Change non-modifiability of an array by using a structure

Question

From Kerrek SB's comment on Why can't a modifiable lvalue have an array type?

You can trivially assign arrays by making them members of a struct and then assigning those lvalues.

What does it mean? Does it mean that if a structure has an array member, then the array member can be modifiable? But the following example doesn't work:

I define a struct type with a member being an array

typedef struct { int arr[3];} MyType;

Then

MyType myStruct;
myStruct.arr = (int[]) {3,2,1};

and got error: assignment to expression with array type.

For your code, you'd need to use: `MyType myStruct; myStruct.arr = (MyType) { { 3, 2, 1 } };` or `myStruct.arr = (MyType) { .arr = { 3, 2, 1 } };` to create a compound literal of the structure type, which can then be assigned to the structure as a whole. You still can't do array assignments, so any assignment to `myStruct.arr` is bogus. — Jonathan Leffler, Aug 14 '17 at 23:02

Iharob Al Asimi · Accepted Answer · 2017-08-15T04:46:48.177

4

No, it means that if you assign an instance of a struct like the one in your example to another struct you are effectively assigning arrays.

struct Array {
    int array[3];
};

struct Array A;
struct Array B = {{0, 1, 2}}; // by Chris Dodd
// This is valid!
A = B;

edited Aug 15 '17 at 04:46

answered Aug 14 '17 at 19:10

Iharob Al Asimi

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and you can initialize `B` with an array initializer declaration like: `struct Array B = {{ 0,1,2,3,4,5,6,7,8,9 }};` – Chris Dodd Aug 14 '17 at 19:14
@ChrisDodd I was to lazy to do so, so I will borrow yours if you don't mind. – Iharob Al Asimi Aug 14 '17 at 19:15
`A = B;` must be inside a function for it to be valid – M.M Aug 14 '17 at 21:24
2

There are sometimes advantages to sticking with the example from the question, where the array size is only 3 — it is simpler to create the compound literal. – Jonathan Leffler Aug 14 '17 at 23:03
@M.M. why has to be inside a function? – Tim Aug 14 '17 at 23:05
1

@Tim because assignments aren't valid at global scope. Only declarations are. – o11c Aug 14 '17 at 23:06

Change non-modifiability of an array by using a structure

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