make a function return a pointer to a function within a class (C++)

Question

I found this question here but now I want to extend it. so I have the following class in one file and it works correctly:

typedef int (*fptr)();

int f1() {
    return 1;
}

int f2() {
   return 2;
}




fptr f( char c ) {
    if ( c == '1' ) {
        return f1;
    }
    else {
        return f2;
    }
}

But when I want to move the functions in another file and make a factory out of them, I get an error that I cannot understand.

it would be like this:

Header:

public class Factories{
   public:
     int f1();
     int f2();
}

CPPFile:

int f1() {
    return 1;
}

int f2() {
   return 2;
}

Header:

public class FactoryClass{
   public:
     fptr f( char c );
}

CPPFile:

typedef int (*fptr)();
fptr f( char c ) {
    if ( c == '1' ) {
        return Factories::f1;
    }
    else {
        return Factories::f2;
    }
}

Here in the returns of this FactoryClass Function I get the error "return value type does not match the function type". Any Ideas would be appreciated.

In C++ you can simply overload a pure virtual function which means you don't need a function pointer. — SPlatten, Apr 13 '16 at 12:14
typedef int (*fptr)(); is not a pointer to the member function of the Factories class, that would be **typedef int (Factories::*fptr)();** — Biruk Abebe, Apr 13 '16 at 12:16

NathanOliver · Accepted Answer · 2016-04-13T12:21:55.633

2

Factories::f1 is not a normal function, it is a member function. Since it is a member function it needs to take a pointer to the object that called it so it can affect that instance. This means the type of a member function includes what type of object it is called from so a pointer to f1 actually has the type

int (Factories::*)()

You will either have to make fptr

typedef int (Factories::*fptr)();

or make the functions static so they act like regular functions.

edited Apr 13 '16 at 12:21

answered Apr 13 '16 at 12:16

NathanOliver

171,901
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Or keep them in the class but make them `static`. – Marvin Sielenkemper Apr 13 '16 at 12:46
@MarvinSielenkemper I said that in my last sentence. – NathanOliver Apr 13 '16 at 12:48
Ah, sorry! This was meant for a different answer. – Marvin Sielenkemper Apr 13 '16 at 12:50

score 1 · Answer 2 · answered Apr 13 '16 at 12:14

1

the member functions Facotires::f1 you are returning just have the wrong signature. If possible, make them (f1 and f2) static. If not, bind the respective class instance (using boost::bind or std::bind)

answered Apr 13 '16 at 12:14

lipp

5,586
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score 1 · Answer 3 · edited May 23 '17 at 12:16

1

The function type changed when you put them in a class. The new type should be typedef int (Factories::*fptr)();

C++: Calling member function via pointer

edited May 23 '17 at 12:16

Community

1
1

answered Apr 13 '16 at 12:15

Kyle A

928
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make a function return a pointer to a function within a class (C++)

3 Answers3