1

The latest Visual Studio 2019 compiling the following code with /Wall command-line switch (which enables all warnings):

struct A{};

void f( const std::array<A, 2> & ) {}

int main() {
    A x, y;
    f( { x, y } );
}

prints the warning:

warning C5246: 'std::array<A,2>::_Elems': the initialization of a subobject should be wrapped in braces

however both GCC and Clang accepts the code with the most pedantic error checking, demo: https://gcc.godbolt.org/z/Ps6K6jK1G

Indeed, all compilers accept the call f( { { x, y } } );

Is MSVC right in suggesting it in favor of the simpler form without extra braces?

Fedor
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  • The warning has been removed in Visual Studio 2022 version 17.2: https://developercommunity.visualstudio.com/t/Outdated-warning-C5246:-the-initializati/1597135 – Fedor May 14 '22 at 11:23

2 Answers2

3

Visual Studio 2019 has the CWG defect #1270 and offers the old initialization behaviour.

std::array is defined as an aggregate that contains another aggregate.

C++ 17 Standard (11.6.1 Aggregates)

12 Braces can be elided in an initializer-list as follows. If the initializer-list begins with a left brace, then the succeeding comma-separated list of initializer-clauses initializes the elements of a subaggregate; it is erroneous for there to be more initializer-clauses than elements. If, however, the initializer-list for a subaggregate does not begin with a left brace, then only enough initializer-clauses from the list are taken to initialize the elements of the subaggregate; any remaining initializer-clauses are left to initialize the next element of the aggregate of which the current subaggregate is an element.

You may imagine std::array<A, 2> as

struct Array {
    A sub[2];
};

According to the quote Array a {{x, y}} is correct, but obsolete, the inner braces is aggregate initialization of the enclosed array.

Useful thread Why is the C++ initializer_list behavior for std::vector and std::array different?.

273K
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0

I believe that Clang and GCC now suppress that error by default so that the warning does not reveal details of the implementation. With that said, VS used to not give me an error using C++11 and Clang did (so they seemed to have flipped on the issue). All documentation suggests the double curly braces are the most appropriate it may make sense to add them to match industry standard code practices.

Max Dylan
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