Copying a class in C++ creates a new instance of that class, but if any variables were declared with "new," those variables don't get copied, only their addresses do. Therefore the original instance and the copy essentially share the same variable.

entangled_copy.cc

// showing how copying an instance of a class
// doesn't copy the dynamically allocated variables,
// it simply copies over their address. So the new
// copy shares the old variable.

#include <iostream>

class Thing{
public:
    Thing(){ pInt = new int; }
    int* pInt;    
};

int main(){
    Thing t1;
    Thing t2 = t1;
    
    std::cout << "Address to t1: " << &t1 << std::endl;  
    std::cout << "Address to t2: " << &t2 << std::endl;  

    
    std::cout << "Address to t1's int: " << t1.pInt << std::endl;
    std::cout << "Address to t2's int: " << t2.pInt << std::endl;
}

// OUTPUT:
// Address to t1: 0x5052c8
// Address to t2: 0x5052c0
// Address to t1's int: 0x505388
// Address to t2's int: 0x505388


// note: t1 and t2 are different places in memory,
// but both of their pInt variables point to the 
// same place in memory. 
// so be careful copying variables in this way!