Do C 's Default Constructors Initialize Built-in Types?

Do Built-in Types Get Initialized by C 's Default Constructor?

In C , the implicit default constructor generated by the compiler is responsible for initializing class members. However, there's a common misconception that this behavior extends to built-in types.

Default Initialization Behavior

The implicit default constructor does not initialize members of built-in types. When such a constructor is used without user-defined initialization, the members of built-in types remain uninitialized.

Exceptions to the Rule

While default constructors typically do not initialize built-in types, there are certain circumstances under which initialization may occur:

Value Initialization:
In C 03 and later, the syntax C() invokes value-initialization for class instances. If the class has no user-declared constructor, value-initialization will zero-initialize built-in type members.

Aggregate Initialization:
Aggregate initialization syntax, such as C c = {}, performs initialization without using a constructor. This results in zero-initialization of built-in type members.

Example:

Consider the following class without a user-declared constructor:

class C { 
public:
  int x;
};

Without Explicit Initialization:

C c; // Compiler-provided default constructor used
// c.x contains garbage

With Explicit Value Initialization:

C c = C(); // Value-initialization used
assert(c.x == 0);

With Aggregate Initialization:

C c = {}; // Aggregate initialization
assert(c.x == 0);

It's important to note that the behavior described above may vary depending on the version of C used and the specific class definition.

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