getting a must have (pointer-to) function type on a variable that is defined as a (pointer-to) function type

Question

I am creating a typedef for a function that will be used to call arbitrary functions that are stored in a string to function pointer map. I am sure that the problem has something to do with how the type is declared and referenced through many objects in my code but the is the only way I can think of how to do what I need to

This is fmap.h here i declare the fmap class and the executefunctions class, the typedef for the function pointer is the on the first line of the public members for fmap

class Web;
class Matlab;
class Word;
class Node;

class ExecFunctions
{
public:
    ExecFunctions(){}
    /**
    Function:       travel
    */
    int travel(Web *concepts, Matlab *mat, Word * words, int reqIdx, string theWord, vector<string> *dependsMet);

};

class FMap
{
public:
    typedef int (ExecFunctions::*ExecFunc)(Web *, Matlab *, Word *, int, string, vector<string> *);
    ExecFunc getFunc(string funcName){
        return theFuncMap.descrToFuncMap[funcName];
    }

private:
    class FuncMap {
    public:
        FuncMap() {
                descrToFuncMap["travel"] = &ExecFunctions::travel;  
        }
        std::map<std::string, ExecFunc> descrToFuncMap;
    };    
};



#endif

next is web.h I am only including what I think are the relevant parts

#include "fmap.h"

class Node
{
    private:
        ....
        //pointer to execcution function
        FMap::ExecFunc func;
        //fmap function used to access execution function pointers
        FMap *functionMap;
            .....    
public:
          FMap::ExecFunc getExecFunc();
};

now what I think is the relavent part of web.cpp

Node::Node(string name, Node *nodeParent)
{
    attrList = new Attr();
    nodeName = name;
        ........
    func = functionMap->getFunc(name);
}

Now finally. This is where I am getting the error. There are three lines of comments before the error line explaining the error I am getting.

void process(Util myUtil, Web *concepts, Matlab *mat, string path)
{
    int funcP
    bool dependsProcessed = false;
    vector<string> *dependsDone = new vector<string>();
    FMap::ExecFunc funcToCall;

    funcToCall = realMeanings[i][j]->getConcept()->getExecFunc();


//the line bellow this comment is where I'm getting the error. Visual Studio says
//that funcToCall must have (pointer-to) function type, and then the VS compiler says
//diabot.cpp(177): error C2064: term does not evaluate to a function taking 6 arguments
    funcPtrRet = funcToCall(concepts, mat, realMeanings[i][j], reqConceptsIdx[i][j], queryWords[i], dependsDone);

    return;
}

any help anyone can give would be greatly appreciated.

Thank you all in advance

Thats a *lot* of code, most of which is probably irrelevant to the problem. Could you attempt to simplify your example? — Oliver Charlesworth, Apr 28 '11 at 07:26
Sorry about that, I have trouble cutting down code to the basic issues. I cut it down a bit. Let me know if its still to much — jeffminton, Apr 28 '11 at 07:35

score 0 · Answer 1 · answered Apr 28 '11 at 07:28

0

I'm not sure I didn't get lost reading the whole code but I believe you must call funcToCall() on a ExecFunctions instance, because ExecFunc is a method-pointer type.

What does it give if you use the following syntax:

ExecFunctions ef;
ef.*funcToCall(...);

or

ExecFunctions* ef = ...;
ef->*funcToCall(...);

answered Apr 28 '11 at 07:28

ereOn

53,676
39
161
238

the first doesn't change the error at all, it still gives a must have (pointer-to) function type error and the second gives both a cannot assign type FMap::ExecFunc to type ExecFunctions and ExecFunctions has no member funcToCall – jeffminton Apr 28 '11 at 07:56

score 0 · Answer 2 · answered Apr 28 '11 at 07:32

Your funcToCall is a pointer-to-member-function but you are calling like a simple function pointer, you need to call it on an instance of the ExecFunctions class , otherwise consider changing ExecFunctions to a namespace and ExecFunc to int ()(Web *, Matlab *, Word *, int, string, vector<string> *);

score 0 · Answer 3 · answered Apr 28 '11 at 07:33

The issue is that member functions and free functions are different in C++, and if you think of it, there is a good reason (hidden implicit this argument). There are different solutions for what you want to do depending on what you really need:

Low level:

Make the functions non-member (move them to namespace level).
Make the functions static (removes the hidden this, and then that type defintion will work. Consider the first option first, C++ does not force you to add all code inside classes, and sometimes functions do not belong to a class. There is no point in creating a meaningless class just to store some free functions.
Change the typedef to reflect that they are members: typedef return_type (type::*member_function)( arg_type ); -- but this will require creation of a type object to call the member function on.

High level:

use std::function (C++0x) or boost::function together with bind: this is the most flexible, as it will allow you to bind not only free functions, and static functions but also member functions by providing the actual object. But: if you don't need the flexibility you can use one of the other options.

getting a must have (pointer-to) function type on a variable that is defined as a (pointer-to) function type

3 Answers3