We use the **`consteval`** specifier in C++ to declare an immediate function. An immediate function is a function that must be evaluated at the compile-time to produce a constant. In other words, an immediate function is executed at compile-time.

# Consteval vs Constexpr
The `constexpr` specifier is the same as the `consteval` specfier, except for the fact that if a function declared with `consteval` does not return a compile-time constant, the function returns an error.



# Declaration
We can declare an immediate function with the `consteval` specifier as shown below:
```C
consteval returnType funcName (parameters)
{
//function body
}
```
* `returnType`: The return type of the immediate function.
* `funcName`: The function identifier of the immediate function.
* `parameters`: The input parameters of the immediate function.

# Code
Consider the code snippet below, which demonstrates the use of the `consteval` specifier:

#include <iostream>

consteval int add(int a, int b) {
    return a + b;
}

int main() {
    
    std::cout << "sum(2 + 5)= " << add(2, 5) << std::endl; 

    const int a = 2;
    const int b = 5;                            
    std::cout << "sum(2 + 5)= " << add(a, b) << std::endl;     

    int a2 = 2;
    int b2 = 5;                            
    //std::cout << "sum(2 + 5)= " << add(a2, b2) << std::endl;  //Error: Call does not produce a constant  

    return 0;
}

# Output
```
sum(2 + 5)= 7
sum(2 + 5)= 7
```
# Explanation
An immediate function `add` is declared in *line 3* using the `consteval` specifier.
* *line 9*: Two constants `2` and `5` are passed to the immediate function `add`. It is a valid invocation, so the function returns the answer.
* *line 13*: Two constants, integers `a` and `b`, are passed to the immediate function `add`. It is a valid invocation, so the function returns the answer. 
* *line 17*: `a2` and `b2` are not constant expressions. Thus, the invocation of the immediate function is not valid and the function returns an error.

What is consteval in C++?

We use the consteval specifier in C++ to declare an immediate function. An immediate function is a function that must be evaluated at the compile-time to produce a constant. In other words, an immediate function is executed at compile-time. Consteval vs Constexpr The constexpr specifier is the same as the consteval specfier, except for the fact that if a function declared with consteval does not return a compile-time constant, the function returns an error. 