CODEWITHSUNDEEP
c++boostpointer-to-member
Roddy
Roddy

Reputation: 68114

Can't pass pointer to derived class function into base class

I'm trying to pass a function from a derived class to a base class function which expects a function pointer, but I'm getting a compiler error. 

[BCC32 Error] E2034 Cannot convert 'void (Bar::*)(int)' to 'void (Foo::*)(int)'

I guess this is as designed, but is there a way around this that doesn't involve unpleasant casting? Can boost::bind help me here?

#define CALL_MEMBER_FN(object,ptrToMember)  ((object).*(ptrToMember))

class Foo
{
public:
  typedef void (Foo::*FFunc)(int i);

  void test(FFunc f)
  {
    CALL_MEMBER_FN(*this,f)(123);
  }

  void barf1(int i) {};
};

class Bar : public Foo
{
public:
  void barf2(int i) {};
};

void ffff()
{
  Foo f;
  f.test(Foo::barf1); // works

  Bar b;
  b.test(Bar::barf1); // works
  b.test(Bar::barf2); // ERROR
}

As a side note, I have this working fine using a virtual function, which is the obvious approach rather than a function pointer, but I'm just trying to get it working this way before attempting some boost::bind trickery later...

Upvotes: 2

Views: 822

Answers (3)

Tsvetomir Dimitrov
Tsvetomir Dimitrov

Reputation: 648

Can you generalise the function pointer a bit more? I mean something like this:

typedef void (Foo::*FFunc)(int i);

to:

typedef void (*FFunc)(int i);

Then you can use bind:

b.test(bind(&Bar::barf2, &b));

Upvotes: 1

Lol4t0
Lol4t0

Reputation: 12557

As well, as my knowledge of c++11 && bost && stl are not the best, I can only advise this:

 template <typename T>
    void test(void (T::*f)(int))
  {
    CALL_MEMBER_FN(static_cast<T&>(*this),f)(123);
  }

All other code same to yours.

Bad casting, but it is hidden

Upvotes: 1

lapk
lapk

Reputation: 3918

b.test( ( Foo::FFunc )&Bar::barf2 );

Sorry, didn't mean to submit it as answer, wanted to comment. Sample code.

EDIT: Well, maybe this is more "pleasant" casting, since it's C++ style? Updated sample.

b.test( static_cast< Foo::FFunc >(&Bar::barf2) );

Upvotes: 1

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