Can I pass a function by parameter?

[Jakob Ovrum via Digitalmars-d]

On Monday, 8 September 2014 at 03:01:40 UTC, AsmMan wrote:
> I got it. Why it doesn't works if foo is a method?

Taking the address of a method/member function yields a delegate, 
not a function pointer. Delegates are fat pointers; they contain 
a pointer to the function as well as a pointer to the context 
(the context is a hidden argument to the function, such as the 
this-reference of a class method). The context pointer is needed 
when calling the function, so they are encapsulated in one 
convenient type, which is the delegate type.

Function pointers can be converted to delegates through a simple 
wrapper function (which is how std.functional.toDelegate achieves 
it), but delegates cannot be wrapped by a function pointer 
without introducing additional parameters to the function in 
order to pass the context pointer. Additionally, the *type* of 
the context pointer is erased, which is immensely handy as it 
means delegates with different contexts can be freely mixed, but 
it means there's no way to know the type of the context (such as 
a class type) just from the delegate, so there is no generic 
`toFunctionPointer` function that would roughly do the inverse of 
`toDelegate`. Each case has to be dealt with on a case-by-case 
basis.

As function pointers can be converted to delegates, delegates are 
the more versatile of the two. I recommend using delegates unless 
there is a good reason not to.