Can you explain what the keyword constexpr means in C++?

The keyword constexpr in C++ is used to indicate that a function or variable can be evaluated at compile time. This allows for increased performance and guarantees that the value will be known during compilation.

Features of constexpr:

• For variables: declares a constant whose value is computed at compile time.
• For functions: the function can be called in contexts where a constant expression is required (e.g., array size).
• constexpr functions must contain only one return statement and be simple enough for the compiler to evaluate the result at compile time.

Example:

constexpr int square(int x) {
    return x * x;
}

int arr[square(5)]; // Array size is computed at compile time

If the function is called with constant arguments, the result will be computed at compile time. If called with variables, the function will work like a regular function.