Execute function within the constructor call-CodePudding

What I would like to achieve is the following: I want to allow the users to only create objects of class A if they provide a function - this function may construct and calculate things and set other members of the object. In short, I need a way to only allow a constructor call that includes a function as an argument that is subsequently executed with said constructor call. Imagining it like filling those blanks right now:

void foo()
{
// do something
}


class A
{
public:
    A(/*magic*/);

/*
...
*/


A:A()
{
    /*magic*/
}

I have read about function pointers but have yet to figure out what this syntax:

int (*const fcnPtr)();

translates into and how I can achieve what I want to achieve with it - assuming this is the "best" way in the first place.

Whatever closes the chain of function definition -> constructor call with link to function -> executing the function during object construction is of interest to me.

Thank you for reading and thank you in advance for any input.

CodePudding user response：

The easiest way to do it is by a template:

class A
{
public:
    int i;

    template<typename F>
    A(F f)
    : i(0) /* and other initializers*/
    { f(); } // no need to pass *this, see below
};

// usage:
A a([&]() { a.i = 0; });

CodePudding user response：

Looking at your given example, you can add a function pointer as parameter of the constructor:

void foo()
{
  std::cout <<" foo called" << std::endl;
}
void bar()
{
    std::cout << "bar called" << std::endl;
}


class A
{
public:
//-----------vvv------->ptr is a pointer to a function that has no parameter and return type void 
    A(void (*ptr)());

};


A::A(void (*ptr)())
{
    //call through ptr 
    ptr(); 
}
int main()
{
    A a(foo); //this will call foo 
    
    A b(bar); //this will cal bar
}