Why do I get error “declaration as array of references” when I use the reference operator without parentheses in a 2D array parameter?

Question

I understand C++ passes arrays by reference even if we don't use the reference operator (&), but since it can be added with no harm (I think), I'm curious as to why this code throws

declaration of 'matrix' as array of references

void function (int &matrix[2][5])  {
    //something
}

int main()  {
    int matrix[2][3] = {{1,2,3}, {4,5,6}};
    function(matrix);
}

while adding parentheses in (&matrix) works:

void function (int (&matrix)[2][5])  {
    //something
}

Answer 1

&matrix[2][5] has a different meaning from (&matrix)[2][5]) .

The former means matrix is a two dimensional array of references to int and the latter means matrix is a reference to a two dimensional array of integers .

Since matrix is defined as a two dimensional array in main , the second form succeeds.

The cdecl tool can be helpful here:

int (&matrix)[2][5] - declare matrix as reference to array 2 of array 5 of int
int &matrix[2][5] - declare matrix as array 2 of array 5 of reference to int

Answer 2

I understand C++ passes arrays by reference even if we don't use the reference operator (&)

This is incorrect. C++ doesn't pass arrays as references. Additionally, there is no such thing as a "reference operator".

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I'm curious as to why this code throws

The code is not "throwing", that term is used in the context of exceptions. You are simply getting a compilation error, as you're attempting to define an array of references to integers.

This happens because

int &matrix[2][5]

is grouped as

int &((matrix[2])[5])

by default. Adding parenthesis makes the compiler parse your type as "reference to an array of integers".