Why Do I Get Compilation Errors When Initializing a std::array with an Initializer List?

Using std::array with Initialization Lists: Resolving the Syntax Mystery

In the realm of C data structures, std::array stands out with its fixed-size memory allocation. While its versatility allows for initialization using initializer lists, some unexpected challenges may arise.

The Syntax Enigma

According to the query, attempts to initialize a std::array using initializer lists, as illustrated below, result in compilation errors:

<code class="cpp">std::array<std::string, 2> strings = { "a", "b" };
std::array<std::string, 2> strings({ "a", "b" });</code>

However, initialization lists function seamlessly with std::vector. This disparity raises the question: is it a misunderstanding about std::array's capabilities or a defect in the GNU Standard C Library implementation?

Unveiling the Solution

Behind the scenes, std::array is constructed as a struct:

<code class="cpp">template<typename T, int size>
struct std::array
{
  T a[size];
};</code>

This structure encapsulates an array, but curiously, it lacks a constructor that accepts an initializer list. Nonetheless, since std::array qualifies as an aggregate in C 11, aggregate initialization becomes an alternative approach.

To accomplish aggregate initialization, an extra set of curly braces is required to target the array within the struct:

<code class="cpp">std::array<std::string, 2> strings = {{ "a", "b" }};</code>

Compiler Anomalies

The C standard suggests that the additional curly braces in the example above are dispensable. However, the compiler error encountered suggests a potential bug in the GCC implementation, which fails to recognize the aggregate initialization.

Conclusion

While this issue may seem perplexing at first glance, the key lies in understanding the underlying structure of std::array and the intricacies of aggregate initialization. The double curly brace syntax resolves the compilation errors and allows for the creation of std::arrays using initializer lists, as intended by the C standard.

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