C++ - Change private member from outside the class -

is code causes undefined behavior? or can run problem this? (copy full class without functions, variables public modifier , modify private memebers throw pointer) example:

#include <iostream>  using namespace std;  class point {  private:      int x;     int y;  public:      point(int x, int y) {         this->x = x;         this->y = y;     }      void print() {         cout << "(" << x << ", " << y << ")" << endl;     }  };  struct pointhack {      int x;     int y;  };  int main() {     point a(4, 5);     a.print();     ((pointhack *) (&a))->x = 1;     ((pointhack *) (&a))->y = 2;     a.print();      return 0; } 

output:

(4, 5) (1, 2) 

(with original member order, of course)

despite classes being layout compatible (see below), code exhibits undefined behavior due fact such pointer casts prohibited the c++ strict aliasing rules¹.

but: replacing casts union makes code standard-compliant; guaranteed work in c++11:

#include <iostream>  using namespace std;  class point {  private:      int x;     int y;  public:      point(int x, int y) {         this->x = x;         this->y = y;     }      void print() {         cout << "(" << x << ", " << y << ")" << endl;     }  };  struct pointhack {     int x;     int y; };  union pu {     point p;     pointhack ph;     pu(int x, int y) : p(x, y) {} };  int main() {     pu u(4,5);     u.p.print();     u.ph.x=1;     u.ph.y=2;     u.p.print();     return 0; } 

this comes fact point , pointhack standard-layout classes² (c++11, §9 ¶7), , share "common initial subsequence" (§9.2, ¶20); such, if both stored in same union (here pu) it's permitted "inspect common initial part of of them"³.

still, answer exercise of style; don't exploit such tricks unless forced to. c++ provides better means access private members if necessary without brutally breaking encapsulation - have getters/setters, protected inheritance, friend classes, ... , in general, if access private class members in ways not intended target class, potentially violating assumptions of class how data modified, can lead erratic behavior of code of methods.

---

notes:

1. in c++ can't have 2 pointers of unrelated types pointing same object; restriction used optimizers assumptions aliasing.
2. notice requirements quite stringent; typically classes aren't c structs don't qualify this. having different access qualifiers can break magic.
3. the idea assigning ph makes "active object" of union, , p.print() 1 "inspecting" "inactive" object.