C++ template class copy-constructor and assignment-operator

Question

I have an implementation of a template class Triple, which is a container holding any three types. My problem is that, my class takes three const references to values as parameter, and the values have to be private (definition), however, I also have to implement the copy-constructor and overloaded assignment operator.

template <typename T1, typename T2, typename T3>
    class Triple
{
public:
    Triple()
    { }
    Triple(const T1 &a, const T2 &b, const T3 &c) : a(a), b(b), c(c)
    { }

    // copy constructor
    Triple(const Triple &triple) {
        a = triple.first();
        b = triple.second();
        c = triple.third();
    }

    // assignment operator
    Triple &operator=(const Triple& other) {
        //Check for self-assignment
        if (this == &other)
            return *this;

        a = other.first();
        b = other.second();
        c = other.third();

        return *this;
    }

  private:
    T1 const& a;
    T2 const& b;
    T3 const& c;
 };

How would you implement the copy-constructor and assignment operator without assigning to const variables?

Note that the problem with the assignment operator is not due to const. It's due to the fact that references must be initialized when they are created (i.e. her in the initialization list of a constructor) and cannot be changed afterwards. The assignment operator changes an already fully created object. — JohnB

Motti Motti · Accepted Answer · 2012-10-15T09:44:14

You should probably not have const references as members since you can't (in general) know that the objects lifetime will outlast the lifetime of your object, a, b and c should almost certainly be of type Tx and not Tx const&.

If you do know this (be sure that you do, it's more probable that you don't understand the implications unless you're an expert C++ developer), then you can have a copy constructor using an initialization list.

Triple(const Triple& other) {
  : a(other.a)
  , b(other.b)
  , c(other.c)
{ }

You can't have assignment operator since assigning to a reference changes the referred to object not the reference, you could simulate references with pointers but since I think this is not what you want I won't spell it out.

In any case the real thing you should be doing is using std::tuple and not reinventing The wheel.

C++ template class copy-constructor and assignment-operator

1 Answers