A constexpr function is a function that is allowed to be called in a constant expression. To make a function a constexpr function, we simply use the constexpr keyword in front of the return type. Constexpr functions are only guaranteed to be evaluated at compile-time when used in a context that requires a constant expression. Otherwise they may be evaluated at compile-time (if eligible) or runtime. Constexpr functions are implicitly inline, and the compiler must see the full definition of the constexpr function to call it at compile-time.
A consteval function is a function that must evaluate at compile-time. Consteval functions otherwise follow the same rules as constexpr functions.
Quiz time
Question #1
Add const and/or constexpr to the following program:
#include <iostream>
// gets tower height from user and returns it
double getTowerHeight()
{
std::cout << "Enter the height of the tower in meters: ";
double towerHeight{};
std::cin >> towerHeight;
return towerHeight;
}
// Returns ball height from ground after "seconds" seconds
double calculateBallHeight(double towerHeight, int seconds)
{
double gravity{ 9.8 };
// Using formula: [ s = u * t + (a * t^2) / 2 ], here u(initial velocity) = 0
double distanceFallen{ (gravity * (seconds * seconds)) / 2.0 };
double currentHeight{ towerHeight - distanceFallen };
return currentHeight;
}
// Prints ball height above ground
void printBallHeight(double ballHeight, int seconds)
{
if (ballHeight > 0.0)
std::cout << "At " << seconds << " seconds, the ball is at height: " << ballHeight << " meters\n";
else
std::cout << "At " << seconds << " seconds, the ball is on the ground.\n";
}
// Calculates the current ball height and then prints it
// This is a helper function to make it easier to do this
void printCalculatedBallHeight(double towerHeight, int seconds)
{
double ballHeight{ calculateBallHeight(towerHeight, seconds) };
printBallHeight(ballHeight, seconds);
}
int main()
{
double towerHeight{ getTowerHeight() };
printCalculatedBallHeight(towerHeight, 0);
printCalculatedBallHeight(towerHeight, 1);
printCalculatedBallHeight(towerHeight, 2);
printCalculatedBallHeight(towerHeight, 3);
printCalculatedBallHeight(towerHeight, 4);
printCalculatedBallHeight(towerHeight, 5);
return 0;
}
Show Solution
#include <iostream>
// This function should not be made constexpr because output and input can only be done at runtime.
// The versions of `operator<<` and `operator>>` that do output and input don't support constexpr.
double getTowerHeight()
{
std::cout << "Enter the height of the tower in meters: ";
double towerHeight{};
std::cin >> towerHeight;
return towerHeight;
}
// Returns height from ground after "seconds" seconds
// This function is made constepxr because it just calculates a value from its inputs and return it.
// Arithmetic can be done at compile-time, and no non-constexpr functions are called.
// Reminder: A constexpr function can be evaluated at compile-time or runtime.
// If its arguments are constexpr, it can be called at compile-time.
// In this case, it's called at runtime because the argument for towerHeight isn't constexpr.
// If a function can be made constexpr, it should be.
// Remember: function parameters are not constexpr, even in a constexpr function
constexpr double calculateBallHeight(double towerHeight, int seconds)
{
constexpr double gravity{ 9.8 }; // constexpr because it's a compile-time constant
// Using formula: [ s = u * t + (a * t^2) / 2 ], here u(initial velocity) = 0
// These variables can't be constexpr since their initializers aren't constant expressions
const double distanceFallen{ (gravity * (seconds * seconds)) / 2.0 };
const double currentHeight{ towerHeight - distanceFallen };
return currentHeight;
}
// This function should not be made constexpr because output and input can only be done at runtime.
// The versions of `operator<<` and `operator>>` that do output and input don't support constexpr.
void printBallHeight(double ballHeight, int seconds)
{
if (ballHeight > 0.0)
std::cout << "At " << seconds << " seconds, the ball is at height: " << ballHeight << " meters\n";
else
std::cout << "At " << seconds << " seconds, the ball is on the ground.\n";
}
// This function should not be made constexpr because output and input can only be done at runtime.
// The versions of `operator<<` and `operator>>` that do output and input don't support constexpr.
void printCalculatedBallHeight(double towerHeight, int seconds)
{
// height can only be const (not constexpr) because its initializer is not a constant expression
const double ballHeight{ calculateBallHeight(towerHeight, seconds) };
printBallHeight(ballHeight, seconds);
}
int main()
{
// towerHeight can only be const (not constexpr) because its initializer is not a constant expression
const double towerHeight{ getTowerHeight() };
printCalculatedBallHeight(towerHeight, 0);
printCalculatedBallHeight(towerHeight, 1);
printCalculatedBallHeight(towerHeight, 2);
printCalculatedBallHeight(towerHeight, 3);
printCalculatedBallHeight(towerHeight, 4);
printCalculatedBallHeight(towerHeight, 5);
return 0;
}
