Noexcept promise to the constructor of the derived class: can this be used without the noexcept promise in the base constructor?

Question

Say I have a class

class C : public B { public: C() noexcept; } 

Does the noexcept specifier noexcept the same promise with the base class? That is, when I consider the possibility of using noexcept, I just look at the behavior of C :: C () or do I also need to consider whether B :: B () can throw exceptions?

For example, if B :: B throws an exception, does this apply to C :: C or to code requesting a new instance of the class? - If you extend to C :: C, this will be one of the reasons to avoid noexcept for the constructor if the base class is no exception to the constructor.

Answer 1

There is a technical ^† requirement that the constructor of the base class be not declared noexcept, but it should not be thrown when called by a derived constructor that is declared noexcept.

So, yes, you need to think about whether the base class constructor can throw (exceptions or otherwise).

I think the best way to ask my question is: where is the next exception?

Calls are made as follows:

 caller -> derived constructor (the noexcept applies to this) -> subobject constructors (includes bases) - derived constructor body (not a call, but part of the derived constructor that is executed after the subobjects are constructed) 

So, if a subobject (whether basic or elemental) generates a browser, it first passes to the derived constructor, which cannot and cannot ^†† swallow the exception, so it will apply to the caller if the constructor was not superfluous. But since it is, std::terminate called.

^† If the base does not throw away as required by the derivative, then it satisfies the requirement and can itself be declared noexcept, so there is a rare reason not to do this. Perhaps if the base class is part of a library that should support C ++ 03, while the derived class can adopt a later standard, that makes sense.

^†† It can catch using the try-block function, but they will always throw again.