警告 C5246: 子对象的初始化应该用大括号括起来

Question

最新的 Visual Studio 2019 使用 /Wall 命令行开关（启用所有警告）编译以下代码：

struct A{};

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

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

打印警告：

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

但是 GCC 和 Clang 都接受带有最迂腐错误检查的代码，演示：https://gcc.godbolt.org/z/Ps6K6jK1G

确实，所有编译器都接受调用 f( { { x, y } } );

MSVC 建议使用没有额外大括号的更简单形式是否正确？

Answer 1

我相信 Clang 和 GCC 现在默认抑制该错误，因此警告不会透露实施细节。话虽如此，VS 过去不会给我使用 C++11 的错误，而 Clang 会（所以他们似乎已经解决了这个问题）。所有文档都表明双花括号是最合适的，添加它们以匹配行业标准代码实践可能是有意义的。

Answer 2

Visual Studio 2019 具有 CWG defect #1270 并提供旧的初始化行为。

std::array 定义为包含另一个聚合的聚合。

C++ 17 标准（11.6.1 聚合）

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.

您可以将 std::array<A, 2> 想象成

struct Array {
    A sub[2];
};

根据引用 Array a {{x, y}} 是正确的，但已过时，内部大括号是封闭数组的 aggregate initialization。

有用的线程Why is the C++ initializer_list behavior for std::vector and std::array different?。

警告 C5246: 子对象的初始化应该用大括号括起来

warning C5246: the initialization of a subobject should be wrapped in braces

c++

compiler-warnings

visual-studio

language-lawyer