In place member initialization and aggregate initialization

Question

I have this simple struct and a function taking it:

struct S
{
    int a;
};

void foo(S){}

foo({5});

This works fine.

But if I change int a; to int a{0}; VisualStudio (2013 and 2015) complains:

error C2664: 'void foo(S)': cannot convert argument 1 from 'initializer list' to 'S'

I can't find corresponding rule for this in the documentation. But both gcc and clang accept this without problem.

@songyuanyao, I see it works, but here http://webcompiler.cloudapp.net/ (which is the same major version) it doesn't. Now I think the behavior depends on some compiler option. — nikitablack, Mar 03 '16 at 11:48
@nikitablack: Your sample with any of `int a{0};` or `int a = 0;` works on webcompiler.cloudapp.net for me... — Jarod42, Mar 03 '16 at 12:40
@Jarod42, songyuanyao, I apologize but I didn't mentioned in the post that you should call `foo()` with aggregate initialization: `foo({5});` — nikitablack, Mar 03 '16 at 13:05
For gcc/clang `foo({5});` compiles in c++14 but not in C++11. I think aggregate rule change from those 2 standards. (`S` is now an aggregate). — Jarod42, Mar 03 '16 at 13:23
See [what-are-aggregates-and-pods-and-how-why-are-they-special-c++14](http://stackoverflow.com/a/27511360/2684539) — Jarod42, Mar 03 '16 at 13:26

score 2 · Accepted Answer · answered Mar 03 '16 at 13:28

struct S
{
    int a;
};

is an aggregate whereas

struct S
{
    int a {0}; // or int a = 0;
};

is not an aggregate in c++11, but is in c++14.

VisualStudio (2013 and 2015) still uses the c++11 rules in this regard.

foo({5}); is valid for aggregate. For non aggregate, it will (try to) call appropriate constructor, but S doesn't have one valid for this argument.

In place member initialization and aggregate initialization

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