A C = {0,0};

执行aggregateA的value initialization.根据标准,可以省略总括初始化的大括号：

8.5.1聚合[dcl.init.aggr] / 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 members of
a subaggregate; it is erroneous for there to be more
initializer-clauses than members. If,however,the initializer-list
for a sub- aggregate does not begin with a left brace,then only
enough initializer-clauses from the list are taken to initialize the
members of the subaggregate; any remaining initializer-clauses are
left to initialize the next member of the aggregate of which the
current subaggregate is a member.

[Example:

06001

is a completely-braced initialization: 1,3,and 5 initialize the first row of the array y[0],namely y[0][0],
y[0][1],and y[0][2]. Likewise the next two lines initialize y[1] and
y[2]. The initializer ends early and therefore y[3]’s elements are
initialized as if explicitly initialized with an expression of the
form float(),that is,are initialized with 0.0. In the following
example,braces in the initializer-list are elided; however the
initializer-list has the same effect as the completely-braced
initializer-list of the above example,

06002

The initializer for y begins with a left brace,but the one for y[0] does not,therefore three elements
from the list are used. Likewise the next three are taken successively
for y[1] and y[2]. — end example ]

下一个

8.5.1聚合[dcl.init.aggr] / 7

If there are fewer initializer-clauses in the list than there are
members in the aggregate,then each member not explicitly initialized
shall be initialized from its brace-or-equal-initializer or,if there
is no brace-or-equal- initializer,from an empty initializer list.

在你的情况下,这意味着第一个0被分配给B [0],第二个0分配给B [1].然后根据8.5.1 / 7,其余的元素是值初始化的.

但是,为了清楚这种情况,您应该使用A C = {{0},0};或者更好

A C{}; // or A C = {};

唯一让我担心的是g警告(-Wextra)：

warning: missing initializer for member ‘main()::A::D’
[-Wmissing-field-initializers] A C {0,0};

但是根据我对上述标准的解释,你应该是OK,D应该被初始化.我甚至测试了一些新的布局,结果是如预期的

#include <iostream>

int main()
{
    struct A { int B[100]; int D;};
    A memory{};
    memory.D = 42; 
    std::cout << memory.D << std::endl;

    // let's place something an A at the location of memory
    A* foo = new (&memory) A{0,0}; 
    // line below outputs 0,so D is erased; not the case if A* foo = new (&memory) A; 
    std::cout << memory.D << std::endl; // outputs 0
}