C++ function call with default argument in std::array?

Question

Now I have a function in C++

void F( std::array<int,3> x )
{
    //...
}

I hope the argument 'x' could have a default value, how can I do this?

If not a function argument, I can simply use

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

But for a function argument, the code

void F( std::array<int,3> x = {1,2,3} )
{
    //...
}

will make compiler error.

I test in MSVC 2012, and got error C2143, C2059, C2447. And also error in g++ 4.6.3

Is there any way make it has a default value?

thanks.

What compiler error? Which compiler? (It might be a bug.. clang3.2 accepts your code; and I think your example complies to the Standard - it's aggregate-initialization) — dyp
– dyp, Commented May 2, 2013 at 11:20
I had add the comiple error. It looks that is my compiler not support this syntax yet... — Heresy
– Heresy, Commented May 2, 2013 at 11:36
I found a stupid workaround...Use lambda expression. void F( std::array<int,3> x = [](){std::array<int,3> x = {1,2,3}; return x; }() ){} This works both on MSVC11 and G++ 4.6.3. — Heresy
– Heresy, Commented May 2, 2013 at 12:24
It looks like this was not implemented yet in gcc 4.6.3, see this SO question. Consider looking at Microsoft Connect and file a bug report if it hasn't been detected yet. AFAIK, Morwenn's answer is wrong and this should be possible according to the Standard. — dyp
– dyp, Commented May 2, 2013 at 12:38
@DyP After reading again the standard, I think my answer is indeed wrong: he should be able to initialize the array the way he does. However, the work-around is not wrong: the standard says that braces can be elided, not that they have to. — Morwenn
– Morwenn, Commented May 2, 2013 at 13:40

Morwenn · Accepted Answer · 2013-05-02 13:44:38Z

11

Your solution should work according to the standard, but is not implemented in some compilers. Most of them can initialize instances of std::array with the syntax x = {{1,2,3}}, not with x = {1, 2, 3}. If you want it to work as of today, your function should be:

void F( std::array<int,3> x = {{1,2,3}} )
{
    //...
}

This is because std::array just has a C array underneath and initialize it with aggregate initialization. The first pair of braces are for the list initialization list while the second pair of braces is for the C-array initialization.

According to the standard (8.5.1.11), the outer braces can be elided in such a case if (and only if) you use the sign = for initialization. However, some compilers still do not support this behaviour (g++ being one of them).

And as a bonus, you can check it online with ideone.

edited May 2, 2013 at 13:44

answered May 2, 2013 at 10:08

Morwenn

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12 Comments

juanchopanza Over a year ago

Note that the standard is not totally clear about the implementation having an array data member, which can lead to confusion.

dyp Over a year ago

I'm not entirely sure about this. The Standard says in [array.overview]/2 that an array is an aggregate that can be initialized like std::array<int,3> a = {1,2,3};. There have to be two braces for direct-init, std::array<int,3> a{{1,2,3}};

Heresy Over a year ago

@Morwenn Thanks for your answer, but the code still get compile error in MSVC 2012.

dyp Over a year ago

@Heresy Initializer-lists are not supported in MSVC 2012, see SO question, MSDN reference

dyp Over a year ago

@Heresy Maybe with the November 2012 CTP version

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