Question

I have to make a member data pointer to get less copy of the data.I have writed some sample code.Some error occurred where I marked “error”.
How to do rightly.

#include <iostream>

class A {
public:
    A() {
        stra="12345";
    }
    virtual ~A() {}
    string stra;
}

class B {
public:
    B(A *pt) {
        strb=&(pt->stra);  //Error.An assignment statement
    }
    virtual ~B() {}
    string A::*strb;       //member data pointer from class A
}

int main() {
    A ma;
    B mb(&ma);
    std::cout<< *(mb.strb) <<std::endl;  //Error.print data
}

Answer 1

In effect, what you have is a standard pointer, not a pointer-to-member. The following will work:

#include <iostream>
#include <string>

using namespace std;


class A {
public:
    A() {
        stra="12345";
    }
    virtual ~A() {}
    string stra;
};

class B {
public:
    B(A *pt) {
        strb=&(pt->stra);
    }
    virtual ~B() {}
    string* strb; // <<<<<<<< THIS
};

int main() {
    A ma;
    B mb(&ma);
    std::cout<< *(mb.strb) <<std::endl;
}

For some discussion on pointers to data members, see C++: Pointer to class data member

I don’t know where you’re going with all this, but having instances of one class keep pointers to things inside instances of another class strikes me as a rather poor design.