c++ - 来自 gcc 原子 header 的对我的类静态字段错误的 undefined reference

Question

全部!

我有一个静态原子字段，定义如下:

private:
  static std::atomic<int> total_requests_;

在名为 FcgiRequestPool 的类中。

在这个类方法中我使用它:

FcgiRequestPool::FcgiRequestPool() {
  // the first born pool will set total request counter to zero.
  static std::once_flag total_requests_init_flag;
  std::call_once(total_requests_init_flag, []() {
    total_requests_ = 0;
  });
}

到目前为止，我没有看到任何犯罪行为。当我编译它时...

set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} --std=c++14 -O2 -Wall -Wextra -pedantic -Werror )

我得到这个错误:

CMakeFiles/libdolly.dir/kanaria/FcgiRequestPool.cpp.o: In function `std::__atomic_base<int>::store(int, std::memory_order)':
/usr/lib/gcc/x86_64-pc-linux-gnu/4.9.3/include/g++-v4/bits/atomic_base.h:478: undefined reference to `FcgiRequestPool::total_requests_'
CMakeFiles/libdolly.dir/kanaria/FcgiRequestPool.cpp.o: In function `std::__atomic_base<int>::fetch_add(int, std::memory_order)':
/usr/lib/gcc/x86_64-pc-linux-gnu/4.9.3/include/g++-v4/bits/atomic_base.h:618: undefined reference to `FcgiRequestPool::total_requests_'
/usr/lib/gcc/x86_64-pc-linux-gnu/4.9.3/include/g++-v4/bits/atomic_base.h:618: undefined reference to `FcgiRequestPool::total_requests_'
CMakeFiles/libdolly.dir/kanaria/FcgiRequest.cpp.o: In function `FcgiRequest::FcgiRequest(std::shared_ptr<FcgiRequestPool>)':
/home/rakul/play/kanaria/git/src/kanaria/FcgiRequest.cpp:13: undefined reference to `FCGX_InitRequest'
CMakeFiles/libdolly.dir/kanaria/FcgiRequest.cpp.o: In function `FcgiRequest::FinishRequest()':
/home/rakul/play/kanaria/git/src/kanaria/FcgiRequest.cpp:30: undefined reference to `FCGX_Finish_r'

由于我显然进行了简短的谷歌搜索，所以在您建议添加 -latomic 之前，这是我的简单测试:

rakul@lucky-star /tmp $ cat test.cpp
#include <atomic>
#include <iostream>

int main() {
    std::atomic<int> i(42);
    std::cout << i.fetch_sub(1) << std::endl;
    return 0;
}
rakul@lucky-star /tmp $ g++ --std=c++14 test.cpp
rakul@lucky-star /tmp $ ./a.out 
42
rakul@lucky-star /tmp $ 

我认为，这应该证明在我的系统(gentoo、gcc 4.9.3 p1.4)上原子能编译、工作并且不需要额外的库。

非常欢迎任何帮助。谢谢!

Answer 1

您只将静态成员变量添加到类的定义中。您还必须在实现文件中实际定义静态变量。

将其添加到 .CPP 文件中:

std::atomic<int> ClassName::total_requests_;

如果没有这个，变量实际上不会分配任何存储或实例化，也无法使用。