Question

I wanted to test how C++ behaves when the return value of a function is an object. I made this little example to watch how many bytes are allocated and determine whether the compiler makes a copy of object (like when object is passed as a parameter) or instead returns some kind of reference.

However, I couldn’t run this very simple program and I have no idea why. Error says: “Debug assertion failed! Expression: BLOCK_TYPE_IS_INVALID” in some dbgdel.cpp file. Project is a win32 console application. But I’m pretty sure that there is something wrong with this code.

class Ctest1
{
public:
   Ctest1(void);
   ~Ctest1(void);

   char* classSpace;
};

Ctest1::Ctest1(void)
{
   classSpace = new char[100];
}

Ctest1::~Ctest1(void)
{
   delete [] classSpace;
}

Ctest1 Function(Ctest1* cPtr){
   return *cPtr;    
}

int _tmain(int argc, _TCHAR* argv[])
{
   Ctest1* cPtr;

   cPtr=new Ctest1();


   for(int i=1;i<10;i++)
      *cPtr = Function(cPtr);


   delete cPtr;

   return 0;
   }

Answer 1

You have violated the Rule of Three.

Specifically, when you return an object, a copy is made, and then destroyed. So, you have a sequence of events like

Ctest1::Ctest1(void);
Ctest1::Ctest1(const Ctest1&);
Ctest1::~Ctest1();
Ctest1::~Ctest1();

That is two objects are created: your original object construction, followed by the implicit copy constructor. Then both of those objects are deleted.

Since both of those objects contain the same pointer, you end up calling delete twice on the same value. BOOM

---

Extra Credit: When I investigate issues like “I wonder how the copies get made”, I put print statements in the interesting class methods, like this:

#include <iostream>

int serial_source = 0;
class Ctest1
{
#define X(s) (std::cout << s << ": " << serial << "\n")
  const int serial;
public:
   Ctest1(void) : serial(serial_source++) {
     X("Ctest1::Ctest1(void)");
   }
   ~Ctest1(void) {
    X("Ctest1::~Ctest1()");
   }
   Ctest1(const Ctest1& other) : serial(serial_source++) {
    X("Ctest1::Ctest1(const Ctest1&)");
    std::cout << " Copied from " << other.serial << "\n";
   }
   void operator=(const Ctest1& other) {
     X("operator=");
     std::cout << " Assigning from " << other.serial << "\n";
   }
#undef X
};

Ctest1 Function(Ctest1* cPtr){
   return *cPtr;    
}

int main()
{
   Ctest1* cPtr;

   cPtr=new Ctest1();


   for(int i=1;i<10;i++)
      *cPtr = Function(cPtr);

   delete cPtr;

   return 0;
}