C/C++编程：future_error详解

喜欢ヅ旅行 2023-02-28 05:47 68阅读 0赞

# 引言 #

遇到这个问题以后第一次没有得到解答 随后经测试和查阅资料知道了它的由来 同时列出三种导致抛出future\_error的原因 希望能有相同问题的朋友能得到帮助

# 什么是future\_error #

![在这里插入图片描述][20200722103954474.png]

这是官网对这个类型的解释

> The class std::future\_error defines an exception object that is thrown on failure by the functions in the thread library that deal with asynchronous execution and shared states

我们可以看到它是在处理异步程序和shared 状态失败的时候会被抛出.而且它本身是一个各个系统上实现不一致的东西,即ub

我们来看看官网的演示demo

#include <future>
    #include <iostream>
     
    int main()
    {
        
        std::future<int> empty;
        try {
        
            int n = empty.get(); // The behavior is undefined, but
                                 // some implementations throw std::future_error
        } catch (const std::future_error& e) {
        
            std::cout << "Caught a future_error with code \"" << e.code()
                      << "\"\nMessage: \"" << e.what() << "\"\n";
        }
    }

我们可以看到在future的状态不是ready的时候我们进行get操作是ub,但是一些实现则会抛错,那么还有哪些情况会抛错呢

我们来简单的看看future的实现

void
          set_value(const _Res& __r)
          {
         _M_future->_M_set_result(_State::__setter(this, __r)); }
    
          void
          set_value(_Res&& __r)
          {
         _M_future->_M_set_result(_State::__setter(this, std::move(__r))); }
    
          void
          set_exception(exception_ptr __p)
          {
         _M_future->_M_set_result(_State::__setter(__p, this)); }
    
    		--------------------------------------------------------------------
    
          _M_set_result(function<_Ptr_type()> __res, bool __ignore_failure = false)
          {
        
    	bool __did_set = false;
            // all calls to this function are serialized,
            // side-effects of invoking __res only happen once
    	call_once(_M_once, &_State_baseV2::_M_do_set, this,
    		  std::__addressof(__res), std::__addressof(__did_set));
    	if (__did_set)
    	  // Use release MO to synchronize with observers of the ready state.
    	  _M_status._M_store_notify_all(_Status::__ready,
    					memory_order_release);
    	else if (!__ignore_failure)
              __throw_future_error(int(future_errc::promise_already_satisfied));
          }

我们可以看到在设置值的过程中函数都调用了M\_set\_result这个函数,而这个函数中使用了call\_once,这个函数第一个参数是once\_flag类型的,作用是这个函数在这段代码中代码仅调用一次,之后调用失败,

void
          _M_do_set(function<_Ptr_type()>* __f, bool* __did_set)
          {
        
            _Ptr_type __res = (*__f)();
            // Notify the caller that we did try to set; if we do not throw an
            // exception, the caller will be aware that it did set (e.g., see
            // _M_set_result).
    	*__did_set = true;
            _M_result.swap(__res); // nothrow
          }

我们可以看到如果成功执行了call\_once,也就是第一次调用call\_once,就会使dis\_set为true,从而进入第一个判断条件,但是第后面调用的时候就会进入第二个判断条件,直接抛出一个future\_error的错误,这也是一个抛错的时机 下面是一个demo

void test(std::promise<int>& para){
        
        para.set_value(10);
        para.set_value(20);
        return;
    }
    
    int main(){
        
        std::promise<int> pro;
        std::future<int> T = pro.get_future();
        std::thread fun(test, std::ref(pro));
        fun.join();
        cout << T.get() << endl;
        return 0;
    }

terminate called after throwing an instance of 'std::future_error'
      what():  std::future_error: Promise already satisfied
    [1]    12731 abort      ./a.out

随着两次的set\_value 当然会抛错了.

再来看一种情况

template<typename _Tp>
      static void
      _S_check(const shared_ptr<_Tp>& __p)
      {
        
        if (!static_cast<bool>(__p))
          __throw_future_error((int)future_errc::no_state);
      }

我们可以看到这个函数中也会抛出future\_error 且判断条件只有一句,很巧,找到了另一个成员函数

bool
    valid() const noexcept {
         return static_cast<bool>(_M_state); }

在官网上我们找到了这样一段解释

> Checks if the future refers to a shared state.  
> This is the case only for futures that were not default-constructed or moved from (i.e. returned by std::promise::get\_future(), std::packaged\_task::get\_future() or std::async()) until the first time get() or share() is called.  
> The behavior is undefined if any member function other than the destructor, the move-assignment operator, or valid is called on a future that does not refer to shared state (although implementations are encouraged to throw std::future\_error indicating no\_state in this case). It is valid to move from a future object for which valid() is false

> 检测future是否指向共享状态  
> 这仅适用于为非默认构造的或是在第一次执行get或者share之前未移动的future(返回 std::promise::get\_future(), std::packaged\_task::get\_future() 或者std::async()算是移动),  
> 如果在未引用共享状态的将来调用除析构函数，移动运算符或valid以外的任何成员函数，则该行为未定义(),尽管在这种情况下有些实现会鼓励抛出一个future\_error来指示no\_state,从valid()返回为false的对象移动的对象是有效的.

下面一个简单的小demo来模拟这种情况

void test(std::promise<int>& para){
        
        para.set_value(10);
        //para.set_value(20);
        return;
    }
    
    int main(){
        
        std::promise<int> pro;
        std::future<int> T = pro.get_future();
        std::thread fun(test, std::ref(pro));
        fun.join();
        cout << T.get() << endl;
        auto Error_T = pro.get_future();
        return 0;
    }

10
    terminate called after throwing an instance of 'std::future_error'
      what():  std::future_error: Future already retrieved
    [1]    14518 abort      ./a.out

在get后执行了get\_future 抛出了错误

参考  
https://en.cppreference.com/w/cpp/thread/future\_error

[20200722103954474.png]: /images/20230209/6083e458eb164b6b8f8e0508ab3a0b87.png