C++11: Correct std::array initialization?

c++ arrays c++11 initialization c++14
Byzantian picture Byzantian · Jan 6, 2013 · Viewed 75.3k times · Source

If I initialize a std::array as follows, the compiler gives me a warning about missing braces 

std::array<int, 4> a = {1, 2, 3, 4};

This fixes the problem:

std::array<int, 4> a = {{1, 2, 3, 4}};

This is the warning message:

missing braces around initializer for 'std::array<int, 4u>::value_type [4] {aka int [4]}' [-Wmissing-braces]

Is this just a bug in my version of gcc, or is it done intentionally? If so, why?

Answer

Pubby picture Pubby · Jan 6, 2013

This is the bare implementation of std::array: 

template<typename T, std::size_t N>
struct array {
    T __array_impl[N];
};

It's an aggregate struct whose only data member is a traditional array, such that the inner {} is used to initialize the inner array.

Brace elision is allowed in certain cases with aggregate initialization (but usually not recommended) and so only one brace can be used in this case. See here: C++ vector of arrays

Related questions

Now that we have std::array what uses are left for C-style arrays?

std::array is vastly superior to the C arrays. And even if I want to interoperate with legacy code, I can just use std::array::data(). Is there any reason I would ever want an old-school array?

Read More
Simpler way to set multiple array slots to one value

I'm coding in C++, and I have the following code: int array[30]; array[9] = 1; array[5] = 1; array[14] = 1; array[8] = 2; array[15] = 2; array[23] = 2; array[12] = 2; //... Is there a way to initialize the array similar to the following? int array[30]; array[9,5,14] = 1; array[8,15,23,12] = 2; //... Note: In the actual code, …

Read More
How to emulate C array initialization "int arr[] = { e1, e2, e3, ... }" behaviour with std::array?

(Note: This question is about not having to specify the number of elements and still allow nested types to be directly initialized.) This question discusses the uses left for a C array like int arr[20];. On his answer, @James Kanze …

Read More