[英]std::thread constructor with no parameter
According to cppreference.com , the std::thread
constructor with no parameter means: 根据cppreference.com ,没有参数的std::thread
构造函数意味着:
Creates new thread object which does not represent a thread. 创建不代表线程的新线程对象。
My questions are: 我的问题是:
thread
using this constructor, how can we "assign" a thread function later? 如果我们使用这个构造函数创建一个thread
,我们如何在以后“分配”一个线程函数? thread
. 为什么我们没有“run(function_address)”方法,以便在没有参数构造时,我们可以为该thread
指定一个“运行”的函数。 thread
with a callable parameter (function, functors, etc.) but call a "run()" method to actually execute the thread later. 或者,我们可以使用可调用参数(函数,函子等)构造一个thread
,但调用“run()”方法以便稍后实际执行该线程。 Why is std::thread
not designed in this way? 为什么std::thread
没有这样设计? Your question suggests there might be some confusion and it would be helpful to clearly separate the ideas of a thread of execution from the std::thread
type, and to separate both from the idea of a "thread function". 你的问题表明可能存在一些混淆,将std::thread
类型的执行线程的思想清楚地分开,并将两者与“线程函数”的概念分开是有帮助的。
std::thread
can be associated with a thread of execution , or it can be "empty" and not refer to any thread of execution . std::thread
类型的对象可以与执行线程相关联,或者它可以是“空”而不是指任何执行线程 。 std::thread
object. 任何函数都可以通过将它传递给std::thread
对象的构造函数在新的执行线程中运行。
- why do we need this constructor? 为什么我们需要这个构造函数?
To construct the empty state that doesn't refer to a thread of execution . 构造不引用执行线程的空状态。 You might want to have a member variable of a class that is a std::thread
, but not want to associate it with a thread of execution right away. 您可能希望拥有一个std::thread
类的成员变量,但不想立即将它与执行线程相关联。 So you default construct it, and then later launch a new thread of execution and associate it with the std::thread
member variable. 所以你默认构造它,然后启动一个新的执行线程并将它与std::thread
成员变量相关联。 Or you might want to do: 或者您可能想要这样做:
std::thread t;
if (some_condition) {
t = std::thread{ func1, arg1 };
}
else {
auto result = some_calculation();
t = std::thread{ func2, arg2, result };
}
The default constructor allows the object t
to be created without launching a new thread of execution until needed. 默认构造函数允许在不需要启动新执行线程的情况下创建对象t
。
And if we create a thread using this constructor, how can we "assign" a thread function later? 如果我们使用这个构造函数创建一个线程,我们如何在以后“分配”一个线程函数?
You "assign" using "assignment" :-) 你使用“作业”“分配”:-)
But you don't assign a "thread function" to it, that is not what std::thread
is for. 但是你没有给它分配一个“线程函数”,这不是std::thread
的用途。 You assign another std::thread
to it: 你为它分配另一个std::thread
:
std::thread t;
std::thread t2{ func, args };
t = std::move(t2);
Think in terms of creating a new thread of execution not "assigning a thread function" to something. 考虑创建一个新的执行线程而不是“为某个东西分配一个线程函数”。 You're not just assigning a function, that's what std::function
would be used for. 你不只是分配一个函数,那就是std::function
将用于什么。 You are requesting the runtime to create a new thread of execution , which will be managed by a std::thread
object. 您正在请求运行时创建一个新的执行线程 ,该std::thread
将由std::thread
对象管理。
- Why don't we have a "run(function_address)" method so that when constructed with no parameter, we can specify a function to "run" for that thread. 为什么我们没有“run(function_address)”方法,以便在没有参数构造时,我们可以为该线程指定一个“运行”的函数。
Because you don't need it. 因为你不需要它。 You start new threads of execution by constructing a std::thread
object with arguments. 您可以通过使用参数构造std::thread
对象来启动新的执行std::thread
。 If you want that thread of execution to be associated with an existing object then you can do that by move-assigning or swapping. 如果您希望该执行线程与现有对象相关联,则可以通过移动分配或交换来执行此操作。
- Or, we can construct a thread with a callable parameter(function, functors, etc.) but call a "run()" method to actually execute the thread later. 或者,我们可以使用可调用参数(函数,函子等)构造一个线程,但调用“run()”方法以便稍后实际执行该线程。 Why std::thread is not designed in this way? 为什么std :: thread不是这样设计的?
Why should it be designed that way? 为什么要这样设计呢?
