c++ - 为什么 std::string 是标准布局类型？

Question

以此处为例:trivial vs. standard layout vs. POD

以下代码通过:

struct T {
public:
    int i;
private:
    int j;
};

static_assert(! std::is_standard_layout<T>::value, "");

但以下不是:

static_assert(! std::is_standard_layout<std::string>::value, "");

所以如果一个类型不是标准布局，那么 std::string 怎么可能是一个呢？

Answer 1

让我们看看标准布局的实际规则:

[C++14: 9/7]: A standard-layout class is a class that:

• has no non-static data members of type non-standard-layout class (or array of such types) or reference,
• has no virtual functions (10.3) and no virtual base classes (10.1),
• has the same access control (Clause 11) for all non-static data members,
• has no non-standard-layout base classes,
• either has no non-static data members in the most derived class and at most one base class with non-static data members, or has no base classes with non-static data members, and
• has no base classes of the same type as the first non-static data member.

std::string 可能没有 public 数据成员(它们会是什么？)，这是您用 T 绊倒的地方> (因为现在您同时拥有 private 和 public 数据成员；请参阅大胆的段落)。

但据我所知，没有实际要求 std::string 成为标准布局。这就是您的实现方式。