Declaring a struct before defining it

Hello.

How to define structures before using them in each other.

main.cpp:

Code:

---

#include <iostream>
using namespace std;

struct Struct2;
struct Struct1;

struct Struct2
{
    Struct1 a;
}

struct Struct1
{
    Struct2 b;
};

int main()
{
}

---

Compiling with command: g++ main.cpp
Errors:
main.cpp:10: error: field `a' has incomplete type
main.cpp:17: error: multiple types in one declaration


Thanks in advance.

for c use gcc and .c extention

C has no automatic typedef

Code:

---

struct Struct2;
struct Struct1;

struct Struct2
{
    struct Struct1* a;
}

---

you can define pointer to undefined struct
but if you want to add struct as object - it should be fully defined, otherwise compiler cannot know its size.

What you trying to do will require from the compiler to build endless loop of structs inside structs. It is impossible to do. This data strycture will require infinite memory size

Code:

---

#include <stdio.h>

struct Struct2;
struct Struct1;

struct Struct1
{
    Struct2 a;
}

struct Struct2
{
    int a;
}

int main()
{
}

---

... It stil lwon't compile. Even though it will not produce an infinite loop.

One of them has to be a pointer to a struct, not an instance of the struct.

Otherwise, the mutual recursion of the struct uses up all available memory!

You only need
struct foo;
in order to be able to declare a pointer to it.
At this stage, this is called an incomplete type.


But you need the whole thing in order to be able to create an instance of that struct.

Ok, thanks.