Manually define only part of copy constructor and assignment operator - c++

Question

I'm wondering if there is a way to implement copy constructors and assignment operators such that only a small modification is needed when these are redefined for a class.
For example, consider a class as such:
class Foo {
private:
int* mInt_ptr;
/* many other member variables
of different types that aren't
pointers */
public:
Foo();
Foo(const Foo&);
Foo& operator=(const Foo&);
~Foo();
};
Now, in order to deal with the pointer mInt_ptr I would need to handle it appropriately in the copy constructor and assignment operator. However, the rest of the member variables are safe to do a shallow copy of. Is there a way to do this automatically?
Once a class becomes large it may become tedious and unwieldy to explicitly write out the operations to copy the non-pointer member variables, so I'm wondering if there is a way to write, say, a copy constructor such as:
Foo::Foo(const Foo& tocopy)
{
mInt_ptr = new int(*tocopy.mInt_ptr);
/* Do shallow copy here somehow? */
}
rather than the explicit form of:
Foo::Foo(const Foo& tocopy)
{
mInt_ptr = new int(*tocopy.mInt_ptr);
mVar1 = tocopy.mVar1;
mVar2 = tocopy.mVar2;
...
...
mVarN = tocopy.mVarN;
}

Answer 1

Generally, don't use raw pointers, for exactly the reason that you're now fighting with. Instead, use a suitable smart pointer, and use copy-swap assignment:
class Foo
{
int a;
Zip z;
std::string name;
value_ptr<Bar> p;
public:
Foo(Foo const &) = default;
Foo & operator=(Foo rhs)
{
rhs.swap(*this);
return *this;
}
void swap(Foo & rhs)
{
using std::swap;
swap(a, rhs.a);
swap(z, rhs.z);
swap(name, rhs.name);
swap(p, rhs.p);
}
};
namespace std { template <> void swap<Foo>(Foo & a, Foo & b) { a.swap(b); } }
The value_ptr could be a full-blown implementation, or something simple such as this:
template <typename T> // suitable for small children,
class value_ptr // but not polymorphic base classes.
{
T * ptr;
public:
constexpr value_ptr() : ptr(nullptr) { }
value_ptr(T * p) noexcept : ptr(p) { }
value_ptr(value_ptr const & rhs) : ptr(::new T(*rhs.ptr)) { }
~value_ptr() { delete ptr; }
value_ptr & operator=(value_ptr rhs) { rhs.swap(*this); return *this; }
void swap(value_ptr & rhs) { std::swap(ptr, rhs.ptr); }
T & operator*() { return *ptr; }
T * operator->() { return ptr; }
};

Answer 2

How about you wrap all the shallow-copy bits in a small helper struct and use the default copy behaviour there.
class Foo {
private:
int* mInt_ptr;
struct helper_t
/* many other member variables
of different types that aren't
pointers */
} mHelper;
public:
Foo();
Foo(const Foo&);
Foo& operator=(const Foo&);
~Foo();
};
Foo::Foo(const Foo& tocopy)
{
mInt_ptr = new int(*tocopy.mInt_ptr);
mHelper = tocopy.mHelper;
}
Using better primitives, as Kerrek suggested, seems like better design though. This is just another possibility.

Answer 3

Regardless if you use raw pointers or smart pointers the Kerrek's solution is right in the sense that you should make a copy constructor, destructor and swap and implement assignment using those:
class Foo
{
private:
int* mInt_ptr;
// many other member variables
// of different types
public:
Foo()
: mInt_ptr(NULL)
// initialize all other members
{}
Foo(const Foo& that)
: mInt_ptr(new int(*that.mInt_ptr) )
// copy-construct all other members
{}
Foo& operator=(const Foo& that)
{
// you may check if(this == &that) here
Foo(that).swap(*this);
return *this;
}
~Foo()
{
delete mInt_ptr;
// and release other resources
}
void swap(Foo& that)
{
std::swap(mInt_ptr, that.mInt_ptr);
// swap all members
}
};
The members are inline here just to keep it compact, usually it is not advisable to burden class definition with inline member definitions.