Virtual explicit conversion operator overriding

Question

I have a class Base defining an explicit operator bool:

struct Base {     virtual explicit operator bool() const {         return true;     } }; 

And I have a subclass Derived, defining an operator bool:

struct Derived : Base {     operator bool() const override {         return false;     } }; 

As you can observe, Derived::operator bool is explicitly not marked explicit, but marked override, so I expected the compiler to complain. However, both gcc and clang seem to agree that this is valid. Was my expectation unreasonable?

Moreover, if I use the classes as follows, TakesBool(base) does not compile (as expected), but TakesBool(derived) does:

void TakesBool(bool b) {}  int main() {     //Base base;     TakesBool(base); // compilation error (as expected)     Derived derived; TakesBool(derived);     return 0; } 

This seems to indicate that Derived has an (non-explicit) operator bool, which, however, is marked override without a virtual declaration. How is this possible?

Answer 1

You might think that the non-explicit operator bool in Derived doesn't override explicit operator bool in Base, no, it does. The explicit specifier doesn't matter, it's not a part of function signature.

From the standard, §10.3/2 Virtual functions [class.virtual]:

(emphasis mine)

If a virtual member function vf is declared in a class Base and in a class Derived, derived directly or indirectly from Base, a member function vf with the same name, parameter-type-list ([dcl.fct]), cv-qualification, and ref-qualifier (or absence of same) as Base::vf is declared, then Derived::vf is also virtual (whether or not it is so declared) and it overrides Base::vf.

So the compiler will complain only when the name, parameter-type-list, cv-qualification or ref-qualifier of the function doesn't match, the explicit specifier won't be considered.

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^{You said "marked override without a virtual declaration", note that the virtual declaration for member function in derived class is superfluous, it's also virtual.}