passing function as constructor parameter to base class constructor while initializing

Question

#include <iostream>
#include<cstdio>
#include<typeinfo>

using std::cout;
using std::endl;

class foo;
class bar
{
    public:
        bar()
        {
        }
        bar(void (*getNextValue)(void)):memberFunctionPointer(getNextValue)
        {
        }
        void calltoderivedslass()
        {
            //  *memberFunctionPointer();
            ((bar*)this)->bar::memberFunctionPointer;
        }
        void (foo::*memberFunctionPointer)();
};

class foo : public bar
{
    public:
        foo( ):bar(static_cast<foo::*memberFunctionPointer>(&foo::hello))
        {
        }
        void hello()
        {
            printf("Hello \n\n");
        }
};
void byebye()
{
    cout << "bye" << endl;
}
int main()
{
    foo testfoo;
    //((testfoo).*(testfoo.memberFunctionPointer))();
    return 0;
}

Error:

classname.cpp: In constructor "bar::bar(void (*)())":
classname.cpp:15:68: error: cannot convert "void (*)()" to "void (foo::*)()" in initialization
classname.cpp: In constructor "foo::foo()":
classname.cpp:29:25: error: expected type-specifier
classname.cpp:29:25: error: expected ">"
classname.cpp:29:25: error: expected "("
classname.cpp:29:30: error: expected unqualified-id before "*" token
classname.cpp:31:2: error: expected "{" at end of input

Expectations:

I want to initialize the base class function pointer to initialize it points to derived class member function. I want to initialize that while creating object of derived class. From base class i want to call derived class function using the acquired function pointer.

Thanks in advance for all.

If you use OOP correctly there is no need to use function pointers... Consider the programmers who will have to maintain you code.. — Ol1v3r, Dec 02 '16 at 12:09
yeah,there is a requirement to use function pointer. A callback has to be called when some error occurs in a DB reading, So i am in need of function pointer, Not to make difficulty in maintaining the code. Thanks :) — Kiruthika Palanivelu, Dec 06 '16 at 15:41

score 2 · Answer 1 · edited May 23 '17 at 12:01

2

Looks like virtual methods to me:

class bar
{
    public:
        bar()
        {
        }
        void calltoderivedslass()
        {
            this->hello();
        }
        virtual void hello() = 0;
};

class foo : public bar
{
    public:
        foo( )
        {
        }
        void hello() override
        {
            printf("Hello \n\n");
        }
};

An other way may be to use the Curiously Recurring Template Pattern (CRTP) to achieve static polymorphism:

template<typename T>
class bar
{
    public:
        bar()
        {
        }
        void calltoderivedslass()
        {
            static_cast<T*>(this)->hello();
        }
};

class foo : public bar<foo>
{
    public:
        foo( )
        {
        }
        void hello()
        {
            printf("Hello \n\n");
        }
};

If you really want to keep pointers to member functions, you may consider std::function bound to this:

class bar
{
    public:
        bar()
        {
        }
        template<typename F>
        bar(F&& getNextValue):memberFunctionPointer(std::forward<F>(getNextValue))
        {
        }
        void calltoderivedslass()
        {
            this->memberFunctionPointer();
        }
        std::function<void()> memberFunctionPointer;
};

class foo : public bar
{
    public:
        foo( ):bar(std::bind(&foo::hello, this))
        {
        }
        void hello()
        {
            printf("Hello \n\n");
        }
};

