c++ - 未调用已初始化静态 thread_local 结构的构造函数和析构函数

Question

下面的代码应该初始化一个静态线程局部和静态结构。

#include <iostream>

struct Tracer {
public:
  Tracer(const char *new_name) : name{new_name} {
    printf("%s : Constructor()\n", this->name);
  }

  ~Tracer() {
    printf("%s : Destructor()\n", this->name);
  }

private:
  const char *name;
};

// 1. Thread-Local Storage Duration
static thread_local Tracer t_thread_local{"Thread-Local Storage Duration"};

// 2. Static Storage Duration
static Tracer t_static{"Static Storage Duration"};

int main() {
  printf("Start Program\n");
}

但是，我没有看到来自静态线程本地结构构造函数/析构函数的预期消息。打印的输出仅显示来自 static 结构的消息。我错过了什么吗？

Static Storage Duration : Constructor()
Start Program
Static Storage Duration : Destructor()

Answer 1

您认为仅声明和定义这些对象就一定会触发预main 构造的假设是不正确的。

[basic.stc.thread/2]: [ Note: A variable with thread storage duration is initialized as specified in [basic.start.static], [basic.start.dynamic], and [stmt.dcl] and, if constructed, is destroyed on thread exit ([basic.start.term]). — end note ]

[basic.start.dynamic/5]: It is implementation-defined whether the dynamic initialization of a non-local non-inline variable with static storage duration is sequenced before the first statement of main or is deferred. If it is deferred, it strongly happens before any non-initialization odr-use of any non-inline function or non-inline variable defined in the same translation unit as the variable to be initialized. [..]

实际上 basic.start.dynamic 中有许多类似的规则。

重点是，您的程序不会做太多事情，而且它肯定不会使用 t_thread_local，因此由编译器决定 t_thread_local 是否会使用确实存在，如果存在，何时存在。