C++ how call template's class constructor

Question

I have such piece of code:

template <class T>
struct Rgb
{
    T *r, *g, *b;

    Rgb(int nr, int ng, int nb)
    {
        r = new T(nr);
        g = new T(ng);
        b = new T(nb);
    }

    Rgb(int n)
    {
        Rgb(n, n, n);
    }

    ~Rgb()
    {
        delete r;
        delete g;
        delete b;
    }
};

I have done it in such way, because I want to be able to create objects like:

    Rgb<std::vector<int>> a(10); // so it creates 3 vectors inside,
        // each with 10 elements or

    Rgb<std::deque<double>> a(2, 5, 4); // so it creates 3 deques inside
        // with different sizes

But the implementation I have given above does not work. Help me please... thanks...

One, you're not following the rule of three. Two, why on earth do you have pointers? Just store the objects. — chris, Feb 01 '13 at 17:06
"does not work" is a bad description. _How_ does it not work? — Some programmer dude, Feb 01 '13 at 17:08
You have to add here what's the compiler saying or what's the problem — eLRuLL, Feb 01 '13 at 17:10
"doesnot work" means *r *g and *b are pointing to 0xccccccccccc — deathmood, Feb 01 '13 at 17:11
[Rule of Three](http://stackoverflow.com/questions/4172722/what-is-the-rule-of-three). Better: [Rule of Zero](http://flamingdangerzone.com/cxx11/2012/08/15/rule-of-zero.html). — R. Martinho Fernandes, Feb 01 '13 at 17:11
By the way, as for the delegation attempt, http://en.wikipedia.org/wiki/C%2B%2B11#Object_construction_improvement — chris, Feb 01 '13 at 17:11

user786653 · Answer 1 · 2013-02-01T17:23:59.417

1

If you compiler supports constructor delegation (which e.g. Clang 3.2 does) you can do the following:

    Rgb(int n) : Rgb(n, n, n)
    {
    }

What you're doing is constructing a temporary Rgb instance in the constructor, which will fail once the uninitialized r, g and b get deleted.

Otherwise I'd recommend either creating a common init function that does the initialization or simply duplicate the code. And as the comments are noting you shouldn't use new here (that's almost always a code smell in modern C++ code). Bringing this together:

template <class T>
struct Rgb
{
    T r, g, b;

    Rgb(int nr, int ng, int nb) : r(nr), g(ng), b(nb)
    { }

    Rgb(int n) : r(n), g(n), b(n)
    { }

    ~Rgb()
    { }
};

edited Feb 01 '13 at 17:23

answered Feb 01 '13 at 17:14

user786653

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VC11 supports constructor delegation if you install the [Nov 2012 CTP](https://www.microsoft.com/en-us/download/details.aspx?id=35515) – Praetorian Feb 01 '13 at 17:17
Thanks)) That solved my problem)) I did not even knew about NOV CTP for VC11)) – deathmood Feb 01 '13 at 17:44

C++ how call template's class constructor

1 Answers1