With an extended usage I'm guessing:

void byebye()
{
    cout << "bye" << endl;
}
int main()
{
    bar testbar(byebye);
    testbar.calltoderivedslass(); // not a derived class, but it works
    return 0;
}

edited May 23 '17 at 12:01

Community

1
1

answered Dec 02 '16 at 12:07

rocambille

15,398
12
50
68

thank you for your answer, When i have use your code, i am getting errors as below, – Kiruthika Palanivelu Dec 02 '16 at 12:26
@KiruthikaPalanivelu There is no "errors as below" ^^ Anyway, did you add `#include ` to use `std::function`? – rocambille Dec 02 '16 at 12:30
thank you for your answer, When i have use your code, i am getting errors as below, error: expected â,â or â...â before â&&â token error: âfunctionâ in namespace âstdâ does not name a type In constructor âbar::bar(F)â: error: class âbarâ does not have any field named âmemberFunctionPointerâ error: âforwardâ is not a member of âstdâ error: expected primary-expression before â>â token error: âgetNextValueâ was not declared in this scope In member function âvoid bar::calltoderivedslass()â: error: âclass barâ has no member named âmemberFunctionPointerâ – Kiruthika Palanivelu Dec 02 '16 at 12:36
@KiruthikaPalanivelu did you include `functional`? What compiler are you using? My code needs C++11 support – rocambille Dec 02 '16 at 12:40
Thread model: posix gcc version 4.6.3 (Ubuntu/Linaro 4.6.3-1ubuntu5) Yes!! I got compilation terminated when i uses that. kiru4.cpp: In constructor âfoo::foo()â: kiru4.cpp:29:20: error: âbindâ is not a member of âstdâ – Kiruthika Palanivelu Dec 02 '16 at 12:48
@KiruthikaPalanivelu I don't think gcc 4.6.3 has enough C++11 support. But you can try virtual methods or CRTP ;) – rocambille Dec 02 '16 at 12:54
16:14: error: expected â,â or â...â before â&&â token 23:9: error: âfunctionâ in namespace âstdâ does not name a type In constructor âbar::bar(F)â: 16:31: error: class âbarâ does not have any field named âmemberFunctionPointerâ 16:53: error: âforwardâ is not a member of âstdâ 16:67: error: expected primary-expression before â>â token 16:69: error: âgetNextValueâ was not declared in this scope In member function âvoid bar::calltoderivedslass()â: 21:19: error: âclass barâ has no member named âmemberFunctionPointerâ In constructor âfoo::foo()â: 29:20: error: âbindâ is not a member – Kiruthika Palanivelu Dec 02 '16 at 13:14

score 0 · Accepted Answer · answered Dec 02 '16 at 12:11

0

Your constructor for bar is wrong.

The constructor bar(void (*getNextValue)(void)) does not expect a member function pointer of a function of foo and therefore initializing bar::memberFunctionPointer is not possible with the type of getNextValue.

You need to change the parameter in the constructor to void (foo::*getNextValue)(void) for this to compile.

Yet the overall design does not really seem right to me... So I think the answer of @wasthishelpful is more helpful ;-)

answered Dec 02 '16 at 12:11

Simon Kraemer

5,700
1
19
49

That error was fixed:) but still this error classname.cpp: In constructor âfoo::foo()â: classname.cpp:29:25: error: expected type-specifier classname.cpp:29:25: error: expected â>â classname.cpp:29:25: error: expected â(â classname.cpp:29:30: error: expected unqualified-id before â*â token classname.cpp:31:2: error: expected â{â at end of input – Kiruthika Palanivelu Dec 02 '16 at 12:37
Simon-kraemer This is my derived class constructor i am trying to pass function pointer to base class constructor foo( ):bar(static_cast(&foo::hello)) please correct me if i am wrong. – Kiruthika Palanivelu Dec 02 '16 at 12:40
Yes you do, but the receiving side implicitly converts the member function back. BTW: You don't need to cast in foo. `&foo::hello` is already the correct type. Have a look at this: https://godbolt.org/g/Bw72R1 And please check the other answer as it is definitely the better solution! – Simon Kraemer Dec 02 '16 at 12:59
Thanks -Simon Kraemer once again. Compilation error is fixed. But i am not getting my desired output. testfoo.calltoderivedslass(); i want my derived class function hello() to be called, but that is not happening. Is any thing wrong with this syntax, ((bar*)this)->bar::memberFunctionPointer; – Kiruthika Palanivelu Dec 02 '16 at 13:11

passing function as constructor parameter to base class constructor while initializing

2 Answers2