Why Can I Initialize an Aggregate with a Private Default Constructor?

Private Constructors and Aggregate Initialization: An Unexpected Interaction

In C , declaring a default constructor as private would naturally lead one to assume that the default construction of the associated type would become inaccessible. However, a peculiar behavior emerges when using brace-enclosed initialization syntax.

Consider the following example:

class C {
    C() = default;
};

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

Unexpectedly, the above code generates errors on all major compilers. This is expected behavior because the default constructor is indeed declared as private. However, using brace-enclosed initialization suddenly allows default construction:

int main() {
    C c{};         // OK
    auto c2 = C{}; // OK
}    

The reason behind this unexpected behavior lies in the intricacies of C standard library. According to the C 14 standard (8.4.2/5), a function is considered "user-provided" if it is user-declared and not explicitly defaulted or deleted on its first declaration.

In the case of the C class above, the default constructor is explicitly defaulted on its first declaration. This means that it is not considered "user-provided." Consequently, the C class has no user-provided constructors, which makes it an aggregate according to the definition in 8.5.1/1:

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).

As a result, brace-enclosed initialization is permitted for the C class, allowing for default construction even though the constructor is declared as private. This behavior can be surprising and could lead to undesired consequences in certain scenarios.

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