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Can virtual functions like X::f() in the following code
struct X { constexpr virtual int f() const { return 0; } }; be constexpr?
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Can virtual functions be constexpr? Yes. Only since C++20, virtual functions can be constexpr .
A constexpr function is a function that can be invoked within a constant expression. A constexpr function must satisfy the following conditions: It is not virtual. Its return type is a literal type.
We allow annotating a function parameter with constexpr with the same meaning as a variable declaration: must be initialized with a constant expression. We add a new keyword, maybe_constexpr , that deduces whether the parameter is known at compile time.
This answer is no longer correct as of C++20.
No. From [dcl.constexpr]/3 (7.1.5, "The constexpr specifier"):
The definition of a
constexprfunction shall satisfy the following requirements:— it shall not be virtual
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