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Talk:Function pointer Source: en.wikipedia.org/wiki/Talk:Function_pointer

Method Pointers

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this is a pointer to object. To access its member's data, one needs to dereference this first and then to dereference that member if it is a pointer. Pointer-to-member operators are not related to this. Citing cppreference: When a non-static class member is used in any of the contexts where the this keyword is allowed (non-static member function bodies, member initializer lists, default member initializers), the implicit this-> is automatically added before the name, resulting in a member access expression (which, if the member is a virtual member function, results in a virtual function call). — Preceding unsigned comment added by 5.20.191.37 (talk) 21:12, 27 December 2022 (UTC)[reply]

Type safety in older languages?

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Function pointers in C and C++ are both type-safe. How are they any less type-safe than, say delegates in C#? I keep hearing this untrue statement. — Preceding unsigned comment added by 130.76.96.111 (talk) 19:13, 6 September 2013 (UTC)[reply]

Object-oriented equivalent?

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Perhaps there could be something here showing how the same thing can be achieved using Object Oriented Programming? Ubermonkey 20:35, 2 Apr 2005 (UTC)

Also a nice implementation of this using a class where the method is passed as a function pointer (so a method pointer i guess). I've been trying to find an example of this everywhere and i can't find it. -Sharth --128.175.13.43 17:54, 26 October 2005 (UTC)[reply]

It would also be nice if the program actually worked.. say.. if it had a definition for the "list" or "listnode" types. Fresheneesz 07:56, 5 June 2006 (UTC)[reply]

One way to do the equivalent of function pointers in Java is to define an interface with a single method. Then one or more classes can implement the interface, and objects of these class types can be passed to other methods. These interface references act something like method pointers:
public interface Foo
{
    public abstract int perf(int a);
}

class MyFoo implements Foo
{
    public int perf(int a)
    {
        return (2*a + 3);
    }
}

public class Test
{
    static Foo  s_func;

    public static void main(String[] args)
    {
        int x;

        setFunc(new MyFoo());
        x = callFunc(10);
    }

    void setFunc(Foo f)
    {
        s_func = f;
    }

    int callFunc(int a)
    {
        return s_func.perf(a);
    }
}
— Loadmaster 01:00, 9 March 2007 (UTC)[reply]

Not using function pointers

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"Imagine rewriting the above code without function pointers. The fsum() and fproduct() function would both require loops iterating through the linked list." Isn't it contradicting to say that when the code does exactly that--or am I missing something? Guardian653 03:55, 2 December 2006 (UTC)[reply]

Agreed. Seems like the author is not NPOV since he calls it a "clearly" "powerful" mechanism. Using it to obfuscate code is arguably not the best application of this feature. 24.6.99.30 12:50, 3 January 2007 (UTC)[reply]

Procedural parameter

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Procedural parameter seems to have a lot in common with this article. If someone agrees, should these two articles mention each other, or maybe even be merged? --Abdull (talk) 11:58, 26 July 2008 (UTC)[reply]

Java

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Java's lack of function pointers should be noted/explained. Fig (talk) 14:01, 30 September 2013 (UTC)[reply]

Technically, any language supporting abstract interfaces has the same capabilities as function pointers. Using an interface containing a single method effectively makes that a typed function pointer. It's not an exact capability, though, because you can't pass just any (type-matching) method as a parameter to a method expecting a method pointer, or assign it to a method pointer; any such method you use as a method pointer must be defined as implementing the method of the abstract interface, and an actual object must be associated with the method being referenced. But the ability to pass or store a reference to a properly defined method (which is just a function pointer deep down) still exists in these languages. This might be one reason that delegates were never added to Java by its designers, since some programmers consider them unnecessary syntactic sugar for an already-existing feature. — Loadmaster (talk) 17:55, 30 September 2013 (UTC)[reply]
funtion of text select 154.160.5.65 (talk) 21:53, 27 February 2024 (UTC)[reply]
I do remember figuring this out some years ago. C has qsort() that takes a function pointer to a comparison function. And Java, as noted, uses an interface to do the same thing, such as for sorting methods. Java has the Comparable interface, which is fine if a class needs one way to compare. But there is also Comparator for more complicated cases. I do find the asymmetry of Comparable funny, though. Gah4 (talk) 07:51, 4 April 2025 (UTC)[reply]
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simple

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The article mentions simple function pointers containing just the address, and also PL/I. There is a complication in PL/I due to internal procedures, and the context. That is, which level of nesting of the variables are seen when the ENTRY variable is used. I believe this complication delayed internal procedures, and the ability to call them, in Fortran for many years. Last I knew, Fortran only allowed one nesting level for internal procedures. (Might have changed.) Do other languages have this complication? Gah4 (talk) 18:34, 3 April 2025 (UTC)[reply]

context

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As noted here, or in the article, languages like PL/I need context. A recent edit indicates that context supplies in internal procedure with access to the current values of variables in enclosing procedures. I believe, though, that sometimes the are not the current values, and that is the complication. They should be the values that were current when the ENTRY variable was given a value. Depending on what one does with recursion, that may have changed in the mean time. Fortran had the ability to pass a function or subroutine name as an actual argument to another routine, since close to the beginning, and in the Fortran 66 standard. Not actually variables, though. But, as implemented no context, as there was no recursion and no internal procedures. When they did add internal procedures, they only allow one nesting depth. And did not allow them as actual arguments. I think that has changed since, but it took a long time. And the need for context was (one of) the complications. Gah4 (talk) 07:33, 5 April 2025 (UTC)[reply]

Thinking makes my head hurt. I changed the text per this comment, but actually, without multitasking/multithreading the values of A’s data seen by B will be the “current” values, because the call tree is static. When B is called, for example, it gets access to the “current” values of variables from A, and these can’t be changed by A until B returns. If we’re multitasking, A can invoke multiple instances if B as separate tasks, and these access the data in A that can change unpredictably while the Bs are running. Peter Flass (talk) 03:15, 6 April 2025 (UTC)[reply]
You can do it with recursion. If a procedure has two internal procedures that reference variables in the containing procedure, you can call one to change some variable, and then use a previous ENTRY variable to the other one. I actually haven't thought about this for a long time, but I used to sometimes read the PLM for PL/I (F), which describes much of this. Should probably look it up before we get a {{cn}} for it. As you know, PL/I was mostly defined before the first compiler was written. They did things that made sense to the user, not necessarily easier to implement. One that I remember, is that procedures with ENTRY statements have to be able to convert the type in RETURN statements to the type of the ENTRY in use. Fortran, instead, EQUIVALENCEs all the return values. Could easily return the wrong type. Gah4 (talk) 06:33, 6 April 2025 (UTC)[reply]