Does C Implicitly Initialize Built-in Types in Default Constructors?

Does Implicit Default Constructor Initialize Built-in Types?

While compiler-generated default constructors are responsible for initializing members of classes, this rule does not apply to built-in types. Implicit default constructors leave built-in type members uninitialized.

However, there are alternative mechanisms for initializing class instances.

Value Initialization

The syntax C() might appear to invoke the default constructor, but in reality, it performs value initialization, which:

• Only triggers the user-declared default constructor if present.
• For classes without user-declared constructors, it skips the default constructor.
• Direct value-initialization of built-in types results in zero-initialization.

Example:

class C { public: int x; };
C c; // Compiler-generated default constructor used, x retains garbage

Explicit Initialization

The explicit () initializer explicitly triggers value initialization for both built-in types and user-declared constructors.

C c = C(); // Value initialization used, x is zero-initialized
C *pc = new C(); // Value initialization used, pc->x is zero-initialized

Aggregate Initialization

Aggregate initialization also initializes class instances without involving constructors.

C c = {}; // x is zero-initialized
C d{}; // C++11 aggregate initialization, x is zero-initialized

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