What brackets in array input mean

Why in some declarations of dynamic arrays we need to put array name in brackets:

(*allocMat)[count++] = row;

3 answers

  • answered 2018-11-08 06:57 Sourav Ghosh

    This is because of operator precedence. The array subscript operator [] has higher priority than the unary dereference operator *. So, unless the parenthesis is used, a statement like

    *allocMat[count++] = row;

    will be parsed as

    * (allocMat[count++]) = row;

    which is not desired.

    To properly evaluate the statement, we need to first dereference the pointer, and then, index onto it, like

    (*allocMat)[count++] = row;

    In the above snippet, allocMat is a pointer to an array. So, unless, the dereference is forced on higher priority, the subscripting operator [], which has higher priority, will be taken into account first and will result in incorrect evaluation.

  • answered 2018-11-08 06:57 Antti Haapala

    Allocmat is presumably a pointer to an array.

    The parentheses are needed to get the indirection correctly. So (*allocMat)[count++] is the same as allocMat[0][count++]. Would you omit the parentheses, *allocMat[count++] would be equal to allocMat[count++][0] which is completely different. This is because operator precedence - [] binds slightly tighter than *.

  • answered 2018-11-08 07:00 4386427

    It's about operator precedence, i.e. which part of the statement is executed first.

    Like in simple math. Is x = a + b*c executed like x = (a + b)*c or like x = a + (b*c)?

    So for your code the question is: Is * "stronger" than [] or is it the opposite?

    Consider just doing:

    *allocMat[count++] = row;

    How would you expect that to be executed?

    Like A:

    (*allocMat)[count++] = row;

    or like B:

    *(allocMat[count++]) = row;

    The answer is that it's executed like B so if you really want A then you need to explicit add the parenthesis.

    An example where you would want A is when allocMat is a pointer to an array.

    An example where you would want B is when allocMat is an array of pointers.