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digitalmars.D - member pointers

reply bc <mi_emayl_adrez hotmail.com> writes:
am i being stupid or is there any chance of adding pointers to member  
functions and pointers to data members to D? i can't see any way of  
getting the same functionality through existing language constructs for  
member functions... alias seems like it might at first but i can't get it  
to work... and the offsetof syntax for data members seems buggy.  
otherwise, congrats on designing a really cool language
Mar 12 2007
parent reply Daniel Keep <daniel.keep.lists gmail.com> writes:
bc wrote:
 
 am i being stupid or is there any chance of adding pointers to member
 functions and pointers to data members to D? i can't see any way of
 getting the same functionality through existing language constructs for
 member functions... alias seems like it might at first but i can't get
 it to work... and the offsetof syntax for data members seems buggy.
 otherwise, congrats on designing a really cool language

Let's see...
 import std.stdio;

 class Foo
 {
     uint xzzxy = 0;

     void bar()
     {
         writefln("Bar, bar, member func, have you any Foo?");
         writefln("The magic number is: %d", this.xzzxy);
     }
 }

 alias void delegate() voiddg; // <-- just for shorthand

 void main()
 {
     Foo baz = new Foo;

     uint* quxxy = &baz.xzzxy; // <-- pointer to an attribute
     *quxxy = 42;

     voiddg dg = &baz.bar; // <-- "pointer" to our member function
     dg();
 }

Delegates are your pointer to member functions. Pointers to data of an object are just as you would expect. Or did you have a different use in mind? -- Daniel -- Unlike Knuth, I have neither proven or tried the above; it may not even make sense. v2sw5+8Yhw5ln4+5pr6OFPma8u6+7Lw4Tm6+7l6+7D i28a2Xs3MSr2e4/6+7t4TNSMb6HTOp5en5g6RAHCP http://hackerkey.com/
Mar 12 2007
parent reply Frits van Bommel <fvbommel REMwOVExCAPSs.nl> writes:
Daniel Keep wrote:
 bc wrote:
 am i being stupid or is there any chance of adding pointers to member
 functions and pointers to data members to D? i can't see any way of
 getting the same functionality through existing language constructs for
 member functions... alias seems like it might at first but i can't get
 it to work... and the offsetof syntax for data members seems buggy.
 otherwise, congrats on designing a really cool language


 
 Delegates are your pointer to member functions.  Pointers to data of an
 object are just as you would expect.  Or did you have a different use in
 mind?

In C++, a "pointer to member" is a pointer to a _class_ member, not an _instance_ member. That pointer can then be combined with an instance and dereference to the member of that class. If the pointer is to a virtual function, calling the function pointed to will call the correct override for that instance. (the pointer type is a combination of type of the class and the type of the member) So there are basically three cases: * pointer to data member: equivalent to typed offset into object * virtual function: equivalent to typed offset into vtable * non-virtual function: equivalent to function pointer (but still requires "this" pointer) As far as I know, D has no direct equivalent to this, though pointers to data members could probably be trivially written as a template struct. One complication with member function pointers is that the latter two cases above must both fit into the same type (so a pointer to a member function must be able to reference both virtual and non-virtual functions). The compiler could probably easily implement this though. One way to make this easier to implement would be to give non-virtual functions vtable offsets as well, so the pointer can be just an offset into the vtable instead of a struct or "tagged union" type of thing. That way they can also remain the same size as a size_t. Note: pointers to member functions can be up to 16 bytes in some popular C++ compilers, though C++ does need to handle a few case that D doesn't like multiple inheritance and classes that have only been forward-declared, as well as classes that aren't allowed to contain a vtable pointer)
Mar 13 2007
parent Kirk McDonald <kirklin.mcdonald gmail.com> writes:
Frits van Bommel wrote:
 Daniel Keep wrote:
 bc wrote:
 am i being stupid or is there any chance of adding pointers to member
 functions and pointers to data members to D? i can't see any way of
 getting the same functionality through existing language constructs for
 member functions... alias seems like it might at first but i can't get
 it to work... and the offsetof syntax for data members seems buggy.
 otherwise, congrats on designing a really cool language


 Delegates are your pointer to member functions.  Pointers to data of an
 object are just as you would expect.  Or did you have a different use in
 mind?

In C++, a "pointer to member" is a pointer to a _class_ member, not an _instance_ member. That pointer can then be combined with an instance and dereference to the member of that class. If the pointer is to a virtual function, calling the function pointed to will call the correct override for that instance. (the pointer type is a combination of type of the class and the type of the member) So there are basically three cases: * pointer to data member: equivalent to typed offset into object * virtual function: equivalent to typed offset into vtable * non-virtual function: equivalent to function pointer (but still requires "this" pointer) As far as I know, D has no direct equivalent to this, though pointers to data members could probably be trivially written as a template struct. One complication with member function pointers is that the latter two cases above must both fit into the same type (so a pointer to a member function must be able to reference both virtual and non-virtual functions). The compiler could probably easily implement this though. One way to make this easier to implement would be to give non-virtual functions vtable offsets as well, so the pointer can be just an offset into the vtable instead of a struct or "tagged union" type of thing. That way they can also remain the same size as a size_t. Note: pointers to member functions can be up to 16 bytes in some popular C++ compilers, though C++ does need to handle a few case that D doesn't like multiple inheritance and classes that have only been forward-declared, as well as classes that aren't allowed to contain a vtable pointer)

It is interesting to note that Pyd exploits D's behavior w.r.t. pointers to virtual member functions to implement its class wrapping. That is, given a base class and a derived class: class Base { void foo() {} } class Derived : Base { void foo() {} } You can call Base.foo on an instance of Derived.foo with some delegate trickery: void delegate() dg; dg.ptr = new Derived; dg.funcptr = &Base.foo; dg(); The reasons Pyd needs to do this are fairly complex, and I don't feel like explaining them in this space. Suffice to say, it's a good reason, and certain parts of Pyd's class wrapping implementation would be even more complex without it. To perhaps put minds at ease, I will say that the context in which it does this is fairly well guaranteed to be safe. (The derived class is generated by Pyd, so I can be pretty sure it's not doing anything to blow this up.) -- Kirk McDonald http://kirkmcdonald.blogspot.com Pyd: Connecting D and Python http://pyd.dsource.org
Mar 13 2007