consteval
- C++ course
- Expert
- Lesson 5/47
- 5 min
Learn mandatory compile-time with C++20 consteval.
consteval
consteval (C++20) forces a function to run only at compile time. It behaves like constexpr but a call with a variable argument is a compile error. Example: consteval int cube(int x) { return x * x * x; }.
cube(3) becomes 27 at compile time. But int n; std::cin >> n; cube(n); is a compile error, because n is not a compile-time constant. Use consteval when the computation must absolutely happen at compile time.
consteval vs constexpr
constexpr is flexible: with a constant argument it runs at compile time, with a variable argument at run time. consteval is strict: it runs only at compile time; a variable argument is a compile error. Choose consteval when you want a computation baked as a constant in the executable.
For example, if you prepare a table, consteval generates the values at compile time; at run time the table is ready. This shortens the boot time.
Practice Write the consteval cube
Write consteval int cube(int x) with body return x * x * x;. In main define constexpr int v = cube(3); and write std::cout << v;.
Starter code
#include <iostream>
// Define the consteval cube function here
int main() {
// define v and print it
return 0;
}Sample solution
#include <iostream>
consteval int cube(int x) {
return x * x * x;
}
int main() {
constexpr int v = cube(3);
std::cout << v;
return 0;
}Expected output
27
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 Complete the keyword
____ int cube(int x) { return x * x * x; }consteval int cube(int x) { return x * x * x; }Explanation: consteval forces compile-time only.
2 Match each call to its result
| Term | Meaning |
|---|---|
cube(1) |
returns 1 |
cube(2) |
returns 8 |
cube(3) |
returns 27 |
cube(4) |
returns 64 |
3 When is consteval cube(int n) a compile error?
int n;
std::cin >> n;
int r = cube(n);- Never
- When n is a variable Correct answer
- When n is constant
- Only in debug mode
Explanation: consteval gives a compile error with a variable argument.