Variable Templates vs Constexpr Variables

Variable templates and constexpr variables are related but have some key differences:

• Variable templates allow you to define a variable that can be instantiated with different types. They provide a way to create variables with parameterized types, similar to how function templates allow functions to work with different types. Variable templates are defined using the template keyword followed by template parameters.
• Constexpr variables, on the other hand, are variables that are evaluated at compile-time and have a constant value. They are defined using the constexpr keyword and must be initialized with a constant expression. Constexpr variables can be of any type, but their value must be known at compile-time.

Here's an example to illustrate the difference:

1// Variable template
2template <typename T>
3constexpr T pi = T(3.14159265358979323846);
4
5// Constexpr variable
6constexpr double e = 2.71828182845904523536;
7
8int main() {
9  // Instantiating the variable template with double
10  double pi_double = pi<double>;
11  
12  // Instantiating the variable template with float
13  float pi_float = pi<float>;
14
15  // Using the constexpr variable
16  double result = e * 2;
17}

In this example, pi is a variable template that can be instantiated with different types, such as double or float. Each instantiation creates a separate variable with the specified type and initializes it with the provided value.

On the other hand, e is a constexpr variable of type double. Its value is fixed and known at compile-time. The key difference is that variable templates allow for type parameterization, while constexpr variables have a fixed type and value determined at compile-time.

Both variable templates and constexpr variables are useful for creating compile-time constants and performing compile-time computations, but variable templates offer additional flexibility in terms of type parameterization.