How to build std::array like data structure from std::initializer_list

Question

I want to implement a data structure with compile time constant size(like std::array). I want to be able to initialize this data structure like this:

MyStruct<3, int> example = {1, 2, 3};

This work fine using constructor like: MyStruct(std::initializer_list<T> elements), but the compiler doesn't enforce the same size for my internal structure and elements, even if they are both known at compile time.

I can't use static_assert because elements.size() is not compile time constant.

Is there any way to enforce at compile time same size for elements as in MyStruct?

Zulan · Accepted Answer · 2013-07-31 14:20:47Z

2

You could try a constructor using variadic templates:

template<std::size_t N, typename E>
struct MyStruct {
    int otherStuff;
    E values[N];

    template<typename ...TT>
    MyStruct(TT&&...t) : values{std::forward<TT>(t)...} {
        static_assert(N == sizeof...(t), "Size mismatch!");
        for (size_t i = 0; i < N; i++) std::cout << values[i] << ",";
        std::cout << std::endl;
    }
};

This does work as expected with:

MyStruct<3, int> example = {1,2,3};
MyStruct<3, int> exampleFail = {1,2}; //error: static assertion failed: Size mismatch!

Please note that there is still a difference between std:array and MyStruct when it comes to list-initialization:

MyStruct<3, int> exampleList{1,2,3}; // works
std::array<int, 3> arr = {1,2,3};    // works, but warning with clang++
std::array<int, 3> arrList{1,2,3};   // works with g++, does not compile with clang++

The reason is, that the single brace only works for std::array because of brace elision, which does not always apply as documented in an official defect report.

answered Jul 31, 2013 at 14:20

Zulan

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2 Comments

Mircea Ispas Over a year ago

Very nice, thanks. I already tried this but there is a warning if I add default constructor - multiple default constructors specified(VS2013 preview)

Zulan Over a year ago

With g++ or clang++ i don't get a warning when I add MyStruct() = default;. You may be able to use some enable_if trickery for that, but I can't come up with something right now that works. This could also allow you to only enable the constructor with the right number of arguments instead of triggering the assertion. See seanmiddleditch.com/journal/2012/03/…

sasha.sochka · Accepted Answer · 2013-07-31 14:09:22Z

1

std::array does not have a constructor! And it doesn't use initializer_list, instead it uses uniform initialization. (similar to POD structures, actually std::array is POD structure).

So what std::array really does is very similar to the following:

template<int size, typename T>
struct array {
   T data[size];
   // and some member function here;
   // Warning! No constructors !
};

When, later, you write

std::array<3, int> arr = {1,2,3};

It is equivalent to

std::array<3, int> arr = {{1,2,3}};

And this is just POD initialization where {1,2,3} is assigned to data.

If you want to enforce having the same size checking in compile time it's possible using std::array itself instead of std::initializer_list.

If you change your initializations from {1,2,3} to std::array<3, int>{1, 2, 3} and constructor argument from std::initializer_list<T> to std::array<SIZE, T> user will be enforced to pass array with size SIZE.

edited Jul 31, 2013 at 14:09

answered Jul 31, 2013 at 13:33

sasha.sochka

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4 Comments

Mircea Ispas Over a year ago

MyStruct has compile time constant size LIKE std::array, it's not std::array. I need contructors to initialize with default values if there are no values provided.

sasha.sochka Over a year ago

But std::array<3, int> arr = {1,2} does initialize with default value arr[2] although you didn't provide it yourself. arr[2] will be equal to 0

Mircea Ispas Over a year ago

But... maybe default value is not 0. Think about a color where I want to use default value 255 or 1.f, depending on color representation.

Mircea Ispas Over a year ago

Thanks for the answer. I won't accept it for now - I need to use clasic syntanx to build MyStruct. Using it like this is very confusing.

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