What is a right assignment statement for member data pointer

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
}

score 2 · Accepted Answer · edited May 23 '17 at 10:24

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.

In the beginning I do so, but there is an error.You are correct, it appears that an error in another place.Thank you. — simon, Dec 01 '12 at 08:55

score 0 · Answer 2 · answered Dec 01 '12 at 08:22

0

string A::*strb; should be string* strb;

B should have a pointer to a string object, not a pointer to an A string(?)

answered Dec 01 '12 at 08:22

Alan

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What is a right assignment statement for member data pointer

2 Answers2