constexpr and 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:

1#include <iostream>
2
3class Base {
4public:
5  virtual constexpr int getValue() const {
6    return 1;
7  }
8};
9
10class Derived : public Base {
11public:
12  constexpr int getValue() const override {
13    return 2;
14  }
15};
16
17int main() {
18  constexpr Base b;
19  constexpr Derived d;
20
21  constexpr int base_value = b.getValue();
22  constexpr int derived_value = d.getValue();
23
24  std::cout << "Base value: " << base_value << '\n';
25  std::cout << "Derived value: " << derived_value;
26}

1Base value: 1
2Derived value: 2

Compile-Time Polymorphism

C++20 allows for compile-time polymorphism with constexpr virtual functions:

1#include <iostream>
2
3class Shape {
4 public:
5  virtual constexpr double area() const = 0;
6};
7
8class Circle : public Shape {
9 private:
10  double radius;
11
12 public:
13  constexpr Circle(double r) : radius(r) {}
14  constexpr double area() const override {
15    return 3.14159 * radius * radius;
16  }
17};
18
19class Square : public Shape {
20 private:
21  double side;
22
23 public:
24  constexpr Square(double s) : side(s) {}
25  constexpr double area() const override {
26    return side * side;
27  }
28};
29
30template <typename T>
31constexpr double getArea(const T& shape) {
32  return shape.area();
33}
34
35int main() {
36  constexpr Circle c(5);
37  constexpr Square s(4);
38
39  // Use a constant expression for areas
40  constexpr double circle_area = getArea(c);
41  constexpr double square_area = getArea(s);
42
43  // Output the values
44  std::cout << "Circle area: " << circle_area << '\n';
45  std::cout << "Square area: " << square_area << '\n';
46}

1Circle area: 78.5397
2Square area: 16

Limitations

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 constexpr with 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:

1#include <iostream>
2
3class Base {
4 public:
5  virtual constexpr int getValue() const {
6    return 1;
7  }
8};
9
10class Derived : public Base {
11 public:
12  constexpr int getValue() const override {
13    return 2;
14  }
15};
16
17void printValue(const Base& obj) {
18  std::cout << "Value: " << obj.getValue() << '\n';
19}
20
21int main() {
22  Base b;
23  Derived d;
24
25  printValue(b);
26  printValue(d);
27}

1Value: 1
2Value: 2

consteval 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:

1#include <iostream>
2
3template <class Derived>
4class Base {
5 public:
6  constexpr int getValue() const {
7    return static_cast<const Derived*>
8      (this)->getValueImpl();
9  }
10};
11
12class DerivedA : public Base<DerivedA> {
13 public:
14  constexpr int getValueImpl() const { return 1; }
15};
16
17class DerivedB : public Base<DerivedB> {
18 public:
19  constexpr int getValueImpl() const { return 2; }
20};
21
22template <class T>
23constexpr int doubleValue(const Base<T>& obj) {
24  return obj.getValue() * 2;
25}
26
27int main() {
28  constexpr DerivedA a;
29  constexpr DerivedB b;
30  constexpr int result1 = doubleValue(a);
31  constexpr int result2 = doubleValue(b);
32  std::cout << "Result1: " << result1 << '\n';
33  std::cout << "Result2: " << result2 << '\n';
34}

1Result1: 2
2Result2: 4

In 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.