I found it quite odd that the following program still compiled fine despite the default constructor being private (4.8.1 g++):
class A{
private:
A() = default;
A(const A&) = default;
};
int main(){
A a;
}
Actually from 8.4.2[2] of the standard (N3242)
An explicitly-defaulted function may be declared constexpr only if it would have been implicitly declared as constexpr. If it is explicitly defaulted on its first declaration,
— it shall be public,
..........
What exactly is the purpose for the default specifier to ignore the access specification? I feel like that could cause an interface issue unwarranted by the class designer that didn't want users to create default values but needed the default constructor in the implementation. I thought that maybe it's because the default constructor is normally public and so the default aims to replicate it - but that doesn't answer why =default on the copy constructor doesn't ignore the private specification.
class A{
private:
A() = default;
A(const A&) = default;
};
int main(){
A a;
A b(a); //error: constexpr A::A(const A&) is private
}
Actually I can't see from the standard where it mentions that explicitly-defaulted copy/move constructors/assignments aren't made public.
