Constexpr and Consteval

constexpr and consteval are both keywords used for compile-time evaluation and optimizations. They are used to indicate that certain expressions or functions can be evaluated at compile time rather than runtime, which can lead to improved performance and code optimization. However, there are some differences between them.

1. constexpr:
   • The constexpr keyword is used to declare that an object or function can be evaluated at compile time.
   • It can be applied to variables, functions, and constructors.
   • constexpr variables must have a value that can be determined at compile time and are required to be initialized with a constant expression.
   • constexpr functions are allowed to have a body containing a set of statements, but they must meet certain requirements:
     • The function must be defined in a way that it can be evaluated at compile time for all possible arguments.
     • The function’s return type and all of its parameters must be literal types (types with certain restrictions).
     • The function body can only contain a limited set of statements, including variable declarations, if-else statements, loops, and other constexpr functions.
     • The function body cannot contain operations that are not allowed in constant expressions, such as dynamic memory allocation or I/O operations.
   • Example:

constexpr int factorial(int n) {
    if (n <= 1)
        return 1;
    else
        return n * factorial(n - 1);
}

constexpr int result = factorial(5);  // Computed at compile time

2.consteval:

• The consteval keyword is used to declare that a function must be evaluated at compile time and that a compile-time error should occur if it cannot be evaluated at compile time.
• It can only be applied to function declarations and definitions.
• consteval functions must have a body containing a single return statement that yields a compile-time constant expression.
• consteval functions cannot contain any other statements or have any parameters or return types that are not literal types.
• Example:

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

constexpr int result = square(5);  // Computed at compile time

consteval int getUserInput() {
    int input;
    std::cin >> input;  // Compile-time error: I/O operations not allowed
    return input;
}

int value = getUserInput();  // Compile-time error

In summary, constexpr is used to indicate that an expression or function can be evaluated at compile time, allowing a broader range of operations but with runtime fallback if needed. On the other hand, consteval is more restrictive and enforces that the function must be evaluated at compile time, resulting in a compile-time error if it cannot be evaluated.