Question

I recently revisited the copy constructor, assignment operator, copy swap idom seen here:
What is the copy-and-swap idiom?
and many other places –

The Above link is an excellent post – but I still had a few more questions –
These questions are answered in a bunch of places, on stackoverflow and many other sites, but I have not seen a lot of consistency –

1 – Should you have try–catch around the areas where we allocate the new memory for a deep copy in the copy constructor ? (Ive seen it both ways)

2 – With regards to inheritance for both the copy constructor and assignment operator, when should the base class functions be called, and when should these functions be virtual?

3 – Is std::copy the best way for duplicating memory in the copy constructor? I have seen it with memcpy, and seen others say memcpy the worst thing on earth.

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Consider the example Below (Thanks for all the feedback), it prompted some additional questions:

4 – Should we be checking for self assignment? If so where

5 – Off topic question, but I have seen swapped used as :
std::copy(Other.Data,Other.Data + size,Data);
should it be:
std::copy(Other.Data,Other.Data + (size-1),Data);
if swap goes from ‘First to Last’ and the 0th element is Other.Data?

6 – Why doesn’t the commented out constructor work (I had to change size to mysize) – is assume this means regardless of the order I write them, the constructor will always call the allocation element first?

7 – Any other comments on my implementation? I know the code is useless but i’m just trying to illustrate a point.

class TBar
{

    public:

    //Swap Function        
    void swap(TBar &One, TBar &Two)
    {
            std::swap(One.b,Two.b);
            std::swap(One.a,Two.a);
    }

    int a;
    int *b;


    TBar& operator=(TBar Other)
    {
            swap(Other,*this);
            return (*this);
    }

    TBar() : a(0), b(new int) {}                //We Always Allocate the int 

    TBar(TBar const &Other) : a(Other.a), b(new int) 
    {
            std::copy(Other.b,Other.b,b);
            *b = 22;                                                //Just to have something
    }

    virtual ~TBar() { delete b;}
};

class TSuperFoo : public TBar
{
    public:

    int* Data;
    int size;

    //Swap Function for copy swap 
    void swap (TSuperFoo &One, TSuperFoo &Two)
    {
            std::swap(static_cast<TBar&>(One),static_cast<TBar&>(Two));
            std::swap(One.Data,Two.Data);
            std::swap(One.size,Two.size);
    }

    //Default Constructor
    TSuperFoo(int mysize = 5) : TBar(), size(mysize), Data(new int[mysize]) {}
    //TSuperFoo(int mysize = 5) : TBar(), size(mysize), Data(new int[size]) {}                *1

    //Copy Constructor
    TSuperFoo(TSuperFoo const &Other) : TBar(Other), size(Other.size), Data(new int[Other.size])        // I need [Other.size]! not sizw
    {
            std::copy(Other.Data,Other.Data + size,Data);        // Should this be (size-1) if std::copy is First -> Last? *2
    }

    //Assignment Operator
    TSuperFoo& operator=(TSuperFoo Other)
    {
            swap(Other,(*this));
            return (*this);
    }

    ~TSuperFoo() { delete[] Data;}

};

Answer 1

1. If you allocate memory then you need to ensure that it is freed in the case of an exception being thrown. You can do this with an explicit try/catch, or you can use a smart pointer such as std::unique_ptr to hold the memory, which will then be automatically deleted when the smart pointer is destroyed by stack unwinding.

2. You very rarely need a virtual assignment operator. Call the base class copy constructor in the member initialization list, and base-class assignment operator first in the derived assignment operator if you are doing a memberwise assignment — if you are doing copy/swap then you don’t need to call the base class assignment in your derived assignment operator, provided that copy and swap are implemented correctly.

3. std::copy works with objects, and will correctly call copy constructors. If you have plain POD objects then memcpy will work just as well. I’d go for std::copy in most cases though — it should be optimized to memcpy under the hood anyway for PODs, and it avoids the potential for errors should you add a copy constructor later.

[Updates for updated question]

1. With copy/swap as written there is no need to check for self-assignment, and indeed no way of doing so — by the time you enter the assignment operator other is a copy, and you have no way of knowing what the source object was. This just means that self-assignment will still do a copy/swap.

2. std::copy takes a pair of iterators (first, first+size) as input. This allows for empty ranges, and is the same as every range-based algorithm in the standard library.

3. The commented out constructor doesn’t work because the members are initialized in the order they are declared, regardless of the order in the member initializer list. Consequently, Data is always initialized first. If the initialization depends on size then it will get a duff value since size hasn’t been initialized yet. If you swap the declarations of size and data then this constructor will work fine. Good compilers will warn about the order of member initialization not matching the order of declarations.