如何使用函数指针初始化类中的对象？

Question

I have 3 classes - a superclass, a subclass, and a container class. My superclass has the following constructor:

Superclass(char* t,int(*i)(myType),int a,int b,int c,int p){
    T=t;
    I=i;
    A=a;
    B=b;
    C=c;
    P=p;
}

And the subclass constructor:

Subclass(char* t,int(*i)(myType),int a,int b,int c,int p)
  : Superclass(t,i,a,b,c,p){;}

The container class contains multiple pointers to objects of the subclass type:

class Container{
  public:
    char x[2000];
    int funct(myType);
    ...
    Subclass* S;
    ...
    Container(){
      S= new Subclass(x,&funct,3,4,2000,0);
      ...
    }
}

I get a compiler error on the above line "S= new ..." with the message:

"Error: No instance of 'Subclass::Subclass' matches the argument list
argument types are: (char[2000],int(Container::*)(myType),int,int,int,int)"

I believe (though I'm not certain) the error has something to do with the function pointer being passed. I've used function ptrs in a similar manner, but it seems to not like that its pointing to a function inside the Container class. Any suggestions?

Thanks in advance for the help.

Answer 1

The function pointer you passed is a class member function, which is of type int(Container::*)(myType) , which causes the problem.

If the funct function doesn't need the information from a Container object, then you can make it static. If it indeed need the access to the object, then you either want to change the type of the parameter to std::function<int(Container&,myType)> and use this function with a object. Or change it to std::function<int(int)> and pass the function through std::bind .

Answer 2

You can't pass a member function as a plain function pointer, unless it's a static member function.

static int funct(myType);

Naturally a static function won't have access to member variables or functions that aren't also static, unless you use them in conjunction with another object pointer.

You could also change the type of the pointer passed to the constructor to be a member pointer, but that's much more involved and isn't generally useful unless you're also passing an object of that type.

Subclass(char* t,Container* cp, int((Container::*)i)(myType),int a,int b,int c,int p)
  : Superclass(t,cp,i,a,b,c,p){;}