C++ Undefined Behavior - When a Null Pointer Just Works

Today, I lost more than half a day debugging a bizarre issue in a C++ project.

The cause? A null pointer.

A piece of code that shouldn't work.

But it did.

Log* log = new Log("[TEXT] ");

log->print("Hello, World!"); // Works
log = nullptr;

log->print("Why does this work?"); // Still prints?!
log->printPrefix("This won't work");

delete log;

The print() method doesn't access any member variables, so the compiler can generate code that appears to work even though calling a non-static member function through a null pointer is undefined behaviour.

Meanwhile, the original object is leaking memory, and delete log does absolutely nothing because log is now nullptr.

And there goes half a day with lots of confusion. 🫠

The lesson

Just because your C++ code works doesn't mean it's correct nor makes sense. Undefined behavior doesn't promise to crash. Sometimes, it just quietly wastes your afternoon.

The complete code:

#include <iostream>

class Log {
  private: std::string prefix;
    public: void print(const std::string & message) {
    std::cout << message << std::endl;
  }

  public: void printPrefix(const std::string & message) {
    std::cout << prefix << message << std::endl;
  }
  Log(const std::string & p): prefix(p) {}
};

int main() {
  Log * log = new Log("[TEXT] ");

  log -> print("Hello, World!");
  log -> printPrefix("Hello, World!");

  log = nullptr;

  log -> print("Why does this work?");
  log -> printPrefix("Why does this work?"); 

  delete log;
  return 0;
}

Coverimage source: https://unsplash.com/de/fotos/eine-katze-schaut-in-die-kamera-L_jzNaicIIM