Everything I wish I knew when learning C

Re:Everything I wish I knew when learning C by mytherin on Monday November 28, 2022 @12:54PM (#33776898)

Perhaps I am missing something in the spec - but trying this in various compilers, it seems that you *can* assign structs holding arrays to one another, but you *cannot* assign arrays themselves.

This compiles:

  struct BigStruct {
    int my_array[4];
  };
  int main() {
    struct BigStruct a;
    struct BigStruct b;
    b = a;
  }

But this does not:

  int main() {
    int a[4];
    int b[4];
    b = a;
  }

That seems like an arbitrary restriction to me.

[ Parent | on HN ]

Re:Everything I wish I knew when learning C by poepie on Monday November 28, 2022 @01:29PM (#33777468)

In the first example a & b are variables, which can be assigned to each other. In the second a & b are pointers, but b is fixed, so you can not assign a value to it.

[ Parent | on HN ]

Re:Everything I wish I knew when learning C by yakubin on Monday November 28, 2022 @07:40PM (#33782085)

They’re not pointers. sizeof a == 4*sizeof(int), not sizeof(int*).

[ Parent | on HN ]

Re:Everything I wish I knew when learning C by chipsa on Tuesday November 29, 2022 @09:48AM (#33788237)

They're pointers, just weird ones. The compiler knows it's an array, so it gives the result of the actual amount of space it takes up. If you passed it into a function, and used the sizeof operator in the function, it'd give `sizeof(int *)`. Because sizeof is a compile-time operation, so the compiler still knows that info for your example.

[ Parent | on HN ]

Re:Everything I wish I knew when learning C by yakubin on Tuesday November 29, 2022 @11:17AM (#33789455)

That jest means it decays into a pointer after being passed as a function argument. In the example given however it’s not a pointer. Just like it wouldn’t be inside a struct.

[ Parent | on HN ]

Re:Everything I wish I knew when learning C by fsociety on Monday November 28, 2022 @05:43PM (#33780954)

Essentially ‘b = a’ in the second example is equivalent to ‘b = &a[0]’ or assigning an array to a pointer.

This is because if you use an array in an expression, it’s value is (most of the time) a pointer to the array’s first element. But the left element is not an expression, therefore it is referring to b the array.

Example one works because no arrays are referred to in the expression side, so this shorthand so to speak is avoided.

Arrays can be a painful edge in C, for example variable length arrays are hair pulling.

[ Parent | on HN ]

Re:Everything I wish I knew when learning C by int_19h on Friday December 02, 2022 @02:17PM (#33835365)

The left side of assignment in C is an expression. it's just not in a context where array-to-pointer decay is triggered.

[ Parent | on HN ]