What is the constexpr keyword in C++?
Compile-time evaluation
The constexpr keyword, short for constant expression, was a feature introduced in C++11 that allows the value of an expression to be evaluated at compile-time.
This feature significantly increases performance since the expression does not need to be computed again during runtime; instead, its value can simply be used where needed.
There are three main applications of constexpr:
constexpr variables
The value of a variable can be computed at compile-time by making it constexpr:
constexpr int num1 = 20;
constexpr int num2 = num1 * 10;
// Both are evaluated at compile-time
The variables num1 and num2 are implicitly const. A constexpr variable has to be initilized with an expression. This expression can only contain literals or other constexpr variables.
constexpr functions
constexpr functions are executed and evaluated during compilation.
A constexpr function must follow the rules below:
-
Its return type cannot be
void, i.e., it must compute and return a value. -
It can only call other
constexprfunctions. -
It can only refer to global
constvariables.
Here is an example:
#include <iostream>using namespace std;constexpr bool is_even(int num){if(num % 2 == 0){return true;}return false;}int main() {constexpr int n = 30;constexpr bool check_even = is_even(n); // Evaluated at compile-timecout << check_even << endl;}
The value of check_even is evaluated at compile-time. If this value is to remain constant throughout the program, having it computed at compile-time is better than computing it at runtime whenever it is required.
constexpr objects
These objects are initialized at compile-time. A constexpr object can be created by following these rules:
-
The constructor must be
constexpr. -
All non-static members must be initialized. Furthermore, they must have
constexprconstructors themselves, e.g., basic data types. -
The constructor must be defined as
defaultordelete; otherwise, its body must be empty. -
Object methods can also be declared
constexpras long as they follow the rules forconstexprfunctions.
A constexpr object has been created below:
#include <iostream>using namespace std;class Coordinate {int x;int y;public:// constexpr constructorconstexpr Coordinate(int x_c, int y_c): x(x_c), y(y_c){}// constexpr methodconstexpr int get_x(){return x;}constexpr int get_y(){return y;}};int main() {Coordinate c = Coordinate(5, 10); // Object created at compile-timeint x = c.get_x();int y = c.get_y(); // Coordinates retrieved at compile-timecout << x << ", " << y << endl;}
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