Initializing Fields in Constructors: Differences Between Initializer List and Constructor Body

In C , constructors provide a convenient way to initialize instance fields during object creation. There are two primary methods for field initialization in constructors: the initializer list and the constructor body.

Initializer List

Thing(int _foo, int _bar): member1(_foo), member2(_bar) {}

The initializer list immediately follows the constructor parameter list and allows direct initialization of fields before the constructor body executes. This method is commonly preferred due to its concise syntax and clarity.

Constructor Body

Thing(int _foo, int _bar) {
    member1 = _foo;
    member2 = _bar;
}

The constructor body uses assignment statements to initialize fields within the function body. This method is less common, as it requires more lines of code and can be prone to errors if the assignment statements are not executed in the desired order.

Key Differences

• Default Initialization: When using the initializer list, non-POD fields that lack default constructors will not be initialized, potentially leading to undefined behavior. In contrast, the constructor body initializes fields to their default values if no explicit assignment is provided.
• Initialization Order: The initializer list initializes fields before the constructor body executes. This ensures that fields are initialized in the correct order, even if the order of the assignment statements in the constructor body changes.
• Error Handling: Using the initializer list helps to prevent errors caused by incorrect assignment statements. For example, if a field is accidentally initialized twice in the constructor body, the initializer list will raise an error.
• Performance: The initializer list is generally more efficient as it directly initializes fields without the overhead of executing the constructor body.

Conclusion

While both the initializer list and constructor body can initialize fields in C constructors, the initializer list is generally preferred due to its clarity, safety, and performance benefits. It ensures that fields are initialized in the correct order and prevents potential errors in the constructor body.