pass function to class as parameter C++ - c++

Question

there is error here(but when i try to send non //class function its workin);
like when i try to add normal void zad1(){somethin...} its works but when i try to add function from class its not :?
//Class that send function//
class Lekcja1 : public ZadanieW {
private:
int numerZad;
public:
Lekcja1(int num) {
this->numerZad = num;
};
~Lekcja1() {};
void tab();
virtual void LekcjaChose();
};
/*void Zad12() {
cout << "dupa" << endl;
}*/
void Lekcja1::tab() {
cout << "dupa" << endl;
};
void Lekcja1::LekcjaChose() {
wyborZadania* obj = new wyborZadania(numerZad,tab);//there is a problem
delete obj;
}
//Class that takin function//
class ZadanieW {
public:
virtual void LekcjaChose() = 0;
};
class wyborZadania{
public:
int _chose;
public:
wyborZadania(int num,void (*tab)()) {
this->_chose = num;
switch (_chose)
{
case 1:
(*tab)();
break;
default:
break;
}
}
~wyborZadania() {}
};

Answer 1

tab is a method of Lekcja1, which is different from a regular function because it needs access to all the data members of Lekcja1.
You can either make Lekcja1::tab a static method, which hides access to data member and makes it a normal function:
class Lekcja1 : public ZadanieW {
//...
public:
static void tab();
//...
};
Or if you need tab to access data members of Lekcja1, then the wyborZadania constructor should take a pointer-to-member of Lekcja1 and an instance of Lekcja1, instead of a function pointer:
wyborZadania(int num,void (Lekcja1::*tab)(), Lekcja1& instance) {
// ...
(instance.*tab)();
// ...
}
If you need that to be more flexible or accept different kinds of classes, then wyborZadania should take an std::function, which is a rich wrapper around a function that will let you bind arguments and data.
wyborZadania(int num,std::function<void()> tab) {
// ...
tab();
// ...
}
And then in LekcjaChose() pass a lambda to call tab():
wyborZadania* obj = new wyborZadania(numerZad,[this](){ tab(); };
demo: https://godbolt.org/z/XfwpSJ

Answer 2

Pointer to member functions are passed differently. You need to pass it like this:
template<class T>
wyborZadania(int num,void (T::*&tab)()) {
this->_chose = num;
switch (_chose)
{
case 1:
(*tab)();
break;
default:
break;
}
}
For this to work Lekcja1::tab() should be declared static, otherwise you also have to pass an object, too. See the live example.

Answer 3

a non-static method cannot be sent as an argument without specifying the object of the class.
Below code uses std::function() to acheive this in simple code.
#include <iostream>
using namespace std;
#include <functional>
//class wyborZadania;
//Class that takin function//
class ZadanieW {
public:
virtual void LekcjaChose() = 0;
};
//Class that send function//
class Lekcja1 : public ZadanieW {
private:
int numerZad;
public:
Lekcja1(int num) {
this->numerZad = num;
};
~Lekcja1() {};
void tab();
virtual void LekcjaChose();
};
void Zad12() {
cout << "dupa" << endl;
}
class wyborZadania {
public:
int _chose;
private:
wyborZadania() {};
public:
wyborZadania(int num, std::function<void()> tab) {
this->_chose = num;
switch (_chose)
{
case 1:
tab();
break;
default:
break;
}
}
~wyborZadania() {}
};
void Lekcja1::tab() {
cout << "dupa1" << endl;
};
void Lekcja1::LekcjaChose() {
wyborZadania* obj = new wyborZadania(numerZad, std::bind(&Lekcja1::tab, this));//there is a problem
//wyborZadania* obj = new wyborZadania(numerZad, Zad12);//there is a problem
delete obj;
}
int main()
{
Lekcja1 obj(1);
obj.LekcjaChose();
}