When is a private constructor not a private constructor?
aggregate-initialization, c++, c++11, default-constructor, language-lawyer
Solution
The trick is in C++14 8.4.2/5 [dcl.fct.def.default]:
... A function is user-provided if it is user-declared and not explicitly defaulted or deleted on its first declaration. ...
Which means that `C`'s default constructor is actually not user-provided, because it was explicitly defaulted on its first declaration. As such, `C` has no user-provided constructors and is therefore an aggregate per 8.5.1/1 [dcl.init.aggr]:
An aggregate is an array or a class (Clause 9) with no user-provided constructors (12.1), no private or protected non-static data members (Clause 11), no base classes (Clause 10), and no virtual functions (10.3).
Problem
Let's say I have a type and I want to make its default constructor private. I write the following: ``` class C { C() = default; }; int main() { C c; // error: C::C() is private within this context (g++) // error: calling a private constructor of class 'C' (clang++) // error C2248: 'C::C' cannot access private member declared in class 'C' (MSVC) auto c2 = C(); // error: as above } ``` Great. But then, the constructor turns out to not be as private as I thought it was: ``` class C { C() = default; }; int main() { C c{}; // OK on all compilers auto c2 = C{}; // OK on all compilers } ``` This strikes me as very surprising, unexpected, and explicitly undesired behavior. Why is this OK?