C: "array type has incomplete element type" when using array of struct without typedef

Question

Problem: The following code snippet compiles well (where both struct types are typedefed):

typedef struct {
    int a;
    float b;
} member_struct;

typedef struct {
    int a;
    double b;
    member_struct c;
} outside_struct;

outside_struct my_struct_array[4];

However, if the typedef of the "outside_struct" is dropped:

typedef struct {
    int a;
    float b;
} member_struct;

struct {
    int a;
    double b;
    member_struct c;
} outside_struct;

struct outside_struct my_struct_array[4];

I get the error: "array type has incomplete element type 'struct outside_struct'". And if I also drop the typedef of "member_struct", I get an extra error: "field 'c' has incomplete type"

Question: Why it happens? Is using typedef strictly necessary here? In my code, I otherwise never use typedef for structure types, so I am looking for a way to avoid that, if possible.

There is no type called struct outside_struct in the second snippet. You have one instance of an anonymous struct called outside_struct - it is not a type — UnholySheep
– UnholySheep, Commented Nov 23, 2021 at 13:44

Vlad from Moscow · Accepted Answer · 2021-11-23 13:54:18Z

3

In this declaration

struct {
    int a;
    double b;
    member_struct c;
} outside_struct;

there is declared the object outside_struct of unnamed structure type. Neither structure with the name struct outside_struct is declared.

So the compiler issues an error in this declaration of an array

struct outside_struct my_struct_array[4];

because in this declaration there is introduced the type specifier struct outside_struct that is not defined. That is in this declaration the type specifier struct outside_struct is an incomplete type.

You may not declare an array with an incomplete element type.

Instead of declaring the object outside_struct of an unnamed structure you need to declare a structure with the same tag name as

struct  outside_struct {
    int a;
    double b;
    member_struct c;
};

answered Nov 23, 2021 at 13:54

Vlad from Moscow

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2 Comments

kmalarski Over a year ago

Thanks! I got confused by the compiler error, as if I had not specified the internal fields of the struct.

joH1 Over a year ago

@kmalarski I guess the array definition also acts as a declaration for struct outside_struct (which does not collide with the struct instance outside_struct because of the struct keyword) and in that sense, the compiler error is accurate, however without knowing that, it can be confusing indeed!

Lundin · Accepted Answer · 2021-11-23 13:45:29Z

2

If you drop the typedef, you need to add a struct tag instead: struct outside_struct { ... };

answered Nov 23, 2021 at 13:45

Lundin

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Comments

nobbody · Accepted Answer · 2021-11-23 14:10:07Z

0

Typedef is used to create an additional name (alias) for another data type.

typedef int myInt; //=>equivalent to "int"
myInt index = 0; //=>equivalent to "int index = 0;"

It's the same logic for struct.

typedef struct myStruct {} myStruct_t; //=> equivalent to "struct myStruct {};"
myStruct_t myStructVariable; //=> equivalent to "struct myStruct myStructVariable;"

syntaxe = "typedef type newAlias;"

"myStruct{}" is a new type that contain all the type that you want (int, char...)

edited Nov 23, 2021 at 14:10

answered Nov 23, 2021 at 14:05

nobbody

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