std::array Initialization with Braces

In C , there are two common ways to create a std::array using initialization lists:

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

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However, if you encounter a compilation error about "expected primary-expression before ',' token" with GCC 4.6.1, it is due to a slight peculiarity in std::array.

Unlike std::vector, which has a constructor that explicitly takes an initializer list, std::array is defined as a struct:

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

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As such, it does not have a constructor that directly accepts an initializer list. Instead, it can be initialized using aggregate initialization.

To correctly aggregate initialize an array inside the std::array struct, an extra set of curly braces is required:

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

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It is worth noting that the C standard suggests that the extra braces should be optional in this scenario. Therefore, the compilation error you experienced with GCC 4.6.1 is likely a bug in the compiler.

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