CODEWITHSUNDEEP
c++c
Metalhead
Metalhead

Reputation: 91

How do I have to read the following statement?

I liked to refresh my C++ skills and tried to programm a little Object, so I came about this expression:

int (*const vectors)[2];

How do I read it? I know that it is declaring a constant pointer pointing to a two dimensional int array.

Thank you!

Upvotes: 2

Views: 144

Answers (2)

chqrlie
chqrlie

Reputation: 144520

You should use the spiral rule to parse int (*const vectors)[2];:

  • start from the identifier vectors
  • handle the unary operators on its right, from left to right, there are none before the ')`.
  • handle the qualifiers and unary operators on its left, from right to left:
    • const means vectors is constant, you cannot modify its value.
    • * means pointer: vector is a constant pointer
  • skip the parentheses on both sides and start again:
    • [2] means array of 2. vector is a constant pointer to one or more arrays of 2
    • finally: int gives the inner element type.

vector is a constant pointer to one or more arrays of 2 int.

Hence vector can be made to point to an array of arrays of 2 int. For example you can use vector to manipulate a 2D matrix this way:

// allocate a 2D identity matrix:
int (*const vectors)[2] = malloc(sizeof(int[2][2]);
vectors[0][0] = vectors[1][1] = 1;
vectors[1][0] = vectors[0][1] = 0;

Note however that vectors must be initialized, not assigned because it is defined as const. If you intend for vectors to point to a 2D matrix that should not be modified, for example as a function argument, the declaration should be:

void print_matrix(const int (*vectors)[2]);

Or

void print_matrix(int const (*vectors)[2]);

Finally, there are subtile differences for the meaning of const in C and C++, but the parsing method describe above applies to both languages.

Upvotes: 4

drRobertz
drRobertz

Reputation: 3638

In a declaration, both prefix * and postfix [] are operators that modify an identifier (or declarator).

The thing is that [] has higher precedence than *, so if you left the parens out, you would declare vectors as an array size 2 of int * const.

The parens are needed to make vectors a const pointer to int [2].

Upvotes: 0

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