Xcode C++ Struct Order

Question

The code is given below is not allowed to compile because of struct's order. song_node struct has playlist variable and playlist struct has song_node variable.

This code is runned on Visual Studio or gcc compile.

struct song_node {
    song* data;
    song_node* next;
    song_node* prev;
    playlist* parent;
};

struct playlist {
    int songnumber;
    char* name = new char[LNAME_LENGTH];
    song_node* head;
    playlist* next;
    playlist* prev;
};

I am new on Xcode. What is the problem in this code?

It's enough to add a `struct playlist;` forward declaration before `song_node`. — Bo Persson, Nov 18 '16 at 11:58

score 1 · Accepted Answer · answered Nov 18 '16 at 11:59

1

Consider a forward declaration for the class playlist:

struct playlist; //Here

struct song_node {
    song* data;
    song_node* next;
    song_node* prev;
    playlist* parent;
};

struct playlist {
    int songnumber;
    char* name = new char[LNAME_LENGTH];
    song_node* head;
    playlist* next;
    playlist* prev;
};

Then the compiler knows about playlistand you can use it in song_node even though it's not implemented (yet).

answered Nov 18 '16 at 11:59

Stack Danny

7,754
2
26
55

Is there another way? It works so thank you but I just wonder why it is working on other platforms but here give the error? – tgbzkl Nov 18 '16 at 12:06
what other platforms are you talking about? – Stack Danny Nov 18 '16 at 12:08
Visual Studio or gcc compile. – tgbzkl Nov 18 '16 at 12:28

score 1 · Answer 2 · answered Nov 18 '16 at 12:00

You need to do a forward declaration.

struct playlist;

struct song_node {
    song* data;
    song_node* next;
    song_node* prev;
    playlist* parent;
};

struct playlist {
    int songnumber;
    char* name = new char[LNAME_LENGTH];
    song_node* head;
    playlist* next;
    playlist* prev;
};

The compiler needs to know that there is something like a playlist when you want to use it within song_node. By forward-declaring playlist you tell the compiler that such an object exists.

Xcode C++ Struct Order

2 Answers2