Initializing fixed number of variables with an initializer list

Question

I want to initialize an object with an initializer list. The problem is, an initializer list is able to contain unpredictable number of elements, but I need to initialize only for variables. The user may send any number of list elements, and I need only four of them.

I wrote the following code, but it looks like very long and inefficient to me. Is there any better way of doing this?

Pixel::Pixel(std::initializer_list<uint8_t> Channels)
{
    switch (Channels.size())
    {
        case 0:
            R = 0;
            G = 0;
            B = 0;
            A = 0;
            break;
        case 1:
            R = *(Channels.begin() + 0);
            G = 0;
            B = 0;
            A = 0;
            break;
        case 2:
            R = *(Channels.begin() + 0);
            G = *(Channels.begin() + 1);
            B = 0;
            A = 0;
            break;
        case 3:
            R = *(Channels.begin() + 0);
            G = *(Channels.begin() + 1);
            B = *(Channels.begin() + 2);
            A = 0;
            break;
        default:
            R = *(Channels.begin() + 0);
            G = *(Channels.begin() + 1);
            B = *(Channels.begin() + 2);
            A = *(Channels.begin() + 3);
    }
}

(Note: I know this can be done with passing the R, G, B, A values with four separate arguments. But my main purpose is to learn how to do this with the initializer list feature.)

If you always have exactly 4 elements, why not take a const std::array<uint8_t, 4>&? — Baum mit Augen
– Baum mit Augen ♦, Commented Dec 16, 2014 at 10:04
What's wrong with good 'ol Pixel::Pixel(uint8_t r = 0, uint8_t g = 0, uint8_t b = 0, uint8_t a = 0)? It's simple and runs on anything on or above C++03 and doesn't force the client code to create an array. — legends2k
– legends2k, Commented Dec 16, 2014 at 10:10
@legends2k That constructor already exists. I want to implement it with the initializer list in order to learn it. I have never used it before and I'm trying to gain some experience. — hkBattousai
– hkBattousai, Commented Dec 16, 2014 at 10:12
@hkBattousai Well, your problem with the initializer list has demonstrated why it's a poor choice in this case. You already have the optimal solution. — eerorika
– eerorika, Commented Dec 16, 2014 at 10:14

Richard Hodges · Accepted Answer · 2014-12-16 10:41:12Z

4

the best I could come up with when using std::initialiser_list

struct Pixel
{
    Pixel(std::initializer_list<uint8_t> rgba)
    : _rgba { rgba }
    {
        switch(_rgba.size()) {

            case 0: _rgba.push_back(0);
            case 1: _rgba.push_back(0);
            case 2: _rgba.push_back(0);
            case 3: _rgba.push_back(0);
            case 4: break;
            default:
                throw std::invalid_argument { "" };
        }

    }

    std::vector<uint8_t> _rgba;
};

... but ...

Probably the correct way to solve this problem is this:

struct Pixel
{
    Pixel(uint8_t r = 0, uint8_t g = 0, uint8_t b = 0, uint8_t a = 0)
    : R(r)
    , G(g)
    , B(b)
    , A(a)
    {}

    uint8_t R,G,B,A;
};

because

It will fail to compile if you provide invalid arguments
It's optimally efficient
It's the least surprising solution for anyone maintaining your code
It automatically supports the initializer_list syntax at the call site

examples:

int main()
{
    Pixel p1 { 10, 20, 5, 255 };
    Pixel p2 { 10, 20, 5 };
    Pixel p3 { 10, 20 };
    Pixel p4 { 10 };
    Pixel p5 { };

    Pixel pv1 ( 10, 20, 5, 255 );
    Pixel pv2 ( 10, 20, 5 );
    Pixel pv3 ( 10, 20 );
    Pixel pv4 ( 10 );
    Pixel pv5;

    return 0;
}

edited Dec 16, 2014 at 10:41

answered Dec 16, 2014 at 10:25

Richard Hodges

70.3k8 gold badges103 silver badges157 bronze badges

Sign up to request clarification or add additional context in comments.

2 Comments

hkBattousai Over a year ago

I didn't know the rule number 4. I can't find it on Google. Can you please give me a link?

Richard Hodges Over a year ago

since c++11 value-initialisation takes place when a named variable is constructed with braces: see point 4 in the explanation: en.cppreference.com/w/cpp/language/value_initialization

Jarod42 · Accepted Answer · 2014-12-16 10:32:19Z

4

You may rewrite you switch as follow:

Pixel::Pixel(std::initializer_list<uint8_t> Channels) : R(0), G(0), B(0), A(0)
{
    switch (Channels.size())
    {
        default: // Too many args
        case 4: A = *(Channels.begin() + 3); // No break: Follow next line
        case 3: B = *(Channels.begin() + 2); // No break: Follow next line
        case 2: G = *(Channels.begin() + 1); // No break: Follow next line
        case 1: R = *(Channels.begin() + 0);
        case 0: break;
    }
}

answered Dec 16, 2014 at 10:32

Jarod42

228k15 gold badges215 silver badges354 bronze badges

1 Comment

hkBattousai Over a year ago

Splendid solution. Thank you.

Collectives™ on Stack Overflow

Initializing fixed number of variables with an initializer list

2 Answers 2

2 Comments

1 Comment

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

2 Comments

1 Comment

Your Answer

Sign up or log in

Post as a guest

Related