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.
Declaration
We can declare an immediate function with the consteval specifier as shown below:
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 constantreturn 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
2and5are passed to the immediate functionadd. It is a valid invocation, so the function returns the answer. - line 13: Two constants, integers
aandb, are passed to the immediate functionadd. It is a valid invocation, so the function returns the answer. - line 17:
a2andb2are not constant expressions. Thus, the invocation of the immediate function is not valid and the function returns an error.
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