constexpr and Virtual Functions
Can I use constexpr with virtual functions?
Yes, as of C++20, virtual functions can indeed be constexpr. This is a significant change from previous C++ standards. Let's explore this in detail:
Basic Usage
In C++20, you can declare virtual functions as constexpr:
#include <iostream>
class Base {
public:
virtual constexpr int getValue() const {
return 1;
}
};
class Derived : public Base {
public:
constexpr int getValue() const override {
return 2;
}
};
int main() {
constexpr Base b;
constexpr Derived d;
constexpr int base_value = b.getValue();
constexpr int derived_value = d.getValue();
std::cout << "Base value: " << base_value << '\n';
std::cout << "Derived value: " << derived_value;
}Base value: 1
Derived value: 2Compile-Time Polymorphism
C++20 allows for compile-time polymorphism with constexpr virtual functions:
#include <iostream>
class Shape {
public:
virtual constexpr double area() const = 0;
};
class Circle : public Shape {
private:
double radius;
public:
constexpr Circle(double r) : radius(r) {}
constexpr double area() const override {
return 3.14159 * radius * radius;
}
};
class Square : public Shape {
private:
double side;
public:
constexpr Square(double s) : side(s) {}
constexpr double area() const override {
return side * side;
}
};
template <typename T>
constexpr double getArea(const T& shape) {
return shape.area();
}
int main() {
constexpr Circle c(5);
constexpr Square s(4);
// Use a constant expression for areas
constexpr double circle_area = getArea(c);
constexpr double square_area = getArea(s);
// Output the values
std::cout << "Circle area: " << circle_area << '\n';
std::cout << "Square area: " << square_area << '\n';
}Circle area: 78.5397
Square area: 16Limitations
While constexpr virtual functions are powerful, there are some limitations:
- The dynamic type must be known at compile-time for the call to be part of a constant expression.
- You can't use
constexprwith polymorphic objects where the dynamic type is determined at runtime.
Runtime Behavior
constexpr virtual functions can still be called at runtime, maintaining normal virtual function behavior:
#include <iostream>
class Base {
public:
virtual constexpr int getValue() const {
return 1;
}
};
class Derived : public Base {
public:
constexpr int getValue() const override {
return 2;
}
};
void printValue(const Base& obj) {
std::cout << "Value: " << obj.getValue() << '\n';
}
int main() {
Base b;
Derived d;
printValue(b);
printValue(d);
}Value: 1
Value: 2consteval and Virtual Functions
It's important to note that while virtual functions can be constexpr, they cannot be consteval. consteval functions are required to produce a constant expression in every context, which is incompatible with the runtime polymorphism of virtual functions.
Alternative Approaches
If you need compile-time polymorphism, consider using the Curiously Recurring Template Pattern (CRTP) instead of virtual functions.
This approach allows for compile-time polymorphism without using virtual functions:
#include <iostream>
template <class Derived>
class Base {
public:
constexpr int getValue() const {
return static_cast<const Derived*>
(this)->getValueImpl();
}
};
class DerivedA : public Base<DerivedA> {
public:
constexpr int getValueImpl() const { return 1; }
};
class DerivedB : public Base<DerivedB> {
public:
constexpr int getValueImpl() const { return 2; }
};
template <class T>
constexpr int doubleValue(const Base<T>& obj) {
return obj.getValue() * 2;
}
int main() {
constexpr DerivedA a;
constexpr DerivedB b;
constexpr int result1 = doubleValue(a);
constexpr int result2 = doubleValue(b);
std::cout << "Result1: " << result1 << '\n';
std::cout << "Result2: " << result2 << '\n';
}Result1: 2
Result2: 4In summary, C++20 allows constexpr virtual functions, enabling compile-time polymorphism while maintaining runtime virtual function behavior. This feature provides more flexibility in designing class hierarchies that can be used in both compile-time and runtime contexts.
However, it's crucial to understand the limitations and ensure that the dynamic type is known at compile-time when using these functions in constant expressions.
For true compile-time polymorphism, alternative design patterns like CRTP are often more suitable.
Compile-Time Evaluation
Learn how to implement functionality at compile-time using constexpr and consteval