Why Doesn\'t std::array Have a Constructor for Value Initialization?

The Constructor Gap in std::array

The lack of a constructor that populates an array with a specific value in the C Standard Template Library (STL) has raised eyebrows. std::array, unlike its dynamic counterpart std::vector, does not provide such a constructor. The omission has left developers questioning whether it's an oversight or intentional.

Argument for Inclusion

Proponents of including a constructor that initializes the array elements argue for its convenience. It would allow for quick and easy initialization, similar to how dynamic containers like std::vector have a constructor that accepts an initial value. This feature would eliminate the need for manual initialization or using std::array::fill(const T& value) after default construction, which has potential limitations.

Design Rationale

However, the absence of this constructor stems from std::array's design as an aggregate. Aggregates in C are types that do not have user-defined constructors, ensuring their behavior is consistent with C-style arrays. This design choice simplifies memory management and allows std::array to be used as a drop-in replacement for raw arrays.

Alternative Solution

While a constructor with value initialization is not available, developers can alternatively use default construction followed by std::array::fill. Default construction leaves the array elements uninitialized (for trivially initializable types), providing a clean slate for the subsequent fill operation. This approach ensures that the array is filled with the desired value, even with trivially constructed elements.

Conclusion

The absence of a constructor that takes a value to initialize an std::array is intentional, driven by the aggregate nature of the class. While dynamic containers may provide this functionality, std::array's aggregate design emphasizes memory efficiency and compatibility with C-style arrays. Developers can use std::array::fill to initialize the array elements with the desired value after default construction, achieving the same effect as a value-initializing constructor.

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