C++ default copy constructor considered harmful
An annoyingly common bug in C++ apps: accidental double-freeing due to the default copy constructor memcpy()ing whole objects and duplicating pointers, which then get freed each time the destructor fires.
class A
{
char* foo;
public:
A() { foo = new char[100]; }
~A() { delete[] foo; }
};
int main()
{
A a; // A() is called
A b = a; // object is copied
// function exit:
// b.~A() is called, deleting b.foo
// a.~A() is called, deleting a.foo, which fails as a.foo == b.foo
}
If you need to hold a reference to an object, it’s better (quicker) to store a pointer:
<code>A* b = &a;</code>Now, when b falls out of scope, nothing gets deleted, which is good, because a will delete the object’s data fine by itself.
To make sure you don’t run into a bug like this, declare a private copy constructor that doesn’t do anything. Now any attempt to copy an instance of the object will fail as nobody can get at the copy constructor.
class A
{
char* foo;
A(const A& other) {}
public:
A() { foo = new char[100]; }
~A() { delete[] foo; }
};
Visual C++ 7 gives me an error C2248: ‘A::A’ : cannot access private member declared in class ‘A’ when I try to copy it after implementing the fake copy constructor.