7.6. Parameters of Variable Type (Generics)

Vala’s generics can be applied to C functions using void pointers as generic value arguments. Memory management and generics tend not to get along well, so it may be beneficial to avoid this situation where possible. In particular, generic structs that own instances of the generic can behave strangely. Also, putting owned structs in a generic collection can break.

Before starting, determine the scope of the type variable: does it apply to the method or class? Generics are paired with a delegate. Bind the delegate as follows:

typedef int (*foo_func)(void *a, void *b, void* context);
[CCode (cname = "foo_func", simple_generics = true)]
public delegate int FooFunc<T> (T a, T b);

7.6.1. Generic Methods

Frequently, a single method is the context for the generic type variable. Simply apply simple_generics to the CCode attribute:

void sort(void **array, int array_length, foo_func compare, void *context);
[CCode (simple_generics = true)]
public void sort<T> (T[] array, FooFunc<T> compare);

Occasionally, this is not a C function, but a function-like macro that takes the type name (e.g., va_arg), in which case, set generic_type_pos to the position of the argument:

#define sort(array, type, compare, context) ...
[CCode (generic_type_pos = 1.1)]
public void sort<T> (T[] array, FooFunc<T> compare);

7.6.2. Generic Classes and Structs

If a data structure is container-like, then it may be possible to bind the structure using generics. However, Vala’s assumptions about generic structures are rather rigid, so this may be impossible.

  • Create a type variable over the class.

  • Decorate all methods that use the type variable with simple_generics.

  • Constructors for classes are expected to take the destructor as an argument if simple_generics is supplied. If the constructor takes no arguments, convert all constructors to static methods with simple_generics.

  • Verify all ownership. When Vala emits simple_generics code of an owned variable, it always passes the destructor. Frequently, C programs are written where the destructor is passed once in the constructor. In this case, set the destructor to be null, and insist that all values be unowned.

7.6.3. The User Pointer Case

Often, C libraries will have a pointer for some user data associated with an object that is left entirely in the hands of the user. This is easily bound.

typedef struct foo Foo;
void *foo_get_userptr(Foo*);
void foo_set_userptr(Foo*,void*);
public class Foo<T> {
    public unowned T? user_data {
        [CCode (cname = "foo_get_userptr", simple_generics = true)] get;
        [CCode (cname = "foo_set_userptr", simple_generics = true)] set;

The only caveat is this is rather infectious: the simple_generics attribute must be applied to all methods use of Foo in other contexts, including arrays of that object and other classes that contain that type. To avoid this, the alternate binding is:

public class Foo {
   [CCode (simple_generics = true)]
   public void set_user_ptr<T> (T value);
   [CCode (simple_generics = true)]
   public T get_user_ptr<T> ();

However, this scheme is less type-safe.