 = factorial<5>;
15  constexpr int fact_10 = factorial<10>;
16}

In this example, we have two variable templates: pi and factorial. The pi variable template is defined with a type parameter T and initialized with a constant expression that represents the value of pi. When instantiated with a specific type, such as double or float, it creates a compile-time constant of that type with the value of pi.

The factorial variable template is defined with a non-type template parameter N and calculates the factorial of N at compile-time. It uses recursion to multiply N with the factorial of N-1 until the base case of factorial<0> is reached, which is defined as a specialization that returns 1.

In the main function, we instantiate the variable templates with specific arguments, such as pi<double>, pi<float>, factorial<5>, and factorial<10>. These instantiations create compile-time constants that can be used in constant expressions. Using variable templates in this way allows you to define compile-time constants with parameterized types or values, providing flexibility and reusability in your code.

It's important to note that the expressions used to initialize variable templates must be constant expressions that can be evaluated at compile-time. This ensures that the variable templates are true compile-time constants and can be used in contexts that require constant expressions.