Question

I have the following code:

class TestClass
{
public:
  TestClass(){};

  std::string GetTestString()
  {
    return (mTestString);
  }
  void SetTestString(const std::string& rTestString)
  {
    mTestString = rTestString;
  }

private:
  std::string mTestString;
};

TestClass* pGlobalVar;

void SomeFunction(TestClass MyClass)
{
  pGlobalVar->SetTestString("cba");
  std::cout << "Changed string:  " << pGlobalVar->GetTestString() << std::endl;
  std::cout << "Copied string:   " << MyClass.GetTestString() << std::endl;

}

int main()
{
  pGlobalVar = new TestClass();
  pGlobalVar->SetTestString("abc");
  std::cout << "Original string: " << pGlobalVar->GetTestString() << std::endl;
  SomeFunction(*pGlobalVar);
  delete (pGlobalVar);
}

This outputs the following:

Original string: abc
Changed string:  cba
Copied string:   abc

As I did not define a copy constructor for my class, I would expect that a flat copy would be made, including the pointer in the std::string. Apparently though the std::string copy constructor is used, since a change to the original string did not change the copy.

Can anyone explain to me why it did not make flat copy?

I’m using Linux with GCC 4.4.6.

Answer 1

As I did not define a copy constructor for my class, I would expect that a flat copy would be made

Since you didn’t define a copy constructor, C++ did for you.

The auto-generated copy constructor calls the constructor for all its member variables (if they have one).

Analogously, auto-generated constructors call all their members’ constructors, and destructors call all their members’ destructors.