8

I understand the basic problem in passing the address of a member function outside of its class. I get the feeling that mem_fn() might be the solution but I am having trouble with the specifics.

I have a member function in class p that is currently declared as

typedef void (*valNamedFlagsCallback)(const int, const bool);
bool valNamedFlags(const OptBlk *operand, const char *description_of_value_or_NULL, const int subscripts[], const char *names[], valNamedFlagsCallback callBack);

In class e I am trying to call valNamedFlags with 

pInstance->valNamedFlags(operand, "Statement types", statementsSubscripts, statementsNames, std::mem_fn(&e::setStatement));

(I started out without the mem_fn() but of course that has the classic "pointers to member functions" problem. I've tried both &e::setStatement and just plain &setStatement.)

FWIW, setStatement is prototyped as

  void setStatement(const int ifcid, const bool isAffirmative);

Everything works if I eliminate the mem_fn() and declare setStatement as static. I'm just pointing that out as a way of saying that I have eliminated all of the other possible issues; my only issue is the "pointers to member functions" problem. Unfortunately, setStatement() needs to be a member function, not a static.

The specific error I am getting in MS VS 2010 is

bool p::valNamedFlags(const OptBlk *,const char *,const int [],const char *[],p::valNamedFlagsCallback)' : cannot convert parameter 5 from 'std::tr1::_Mem_fn3<_Rx,_Pmf,_Arg0,_Arg1,_Arg2>' to 'p::valNamedFlagsCallback'

I would like to keep the callback declaration independent of class e; that is, I do not want to go to 

typedef void (*e::valNamedFlagsCallback)(const int, const bool);

because I want to keep p more generalized than that.

Is mem_fn() the right solution or am I way off base? If so, how should I be declaring the callback in the valNamedFlags() prototype?

Or should I be pursuing a different approach?

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4
  • Have a look at std::bind Commented Feb 10, 2016 at 17:04
  • ^ ... and std::function when you're already at it. Commented Feb 10, 2016 at 17:05
  • 1
    Thanks. I have looked at a lot of Web pages and my head is swimming. I need some specifics. That's why I came here with this question. Commented Feb 10, 2016 at 17:07
  • Nobody managed to answer your question with mem_fn()? That's sad... Related: stackoverflow.com/questions/75551158/… Commented Feb 23, 2023 at 23:14

3 Answers 3

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6

Actually, you'll have a problem doing bind-like stuff as some suggested, since your callback is defined as a simple function pointer, and not a callable.

If you can afford to do it, you can change your

typedef void (*valNamedFlagsCallback)(const int, const bool);

to something like

typedef std::function<void(int, bool)> valNamedFlagsCallback

(also noting that const on value parameters doesn't affect the signature of the function), then you can use std::bind(),

using namespace std::placeholders;

pInstance->valNamedFlags(operand,
      "Statement types", 
      statementsSubscripts,
      statementsNames,
      std::bind(&E::setStatement, e, _1, _2));

or you can use lambdas:

pInstance->valNamedFlags(operand,
  "Statement types", 
  statementsSubscripts,
  statementsNames,
  [&](int i, bool b) { e->setStatement(i, b); });

If you must keep it as a simple function, then you'll have to send one which references the right object as a global/static variable.

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3 Comments

Thanks much! This is the level of help I need. Forgot to mention that one problem is that I have found a lot of examples of mem_fn() and bind() on the Web but none are for a function prototype and I have been unable to make that leap on my own. Can't see why I would not want to go to your std::function typedef. Not sure exactly how to specify the reference now in the call. Tried std::bind<e>(&e::setStatement) and I am getting dozens of inscrutable errors on MS templates ...
Oh, also, I am doing alpha testing in MS VS 2010 but my ultimate environment is a compiler that only offers partial C++11 support and so I cannot use lambdas.
@Charles That's not your fault, because I had a typo. Try again with revised code, please, and we'll see what else needs to be worked out.
5

You need to bind an instance on it to call via a member function pointer. (i.e. std::bind(&e::setStatement, eInstance, _1, _2), suppose eInstance is a pointer to an object of class e).

using namespace std::placeholders;  // for _1, _2, _3...
pInstance->valNamedFlags(operand, "Statement types", statementsSubscripts, statementsNames, std::bind(&e::setStatement, eInstance, _1, _2));

Note that the return value of std::bind (which is unspecified) doesn't match the free function pointer type valNamedFlagsCallback, one of the solutions is to use std::function.

typedef std::function<void(const int, const bool)> valNamedFlagsCallback;

Simplified demo

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7 Comments

Actually, will bind() return something that's convertible to a simple function pointer, as he's defined it?
@YamMarcovic - good point. valNamedFlags has to take the result of the bind expression as its final argument; in practice, that means it should either be a template or it should take std::function<void(int,bool)> where it currently takes valNamedFlagsCallback.
Thanks. Sorry to be so dense. I changed the prototype as you suggested to use std::function<void(int,bool)> callBack and I have been playing with the call -- something like std::bind(setStatement, _1, _2). I am getting literally hundreds of errors that I cannot make heads or tails of. Any more clues? Thanks much!
Oh, also tried std::bind(setStatement, this, _1, _2) and still getting hundreds of errors.
@Charles -- you should probably post either of those as a new question, including the code that's failing.
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0

Got it!!! THANKS ALL!!!

Prototype:

bool valNamedFlags(const OptBlk *operand, const char *description_of_value_or_NULL, const int subscripts[], const char *names[], std::function<void(int,bool)> callBack);

Call:

valNamedFlags(..., std::bind(&e::setStatement, this, _1, _2));

Actually runs and calls setStatement with the proper arguments.

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1 Comment

Just for completeness let me add that MS VS 2010 takes std::bind and so forth with no problem but my other compiler which is much more of a stickler for standards-conformant syntactic perfection requires std::tr1::bind() and so forth.

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