friend function pointer parameter in constructor-CodePudding

i'm trying to create a costructor in the form

class A {
public:
    A(void (*initializer)(A&))
    {
        initializer(*this);
    }
};

where initializer take a reference to the instance he's passed and can make custom operation on variable it is initializing. Is there a way to make such a function friend so it can access to private variables too?

CodePudding user response：

Here is a simple way:

class Impl
{
public:
    int x;
    int y;
};

class A
{
public:
    A(void (*initializer)(Impl&))
    {
        initializer(_impl);
    }

private:
    Impl _impl;
};

This hides _impl from everyone except initializer. And since nobody other than initializer can access _impl at all, the members of of class Impl can be public, making it easy for initializer to mutate them.

CodePudding user response：

Other than listing all the functions you want to pass pointers to as friends separately, no. An alternative may be to separate the data into a base class with public members you inherit privately from:

struct AData
{
    int m_privateInt;
    ...
};

class A : private AData {
public:
    A(void (*initializer)(AData&))
    {
        initializer(*this);
    }
};

You may need to also pass a reference to A, if you want to access its member functions in initializer.