The std::thread
type is for managing a thread of execution not holding a callable object for later use. std::thread
类型用于管理不包含可调用对象的执行线程供以后使用。 If you want to create a callable object that can be later run on a new thread of execution there are lots of ways to do that in C++ (using a lambda expression, or std::bind
, or std::function
, or std::packaged_task
, or a custom functor type). 如果你想创建一个可以在新的执行线程上运行的可调用对象,那么在C ++中有很多方法可以做到这一点(使用lambda表达式,或std::bind
,或std::function
,或std::packaged_task
,或自定义函子类型)。 The job of std::thread
is to manage a thread of execution not to hold onto a callable object until you want to call it. std::thread
的工作是管理一个执行线程,直到你想要调用它为止。
The default constructor is provided such that an "empty" thread
object can be created. 提供默认构造函数,以便可以创建“空” thread
对象。 Not all thread
objects will be associated with a thread of execution at the time of construction of said object. 并非所有thread
对象都与构造所述对象时的执行线程相关联。 Consider when the thread
is a member of some type and that type has a default constructor. 考虑何时thread
是某种类型的成员,并且该类型具有默认构造函数。 Consider another case that the thread
type has no concept of a "suspended" thread, ie it can't be created in a suspended state. 考虑另一种情况,即thread
类型没有“挂起”线程的概念,即它不能在挂起状态下创建。
The thread
type doesn't have a "run" method of some sort since the one of the original design decisions (IIRC) was to have a "strong" association between the thread
object and the thread of execution. thread
类型没有某种“运行”方法,因为原始设计决策(IIRC)之一是在thread
对象和执行线程之间建立“强”关联。 Allowing thread
s to be "moved" makes that intent clearer (in my opinion). 允许thread
被“移动”使得该意图更清晰(在我看来)。 Hence moving an instance of a thread
object to an "empty" object is clearer than attempting to "run" a thread
. 因此,将thread
对象的实例移动到“空”对象比尝试“运行” thread
更清楚。
It is conceivable that you can create a wrapper class of some sort that offers the "run" method, but I think this may be a narrower use case, and that can be solved given the API of the std::thread
class . 可以想象你可以创建一个提供“run”方法的某种包装类,但我认为这可能是一个较窄的用例,并且可以在std::thread
类的API下解决 。
EDIT: Maybe lets firsts comment on the very last part: 编辑:也许让我们在最后一部分评论:
3.2 Why std::thread is not designed in this way? 3.2为什么std :: thread不是这样设计的?
I don't know why exactly (there are surely advantages and disatvantages - see Jonathan Wakely's answer for a more details about the rational behind it), but it seems that c++11 std::thread
is modelled much closer to pthreads than eg QT's or Java's QThread/Thread
classes, which might be the source of your confusion. 我不知道究竟是为什么(肯定有优点和缺点 - 请参阅Jonathan Wakely的答案,了解有关它背后的理性的更多细节),但似乎c ++ 11 std::thread
的模型更接近pthreads而不是例如QT或Java的QThread/Thread
类,可能是您混淆的根源。
As to the rest of your questions: 至于你的其他问题:
1.1 why do we need this constructor? 1.1为什么我们需要这个构造函数?
You might want to create a std::thread
variable but don't directly start a thread (eg a class member variable or an element of a static array, es shown by alangab). 您可能想要创建一个std::thread
变量,但不要直接启动一个线程(例如一个类成员变量或一个静态数组的元素,由alangab显示)。 It's not much different to an std::fstream
that can be created without a filename. 与没有文件名创建的std::fstream
没什么不同。
1.2 And if we create a thread using this constructor, how can we "assign" a thread function later? 1.2如果我们使用这个构造函数创建一个线程,我们如何在以后“分配”一个线程函数?
For example: 例如:
std::thread myThread;
// some other code
myThread = std::thread(foo());
- Why don't we have a "run(function_address)" method so that when constructed with no parameter, we can specify a function to "run" for that thread. 为什么我们没有“run(function_address)”方法,以便在没有参数构造时,我们可以为该线程指定一个“运行”的函数。
I don't know, why it was designed like this, but I don't see the benefit a run
method would have compared to above syntax. 我不知道,为什么它是这样设计的,但是我没有看到run
方法与上面的语法相比有什么好处。
3.1 Or, we can construct a thread with a callable parameter(function, functors, etc.) but call a "run()" method to actually execute the thread later. 3.1或者,我们可以使用可调参数(函数,函子等)构造一个线程,但调用“run()”方法以便稍后实际执行该线程。
You can simulate this by creating a lambda or std::function
object and create the thread when you want to run the function. 您可以通过创建lambda或std::function
对象来模拟这一点,并在您想要运行该函数时创建该线程。
auto myLambda = [=]{foo(param1, param2);};
// some other code
std::thread myThread(myLambda);
If you want to use the syntax you describe, I'd recommend to write your own Thread
wrapper class (should only take a few dozen lines of code) that (optionally) also ensures that the thread is either detached or joined upon destruction of the wrapper, which is - in my opinion - the main problem with std::thread
. 如果你想使用你描述的语法,我建议你编写自己的Thread
包装器类(应该只需要几十行代码)(可选)确保线程在拆除时被分离或连接包装器,在我看来 - 这是std::thread
的主要问题。
The default constructor gives you then possibility to create array of threads: 默认构造函数为您提供了创建线程数组的可能性:
thread my_threads[4];
for (int i=0; i<4; i++)
{
thread temp(func,...);
my_threads[i]=move(temp);
}
the thread created with default costructor "become" a "real" thread with the move costructor. 使用默认的costructor创建的线程“成为”移动构造函数的“真实”线程。
You can use thread with standard container if you need/like. 如果需要/喜欢,可以使用标准容器的线程。
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