Why Does a Private Default Constructor Prevent Object Creation in C But Braced Initialization Works?

When is a Private Constructor Not a Private Constructor?

In C , making a constructor private seems like a straightforward way to prevent object creation outside the class. However, unexpected behavior arises when a default constructor is declared private.

Consider the following code:

class C {
    C() = default;
};

int main() {
    C c; // Error: Private constructor
    auto c2 = C(); // Error: Private constructor
}

Surprisingly, this code fails to compile due to the private default constructor. However, the following code:

class C {
    C() = default;
};

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

compiles successfully.

The reason for this peculiar behavior lies in the C standard. According to 8.4.2/5 [dcl.fct.def.default], a function is not user-provided if it is explicitly defaulted on its first declaration. Therefore, in our initial example, the default constructor is not user-provided.

This lack of user-provided constructors makes the class C an aggregate as per 8.5.1/1 [dcl.init.aggr], which:

• Has no user-provided constructors
• Has no private or protected non-static data members
• Has no base classes
• Has no virtual functions

For aggregates, the braced-init syntax is considered a constructor call and not a declaration, which is why it succeeds in the latter code snippet.

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