元素少于结构的 C++ 初始值设定项列表

Question

当我使用初始化列表创建结构时，但初始化列表包含的元素少于我的结构，我看到剩余的元素被初始化为零。

这是未定义的行为吗？我看到零是因为我的编译器 (VS2015) 决定为我将内存归零吗？

或者有人可以指点我解释 C++ 中这种行为的文档吗？

这是我的代码:

struct Thing {
    int value;
    int* ptr;
};


void main() {
    Thing thing { 5 };
    std::cout << thing.value << " " << thing.ptr << std::endl;
}

这是它打印的内容:

5 00000000

最后一个元素是在没有初始化器的情况下被置零的元素。

Answer 1

这是定义的行为。根据aggregate initialization的规则，其余成员 ptr 将为 value-initialized ，即 zero-initialized到 NULL 指针。

(强调我的)

If the number of initializer clauses is less than the number of members and bases (since C++17) or initializer list is completely empty, the remaining members and bases (since C++17) are initialized by their default initializers, if provided in the class definition, and otherwise (since C++14) by empty lists, in accordance with the usual list-initialization rules (which performs value-initialization for non-class types and non-aggregate classes with default constructors, and aggregate initialization for aggregates). If a member of a reference type is one of these remaining members, the program is ill-formed.