int a[2][2][2] = { { {5,8} },{ {9,12} }
                 };

如果您将其写为,这也将起作用

int a[2][2][2] = {5,6,7,8,9,10,11,12};

但这确实不是一个好习惯.

C标准说的
§6.7.9(P17)：

When no designations are present,subobjects of the current object are initialized in order according to the type of the current object: array elements in increasing subscript order,structure members in declaration order,and the first named member of a union¹⁴⁹⁾. […]

P26

EXAMPLE 3 The declaration

06002

is a definition with a fully bracketed initialization: 1,3,and 5 initialize the first row of y (the array object y[0]),namely y[0][0],y[0][1],and y[0][2]. Likewise the next two lines initialize y[1] and y[2]. The initializer ends early,so y[3] is initialized with zeros. Precisely the same effect could have been achieved by

06003

The initializer for y[0] does not begin with a left brace,so three items from the list are used. Likewise the next three are taken successively for y[1] and y[2].