std::array implicit initialization in function call

Question

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When can outer braces be omitted in an initializer list?

I'm using std::array in Visual Studio 2010, which is actually std::tr1::array and I'm running into an annoying issue. I have a function which takes an array as an argument, for instance.

void do_something(std::tr1::array<int, 5> data)

Calling the function like do_something({1,2,3}); doesn't work and results in a compiler error, however

std::tr1::array<int, 5> data = {1,2,3};
do_something(data);

does. I don't really see why the former wouldn't work. The errors I get tell me I'm missing a ) before the {. This leads me to believe it's not expecting an initialization list, but I don't see why not. Am I misusing initialization lists?

edit: std::tr1::array isn't necessary, std::array works fine..

score 3 · Accepted Answer · edited May 23 '17 at 12:27

3

Try this

do_something({{1,2,3}});

std::array uses 2 sets of braces to initialize, but in certain cases (such as that example you posted) you can elide the outer spurious braces. This might help: C++11: Correct std::array initialization?

edited May 23 '17 at 12:27

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answered Jan 07 '13 at 06:35

Pubby

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same here, same compiler.. – Karthik T Jan 07 '13 at 06:36
It works on my GCC: http://stacked-crooked.com/view?id=0db4e5670d751282ca5cd244708bd38a I suppose MSVC doesn't support brace initialization fully. – Pubby Jan 07 '13 at 06:38
yeah, OP mentioned VS2010 I guess it's not supported so far – billz Jan 07 '13 at 06:39
Yeah looks like VS2010 doesn't support it, I just assumed it would because it does outside a function call. Oh well! – user1520427 Jan 07 '13 at 06:40

Nawaz · Answer 2 · 2013-01-07T06:57:28.897

The extra braces is needed because std::array is an aggregate and POD, unlike other containers in the standard library. std::array doesn't have user-defined constructor. It's first data member is an array of size N (which you pass as template argument), and this member is directly initialized with initializer. The extra braces is needed for the internal array which is directly being initialized.

Think of this as:

struct A
{
     int data[2];
};

How many braces you need when you create an instance of A ?

A a{1,2};      //wrong as per the Standard (the compiler might allow it)
A b{ {1,2 } }; //correct

In b, the outer pair of {} is used for the struct, and inner pair of {} is used for the member-array.

Hope that helps.

That makes sense, but apparently not to my compiler! Guess it's not fully supported in VS2010. — user1520427, Jan 07 '13 at 06:42

std::array implicit initialization in function call

2 Answers2