Why Does std::array Lack a Value-Filling Constructor?

Lack of Value-Filling Constructor in std::array: An Oversight or Intentional Design?

Despite the existence of a fill method in std::array that allows for assigning a single value to all elements, the absence of a constructor that takes a value as an argument has raised questions among developers. While dynamic containers like std::vector offer such a constructor, why does std::array lack this feature?

Behind the Design Decision

std::array is designed as an aggregate, meaning that it does not have any user-declared constructors. Aggregates are data structures that consist solely of members with no user-defined constructors and no base classes. Their construction is handled automatically by the compiler.

Default Construction and Aggregate Initialization

Default construction of an aggregate type, like std::array, results in uninitialized memory. Unlike with classes, default construction does not invoke member-wise initialization. If the type is trivially initializable, the memory will have indeterminate values.

Alternative Approaches

To fill an std::array with a specific value, the fill method can be employed after the array has been default-constructed. Default construction initializes the array with uninitialized values, rather than zeroing it out as in normal construction. This allows for filling with non-zero values like the example of initializing all elements with -1.

Conclusion

The absence of a value-filling constructor in std::array is not an oversight but an intentional design decision. std::array's status as an aggregate prevents user-declared constructors, but default construction and subsequent use of the fill method provide alternative ways to initialize an array with a single value.

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