4.7. Delegates

C permits the definition of function pointers, which are pointers to code matching a certain signature that may be executed. The major problem with this is that it does not pass information from the caller, through the library, to the callback. In other languages, a closure is an encapsulation of code and state. C programmers sometimes emulate this behaviour by passing a void pointer of “user data” or “context” that acts as the state portion of the closure.

Vala supports both of these modes: a delegate may be targeted (i.e., a closure) or targetless (i.e., a function pointer). This is controlled by the has_target value, which defaults to true. The position of the target is assumed to be the last value in the argument list, which is typically where most C programs put it, though they occasionally place it first.

typedef int(*compute_func)(int a, int b);
typedef double(*analyze_func)(int a, int b, void *userdata);

[CCode (cname = "compute_func", has_target = false)]
public delegate int ComputeFunc (int a, int b);
[CCode (cname = "analyze_func")]
public delegate double AnalyzeFunc (int a, int b);

If the position of the context is not the last parameter, set the CCode attribute, delegate_target_pos, as per Changing the Position of Parameters.

It is common for C programmers not to create a typedef for a function pointer, instead opting to include it directly. Create a delegate and do not set the cname. If possible, contribute a patch to the library to create a typedef.