Why does forward declaration of struct not work?
c, forward-declaration, struct
Solution
forward declaration only informs the compiler that there is something that is called `foo` it does nothing says about size. you can use `foo*` since this is a pointer of known size but not `foo` itself because the size is unknwon, so the compiler does not know how the memory layout of `bar`should look like.
And the compiler only do a single pass through your document. so it cannot know the strucutre that is defined ahead.
Problem
I wrote a small code in C in which two struct types were defined which have members of each other in their definition. Case 1: If the struct foo is defined before struct bar, the code is compiled as expected. Case 2: If struct foo is defined after struct bar it will not compile which is also expected as there is no way to know the memory requirement of a struct foo variable. But I was expecting it will compile if a forward declaration of struct foo is used in case 2. But it does not work. What am I missing? ``` #include<stdio.h> #include<stdlib.h> struct foo; // forward declaration struct bar { int a; struct bar *next; struct foo ch; }; struct foo { struct bar *n; struct foo *nx; }; int main() { struct bar p; return(0); } ```