c++ - 为什么 NULL 不能作为 std::thread 执行函数的 void* 参数的参数？

Question

这是一个最小的案例:

#include <iostream>
#include <functional>
#include <thread>
void fun(void *args) {}
int main() {
  std::thread thread_id(fun, NULL);
}

g++ 编译失败:

[firstlove@manjaro misc]$ g++ thread_and_shared_resources.cpp -lpthread -std=c++17
In file included from thread_and_shared_resources.cpp:3:
/usr/include/c++/9.3.0/thread: In instantiation of 'std::thread::thread(_Callable&&, _Args&& ...) [with _Callable = void (&)(void*); _Args = {long int}; <template-parameter-1-3> = void]':
thread_and_shared_resources.cpp:6:34:   required from here
/usr/include/c++/9.3.0/thread:120:44: error: static assertion failed: std::thread arguments must be invocable after conversion to rvalues
  120 |           typename decay<_Args>::type...>::value,
      |                                            ^~~~~
/usr/include/c++/9.3.0/thread: In instantiation of 'struct std::thread::_Invoker<std::tuple<void (*)(void*), long int> >':
/usr/include/c++/9.3.0/thread:131:22:   required from 'std::thread::thread(_Callable&&, _Args&& ...) [with _Callable = void (&)(void*); _Args = {long int}; <template-parameter-1-3> = void]'
thread_and_shared_resources.cpp:6:34:   required from here
/usr/include/c++/9.3.0/thread:243:4: error: no type named 'type' in 'struct std::thread::_Invoker<std::tuple<void (*)(void*), long int> >::__result<std::tuple<void (*)(void*), long int> >'
  243 |    _M_invoke(_Index_tuple<_Ind...>)
      |    ^~~~~~~~~
/usr/include/c++/9.3.0/thread:247:2: error: no type named 'type' in 'struct std::thread::_Invoker<std::tuple<void (*)(void*), long int> >::__result<std::tuple<void (*)(void*), long int> >'
  247 |  operator()()
      |  ^~~~~~~~
[firstlove@manjaro misc]$ g++ thread_and_shared_resources.cpp -lpthread -std=c++17
In file included from thread_and_shared_resources.cpp:3:
/usr/include/c++/9.3.0/thread: In instantiation of 'std::thread::thread(_Callable&&, _Args&& ...) [with _Callable = void (&)(void*); _Args = {long int}; <template-parameter-1-3> = void]':
thread_and_shared_resources.cpp:6:34:   required from here
/usr/include/c++/9.3.0/thread:120:44: error: static assertion failed: std::thread arguments must be invocable after conversion to rvalues
  120 |           typename decay<_Args>::type...>::value,
      |                                            ^~~~~
/usr/include/c++/9.3.0/thread: In instantiation of 'struct std::thread::_Invoker<std::tuple<void (*)(void*), long int> >':
/usr/include/c++/9.3.0/thread:131:22:   required from 'std::thread::thread(_Callable&&, _Args&& ...) [with _Callable = void (&)(void*); _Args = {long int}; <template-parameter-1-3> = void]'
thread_and_shared_resources.cpp:6:34:   required from here
/usr/include/c++/9.3.0/thread:243:4: error: no type named 'type' in 'struct std::thread::_Invoker<std::tuple<void (*)(void*), long int> >::__result<std::tuple<void (*)(void*), long int> >'
  243 |    _M_invoke(_Index_tuple<_Ind...>)
      |    ^~~~~~~~~
/usr/include/c++/9.3.0/thread:247:2: error: no type named 'type' in 'struct std::thread::_Invoker<std::tuple<void (*)(void*), long int> >::__result<std::tuple<void (*)(void*), long int> >'
  247 |  operator()()
      |  ^~~~~~~~

为什么这里不能传递NULL？使用 NULL 对 fun 的独立调用工作得很好:

fun(NULL) // compiles happily

Answer 1

NULL 在您的实现中扩展为 0，并且 0 是一个整数，而不是指针。
在某些上下文中它可以转换为指针，但在其他上下文中则不能；这是后者的一个例子。

使用nullptr，它解决了很多类似的问题。