How Do I Initialize a 2D `std::array` in C ?

Initialization of 2D std::array

Despite its appearance, a 2D std::array in C is not initialized in the same way as a 2D C array. The fundamental reason for this is that a std::array is a class, not an array.

To initialize a 2D std::array, you must use both class braces and member array braces:

std::array<std::array<int, 3>, 2> a2 { { { {1, 2, 3} }, { { 4, 5, 6} } } };

The outer braces {} initialize the class itself, while the inner braces {{}} initialize the member C array contained within the class.

Comparison to C array initialization

To clarify the difference, here's a comparison to C array initialization:

struct B {
  int array[3];
};
struct A {
  B array[2];
};

B b = {{1,2,3}};
A a = {{
     {{1,2,3}},
     {{4,5,6}}
}};

In C, the outer braces {} initialize the struct itself, while the inner braces {{}} initialize the member array within the struct. This syntax is analogous to the initialization of a 2D std::array in C , except that in the latter case, the std::array class requires additional outer braces to initialize the class itself.

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