Why cannot I declare class outside of class in Vala (error: redefinition of struct)? - vala

Question

The example below, test4.vala, compiles and runs:
//// compile with: valac test4.vala
//~ public class TestClass : GLib.Object { // error: redefinition of ‘struct _TestClass’
//~ public int x = 0;
//~ public int y = 0;
//~ public int z = 0;
//~ }
public Test App;
public class Test : GLib.Object {
public class TestClass : GLib.Object { //current
public int x = 0; //current
public int y = 0; //current
public int z = 0; //current
} //current
public TestClass mytc;
public void SetVars() {
mytc = new TestClass();
stdout.printf("SetVars called, %p\n", mytc);
}
public Test(string[] args){
stdout.printf("Test() ctor: ok\n");
stdout.flush();
}
public static int main (string[] args) {
App = new Test(args);
App.SetVars();
stdout.printf("main called\n");
return 0;
}
}
However, if I comment the lines marked "current", and uncomment the commented code, I get this error:
$ valac test4.vala && ./test4
/tmp/test4.vala.c:64:8: error: redefinition of ‘struct _TestClass’
struct _TestClass {
^~~~~~~~~~
/tmp/test4.vala.c:20:16: note: originally defined here
typedef struct _TestClass TestClass;
^~~~~~~~~~
error: cc exited with status 256
Compilation failed: 1 error(s), 0 warning(s)
I still trying to grok Vala, but this kinda leaves me puzzled - why cannot I compile an additional class outside the one carrying main on a same level with it - but I have to instead "include" this other class in the main app class?

Answer 1

It has to do with how GObject works and its naming conventions. The GObject manual has more details, so I'm not going to go into much depth here…
When you create an object, let's call it Foo, in Vala, in the generated C two structs will be created: Foo and FooClass. The former is what people will mostly use in the API and represents an instance of Foo, whereas the latter is used to hold information about the Foo class itself; virtual function pointers are the big thing.
So, with your above code, the generated code will contain Test and TestClass for the outer class, and TestTestClass and TestTestClassClass for the inner class. Once you uncomment the rest of the code, it will try to generate TestClass and TestClassClass, the former of which will conflict with the *Class struct for the outer class that already existed.
You could reproduce the issue a bit more easily with just:
public class Test : GLib.Object { }
public class TestClass : GLib.Object { }
Basically, don't call a class *Class.