In this article, we will discuss the immediate function used in C++.

Immediate Function:

• In C++20, an immediate function is a function where every call to the function either directly or indirectly produces a compile-time constant expression.
• These functions are declared by using a consteval keyword before their return type.

Below given some important terms related to Immediate Function:

constexpr function: 

• The constexpr specifier declares that it is possible to evaluate the value of a function or variable at compile time.
• Such variables and functions can then be used where only compile-time constant expressions are allowed.
• These functions are used to improve the performance of the program by doing computations at compile time instead of run time.
• These functions can really be helpful, where executing a program multiple times as the constant expressions will only be evaluated once during the compile time.

Below is the C++ program illustrating the use of constexpr function:

// C++ program demonstrates the use of
// constexpr
#include <iostream>
using namespace std;

// Constexpr function if replaced with
// consteval, program works fine
constexpr int fib(int n)
{
    // Base Case
    if (n <= 1)
        return n;

    // Find the Fibonacci Number
    return fib(n - 1) + fib(n - 2);
}

// Driver Code
int main()
{
    // Constant expression evaluated
    // at compile time
    const int val = fib(22);

    cout << "The fibonacci number "
         << "is: " << val << "\n";

    return 0;
}

// C++ program demonstrates the use of

// constexpr

#include <iostream>

using namespace std;

​

// Constexpr function if replaced with

// consteval, program works fine

constexpr int fib(int n)

{

    // Base Case

    if (n <= 1)

        return n;

​

    // Find the Fibonacci Number

    return fib(n - 1) + fib(n - 2);

}

​

// Driver Code

int main()

{

    // Constant expression evaluated

    // at compile time

    const int val = fib(22);

​

    cout << "The fibonacci number "

         << "is: " << val << "\n";

​

    return 0;

}

The fibonacci number is: 17711

Explanation:

• In the above example, fib() is called with 22.
• That's why, it is a constant expression and so, it can be evaluated at compile time.
• The program shows no error either at using constexpr or consteval.
• But, if the constant expression is used while using the consteval keyword, the program will produce an error.

consteval function:

• In consteval function, every call to the function must directly or indirectly produce a compile-time constant expression.
• The consteval function is the same as constexpr function except that if the call to a consteval function doesn't evaluate to a compile-time constant expression, then the program gives an error while it is not so in the case of a constexpr function.
• The constexpr specifies that the value of a variable or function can appear in constant expressions.
• The key to note here is that it says, a function can appear in constant expressions, it doesn't say that the function has to be, while a consteval specifies that a function is an immediate function, that is, every call to the function must produce a compile-time constant.

Below is the C++ program illustrating the use of consteval function:

// C++ program illustrating the use
// of the consteval function

#include <iostream>
using namespace std;

// If constexpr replaces with consteval
// program gives an error in C++20
constexpr int fib(int n)
{
    // Base Case
    if (n <= 1)
        return n;

    return fib(n - 1) + fib(n - 2);
}

// Driver Code
int main()
{
    // This expression is not evaluated
    // at compile time
    const int val = fib(rand() % 20);

    cout << "The fibonacci number is: "
         << val << "\n";

    return 0;
}

// C++ program illustrating the use

// of the consteval function

​

#include <iostream>

using namespace std;

​

// If constexpr replaces with consteval

// program gives an error in C++20

constexpr int fib(int n)

{

    // Base Case

    if (n <= 1)

        return n;

​

    return fib(n - 1) + fib(n - 2);

}

​

// Driver Code

int main()

{

    // This expression is not evaluated

    // at compile time

    const int val = fib(rand() % 20);

​

    cout << "The fibonacci number is: "

         << val << "\n";

​

    return 0;

}

The fibonacci number is: 2

Explanation:

• In the above example, rand() is used and rand() gets evaluated at runtime not at compile time and so because of that, the expression is no longer a constant expression, that's why our consteval function will now produce an error.
• While the constexpr function still works fine and the reason is that it doesn't have to be at compile time.

Conclusion:

• From the above discussion, it can be concluded that the Immediate function is consteval function, which works fine only when every call to the function must directly or indirectly produce a compile-time constant expression otherwise gives an error.
• These functions are declared by using a consteval keyword before their return type and are used to reduce the time, consumed in evaluating the constant expressions as they evaluate the constant expressions only once during the compile-time and not during every run/execution of the program.
• Hence, it saves a considerable amount of time when there is a need to execute a program with some constant expressions multiple times.