[英]Error about std::promise in C++
I am trying to pass my class instance into threads and the return the processed objects from threads. 我试图将我的类实例传递给线程,并从线程返回处理后的对象。 I've googled about C++ multithreading, and found that std::promising can be helpful.
我搜索了C ++多线程,发现std :: promising可能会有所帮助。
However, I am stuck at the very beginning. 但是,我一开始就陷入困境。 Here is my code:
这是我的代码:
void callerFunc()
{
//...
std::promise<DataWareHouse> data_chunks;
// DataWareHouse is my customized class
//data_chunks has a vector<vector<double>> member variable
std::thread(&run_thread,data_chunks);
// ............
}
void run_thread(std::promise<DataWareHouse> data_chunks)
{
// ...
vector<vector<double>> results;
// ...
data_chunks.set_value(results);
}
The above code generates an error: 上面的代码生成错误:
`error C2248: 'std::promise<_Ty>::promise' : cannot access private member declared in class 'std::promise<_Ty>'`
May I know what am I wrong and how to fix it? 我可以知道我在做什么以及如何解决吗?
Many thanks. 非常感谢。 :-)
:-)
Your first problem is that you are using std::thread
-- std::thread
is a low level class which you should build higher abstractions up on. 您的第一个问题是您正在使用
std::thread
- std::thread
是一个低级类,应在其上构建更高的抽象。 Threading is newly standardized in C++ in C++11, and all of the rough parts are not filed off yet. 线程在C ++ 11中的C ++中是新标准化的,并且所有粗略部分都尚未提出。
There are three different patterns for using threading in C++11 that might be useful to you. 在C ++ 11中使用线程的三种不同模式可能对您有用。
First, std::async
. 首先,
std::async
。 Second, std::thread
mixed with std::packaged_task
. 其次,
std::thread
与std::packaged_task
混合。 And third, dealing with std::thread
and std::promise
in the raw. 第三,处理原始的
std::thread
和std::promise
。
I'll illustrate the third, which is the lowest level and most dangerous, because that is what you asked for. 我将说明第三个级别,它是最低级别和最危险的级别,因为这是您所要求的。 I would advise looking at the first two options.
我建议您看一下前两个选项。
#include <future>
#include <vector>
#include <iostream>
typedef std::vector<double> DataWareHouse;
void run_thread(std::promise<DataWareHouse> data_chunks)
{
DataWareHouse results;
results.push_back( 3.14159 );
data_chunks.set_value(results);
}
std::future<DataWareHouse> do_async_work()
{
std::promise<DataWareHouse> data_chunks;
std::future<DataWareHouse> retval = data_chunks.get_future();
// DataWareHouse is my customized class
//data_chunks has a vector<vector<double>> member variable
std::thread t = std::thread(&run_thread,std::move(data_chunks));
t.detach(); // do this or seg fault
return retval;
}
int main() {
std::future<DataWareHouse> result = do_async_work();
DataWareHouse vec = result.get(); // block and get the data
for (double d: vec) {
std::cout << d << "\n";
}
}
With std::async
, you'd have a function returning DataWareHouse
, and it would return a std::future<DataWareHouse>
directly. 使用
std::async
,您将具有一个返回DataWareHouse
的函数,它将直接返回std::future<DataWareHouse>
。
With std::packaged_task<>
, it would take your run_thread
and turn it into a packaged_task
that can be executed, and a std::future
extracted from it. 使用
std::packaged_task<>
,它将占用您的run_thread
并将其转换为可以执行的packaged_task
,并从中提取std::future
。
std::promise<>
is not copyable, and in calling run_thread()
you are implicitly trying to invoke the copy constructor. std::promise<>
是不可复制的,并且在调用run_thread()
您隐式地尝试调用复制构造函数。 The error message is telling you that you cannot use the copy constructor since it is marked private. 该错误消息告诉您,由于标记为私有,因此无法使用复制构造函数。
You need to pass a promise by reference ( std::promise<DataWareHouse> &
). 您需要通过引用传递一个承诺(
std::promise<DataWareHouse> &
)。 This is safe if callerFunc()
is guaranteed not to return until run_thread()
is finished with the object (otherwise you will be using a reference to a destroyed stack-allocated object, and I don't have to explain why that's bad). 如果这是安全的
callerFunc()
保证不会返回,直到run_thread()
完成与对象(否则你将使用到销毁堆栈分配对象的引用,而我也没有解释为什么这是很糟糕)。
You're trying to pass the promise to the thread by value; 您正在尝试通过值将承诺传递给线程; but you need to pass by reference to get the results back to the caller's promise.
但是您需要通过引用传递结果,以将结果返回给呼叫者的承诺。
std::promise
is uncopyable, to prevent this mistake. std::promise
无法复制,以防止发生此错误。
std::thread(&run_thread,std::ref(data_chunks));
^^^^^^^^
void run_thread(std::promise<DataWareHouse> & data_chunks)
^
The error is telling you you cannot copy an std::promise
, which you do here: 错误提示您无法复制
std::promise
,您可以在此处执行以下操作:
void run_thread(std::promise<DataWareHouse> data_chunks)
and here: 和这里:
std::thread(&run_thread,data_chunks); // makes copy of data_chunks
You should pass a reference: 您应该传递参考:
void run_thread(std::promise<DataWareHouse>& data_chunks);
// ^
And then pass an std::reference_wrapper
to the thread, otherwise it too will attempt to copy the promise. 然后将
std::reference_wrapper
传递给线程,否则它也会尝试复制promise。 This is easily done with std::ref
: 这可以通过
std::ref
轻松完成:
std::thread(&run_thread, std::ref(data_chunks));
// ^^^^^^^^
Obviously data_chunks
must be alive until the thread finished running, so you will have to join the thread in callerFunc()
. 显然,在线程完成运行之前,
data_chunks
必须处于活动状态,因此您必须在callerFunc()
加入线程。
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