Why Can a Private Default Constructor Be Implicitly Invoked in C ?

When is a Private Constructor Not a Private Constructor?

In C , declaring a default constructor private may seem like a straightforward way to prevent object instantiation. However, surprisingly, a private default constructor can still be invoked implicitly.

Question:

Consider the following code:

class C {
    C() = default;
};

int main() {
    C c; // error: default constructor private
    auto c2 = C(); // error: default constructor private
}

Why does the constructor appear to be private but can be called implicitly with brace initialization?

Answer:

The key lies in the C 14 specification. A user-provided constructor is one that is explicitly declared without being defaulted or deleted. Since C's default constructor was explicitly defaulted on its first declaration, it is not considered user-provided.

As a result, C lacks user-provided constructors and becomes an aggregate, according to 8.5.1/1 of the specification. An aggregate is defined as a class with no private or protected non-static data members, no base classes, no virtual functions, and no user-provided constructors. This explains why brace initialization can successfully create objects of C, despite the default constructor being technically private.

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