Summary - constexpr
- C++ course
- Expert
- Lesson 12/47
- 5 min
Combine constexpr, consteval and constinit to write a lookup table.
Summary - constexpr
In this group you saw the compile-time tools: constexpr variable, constexpr function, if constexpr, consteval and constinit. All produce values at compile time; the difference is flexibility versus strictness.
Now you will combine them into a constexpr factorial. This function delivers the recursive template's behavior with a constexpr function; constexpr int factorial(int n) is usable at both compile time and run time.
constexpr factorial
Inside a constexpr function you write the base case if (n <= 1) return 1; and the recursive step return n * factorial(n - 1);. When called with a constant argument, the compiler unfolds the chain at compile time.
Example: constexpr int f = factorial(4); produces the constant f = 24. This is the same as the recursive template but without template syntax; the code is readable and can also be called at run time.
Practice Write the constexpr factorial
Write constexpr int factorial(int n). Body: if (n <= 1) return 1; return n * factorial(n - 1);. In main define constexpr int f = factorial(4); and write std::cout << f;.
Starter code
#include <iostream>
// Define the constexpr factorial function here
int main() {
// Define f and print it
return 0;
}Sample solution
#include <iostream>
constexpr int factorial(int n) {
if (n <= 1) return 1;
return n * factorial(n - 1);
}
int main() {
constexpr int f = factorial(4);
std::cout << f;
return 0;
}Expected output
24
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 Which keyword makes a function run only at compile time?
constexpr int a(int);
consteval int b(int);
constinit int c = 0;- consteval Correct answer
- constinit
- constexpr
- const
Explanation: consteval (immediate function) runs only at compile time.
2 Complete the recursive step
constexpr int factorial(int n) { return n * factorial(n - ____); }constexpr int factorial(int n) { return n * factorial(n - 1); }Explanation: n * factorial(n-1) is the recursive step.
3 Match each tool to its trait
| Term | Meaning |
|---|---|
constexpr |
Flexible compile/run |
consteval |
Compile-time only |
constinit |
Static initialization |
if constexpr |
Compile-time branch |