C++ compilation does not preserve abstraction

evil_client.cpp

struct A {
  int x; //
  void print() const;
};

void break_encapsulation(A& a) {
  a.print();
  a.x = 1;
  a.print();
}

int main() {
  A a;
  break_encapsulation(a);
}

Makefile

CXX := g++
CXXFLAGS := -std=c++11
prog: evil_client.o module.o
	$(CXX) $(CXXFLAGS) -o $@ $^

# Redundant, but included for completeness
%.o: %.cpp
	$(CXX) $(CXXFLAGS) -c -o $@ $^

module.cpp

#include <cstdio>
using namespace std;

class A {
  int x{0}; //
public:
  int getX() const { return x; }
  void print() const;
};

void break_encapsulation(A& a);
void A::print() const {
  printf("this.x: %d\n", getX());
}

This example shows (one reason that) private: in C++ does not protect when linking against arbitrary external code — so any privacy guarantees given by private are not preserved by compilation.

Of course, this code is undefined behavior (because it has conflicting definitions of A, which violates the one-definition rule), but typical implementations will indeed break encapsulation, and the UB can be avoided by moving break_encapsulation to C or assembly, or replacing a.x = 1; with *reinterpret_cast<int&>(&a) = 1; and sharing the definition of A.