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c++virtualconstexprfinal

C++ constexpr final virtual functions

I'm implementing a pure virtual function in a C++ derived class, and mark the method final. I would also like to mark the function constexpr, but it appears the standard doesn't allow this.

Is there any practical reason why it would be difficult for compilers to implement this? Or is such a feature omitted due to the usual "this feature wasn't deemed to be important enough to add to the standard" thinking?

Based on the comments, it seems my original question didn't make it clear I am asking about constexpr final functions. The following code illustrates what I am trying to compile:

struct A {
    virtual int foo() const;
};

struct B : public A {
    // Note that this function is final, and is therefore no longer
    // polymorphic in types derived from B (e.g., the function call
    // no longer requires a lookup in the virtual method table
    // when the compile-time type is known to be derived from B)
    constexpr int foo() const final {
        return 0;
    }
};

Note that this was flagged as a duplicate of Can virtual functions be constexpr?, since in C++20, the above code would compile with or without the final keyword.

Solution