constexpr functions
- C++ course
- Expert
- Lesson 4/47
- 5 min
Learn compile-time computation with constexpr functions.
constexpr functions
A constexpr function is an ordinary function that can compute at compile time. Unlike TMP it needs no template; the constexpr keyword is enough. Example: constexpr int square(int x) { return x * x; }.
Called with a constant argument it runs at compile time; with a variable argument it runs at run time. constexpr int s = square(5); produces the constant s = 25 at compile time. This is far more readable than TMP.
Assigning to a constexpr variable
You can assign the result of a constexpr function to a constexpr variable. This variable is a compile-time constant; it can be used as an array size or a template argument. Example: constexpr int area = square(6); creates the constant area = 36.
You can print these constants with std::cout. The compiler resolves square(6) to 36 at compile time; the executable embeds 36.
Practice Write the constexpr square
Write constexpr int square(int x) with body return x * x;. In main define constexpr int area = square(6); and write std::cout << area;.
Starter code
#include <iostream>
// Define the constexpr square function here
int main() {
// define area and print it
return 0;
}Sample solution
#include <iostream>
constexpr int square(int x) {
return x * x;
}
int main() {
constexpr int area = square(6);
std::cout << area;
return 0;
}Expected output
36
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 What does constexpr int square(int x) enable?
constexpr int square(int x) { return x * x; }- Compile-time computation Correct answer
- Macro only
- Class membership
- Run time only
Explanation: constexpr enables compile-time computation with constant arguments.
2 Complete the return statement
constexpr int square(int x) { return x * ____; }constexpr int square(int x) { return x * x; }Explanation: Write return x * x; the multiplier for square is x.
3 Match each call to its result
| Term | Meaning |
|---|---|
square(2) |
returns 4 |
square(5) |
returns 25 |
square(6) |
returns 36 |
square(10) |
returns 100 |