constructor is implicitly deleted because its exception-specification does not match the implicit exception-specification

Question

Can anyone explain me why using constructor marked as default will have the error but the other one doesn't have. My understanding is they are very much similar. Thanks.

class B {
public:
    B() {
        throw int(42);
    }
};

class A {
public:
    A() noexcept = default; // error: use of deleted function ‘A::A()’
                            // note: ‘A::A() noexcept’ is implicitly deleted because
                            // its exception-specification does not match the implicit exception-specification ‘’
    //A() noexcept : m_b{}{}
    B m_b;
};

Answer 1

Originally, the declaration of the constructor was already ill-formed, the idea being that the defaulted function ought to behave identically to the implicit declaration. Your explicit implementation has no such restriction: noexcept functions are allowed to call potentially throwing functions, and the behavior is even well-defined if they throw.

In C++14 it was relaxed to be merely deleted in that case. Later it was realized that even that was too restrictive (and difficult to implement correctly), so it was relaxed again: now the exception specification can be anything (but there’s still an automatic one for defaulted functions).

Both of these changes were retroactive, so you’ll see a dependence on compiler version more than language version.

Note that none of this depends on the actual throw in B::B: that constructor is potentially throwing regardless.