c - Why can't I omit the dimensions altogether when initializing a multi-dimensional array?

Question

In Visual Studio 2010, this initialization works as expected:

char table[2][2] = {
                       {'a', 'b'},
                       {'c', 'd'}
                   };

But it does not seem legal to write something like:

char table[][] = {
                     {'a', 'b'},
                     {'c', 'd'}
                 };

Visual Studio complains that this array may not contain elements of 'that' type, and after compiling, VS reports two errors: a missing index and too many initializations.

QUESTION: Why can't I omit the dimensions altogether when initializing a multi-dimensional array?

Answer 1

Only the innermost dimension can be omitted. The size of elements in an array are deduced for the type given to the array variable. The type of elements must therefore have a known size.

• char a[]; has elements (e.g. a[0]) of size 1 (8bit), and has an unknown size.
• char a[6]; has elements of size 1, and has size 6.
• char a[][6]; has elements (e.g. a[0], which is an array) of size 6, and has an unknown size.
• char a[10][6]; has elements of size 6. and has size 60.

Not allowed:

• char a[10][]; would have 10 elements of unknown size.
• char a[][]; would have an unknown number of elements of unknown size.

The size of elements is mandatory, the compiler needs it to access elements (through pointer arithmetic).