Does static constexpr variable inside a function make sense?

Yes, a static constexpr variable in a function is indeed meaningful.

First, let's understand the roles and significance of the static and constexpr keywords in this context:

1. Static: When a variable is declared as static, its lifetime spans from the start of the program until its termination. Additionally, a static variable within a function is initialized only once—specifically on the first function call—and subsequent calls retain the state from the previous invocation.

2. constexpr: This keyword, introduced in C++11, denotes that the value of a variable or function is a constant expression, determinable at compile time. This is highly useful for optimization and compile-time error checking.

When combined, a static constexpr variable in a function serves the following purposes:

• Performance Optimization: Since the variable is constexpr, its value is determined at compile time, eliminating the need for recomputation at runtime. Additionally, due to its static nature, the variable has only one instance in memory and is not reinitialized upon subsequent function calls.

• Reuse of Constants: A static constexpr variable provides a commonly used, unchanging value within the function, without requiring reinitialization on each function call. This is particularly useful when working with constant configuration data or reusing the result of an invariant computation.

For example, consider the following function, which calculates the post-tax amount for a fixed tax rate:

cpp
double postTaxAmount(double preTaxAmount) {
    static constexpr double taxRate = 0.05; // 5% tax rate
    return preTaxAmount * (1 - taxRate);
}

In this example, the tax rate (taxRate) is declared as a static constexpr variable, with its value known at compile time and initialized only once during the program's execution. This avoids the need to recompute the tax rate on each call to postTaxAmount, thereby improving efficiency.

In conclusion, a static constexpr variable within a function is not only meaningful but also highly useful when aiming to improve efficiency and code clarity.