I am using multithreading in my QT program. I need to pass data to the worker object that lives in the worker thread from the main gui thread. I created a setData function in a QObject subclass to pass all the necessary data from the main gui thread. However I verified the function is called from the main thread by looking at QThread::currentThreadId() in the setData function. Even though the worker object function is called from the main thread does this ensure that the worker thread still has its own copy of the data as is required for a reentrant class? Keep in mind this is happening before the worker thread is started.
Also if basic data types are used in a class without dynamic memory and no static global variables is that class reentrant as long as all of its other member data is reentrant? (it's got reentrant data members like qstrings, qlists etc in addition the the basic ints bools etc)
Thanks for the help
Edited new content:
My main question was simply is it appropriate to call a QObject subclass method living in another thread from the main gui thread in order to pass my data to the worker thread to be worked on (in my case custom classes containing backup job information for long-pending file scans and copies for data backup). The data pass all happens before the thread is started so there's no danger of both threads modifying the data at once (I think but I'm no multithreading expert...) It sounds like the way to do this from your post is to use a signal from the main thread to a slot in the worker thread to pass the data. I have confirmed my data backup jobs are reentrant so all I need to do is assure that the worker thread works on its own instances of these classes. Also the transfer of data currently done by calling the QObject subclass method is done before the worker thread starts - does this prevent race conditions and is it safe?
Also here under the section "Accessing QObject Subclasses from Other Threads" it looks a little dangerous to use slots in the QObject subclass...
OK here's the code I've been busy recently...
Edited With Code:
void Replicator::advancedAllBackup()
{
updateStatus("<font color = \"green\">Starting All Advanced Backups</font>");
startBackup();
worker = new Worker;
worker->moveToThread(workerThread);
setupWorker(normal);
QList<BackupJob> jobList;
for (int backupCount = 0; backupCount < advancedJobs.size(); backupCount++)
jobList << advancedJobs[backupCount];
worker->setData(jobList);
workerThread->start();
}
The startBackup function sets some booleans and updates the gui.
the setupWorker function connects all signals and slots for the worker thread and worker object.
the setData function sets the worker job list data to that of the backend and is called before the thread starts so there is no concurrency.
Then we start the thread and it does its work.
And here's the worker code:
void setData(QList<BackupJob> jobs) { this->jobs = jobs; }
So my question is: is this safe?
There are some misconceptions in your question.
Reentrancy and multithreading are orthogonal concepts. Single-threaded code can be easily forced to cope with reentrancy - and is as soon as you reenter the event loop (thus you shouldn't).
The question you are asking, with correction, is thus: Are the class's methods thread-safe if the data members support multithreaded access? The answer is yes. But it's a mostly useless answer, because you're mistaken that the data types you use support such access. They most likely don't!
In fact, you're very unlikely to use multithread-safe data types unless you explicitly seek them out. POD types aren't, most of the C++ standard types aren't, most Qt types aren't either. Just so that there are no misunderstandings: a QString is not multithread-safe data type! The following code is has undefined behavior (it'll crash, burn and send an email to your spouse that appears to be from an illicit lover):
QString str{"Foo"};
for (int i = 0; i < 1000; ++i)
QtConcurrent::run([&]{ str.append("bar"); });
The follow up questions could be:
Are my data members supporting multithreaded access? I thought they did.
No, they aren't unless you show code that proves otherwise.
Do I even need to support multithreaded access?
Maybe. But it's much easier to avoid the need for it entirely.
The likely source of your confusion in relation to Qt types is their implicit sharing semantics. Thankfully, their relation to multithreading is rather simple to express:
Any instance of a Qt implicitly shared class can be accessed from any one thread at a given time. Corollary: you need one instance per thread. Copy your object, and use each copy in its own thread - that's perfectly safe. These instances may share data initially, and Qt will make sure that any copy-on-writes are done thread-safely for you.
Sidebar: If you use iterators or internal pointers to data on non-const instances, you must forcibly detach() the object before constructing the iterators/pointers. The problem with iterators is that they become invalidated when an object's data is detached, and detaching can happen in any thread where the instance is non-const - so at least one thread will end up with invalid iterators. I won't talk any more of this, the takeaway is that implicitly shared data types are tricky to implement and use safely. With C++11, there's no need for implicit sharing anymore: they were a workaround for the lack of move semantics in C++98.
What does it mean, then? It means this:
// Unsafe: str1 potentially accessed from two threads at once
QString str1{"foo"};
QtConcurrent::run([&]{ str1.apppend("bar"); });
str1.append("baz");
// Safe: each instance is accessed from one thread only
QString str1{"foo"};
QString str2{str1};
QtConcurrent::run([&]{ str1.apppend("bar"); });
str2.append("baz");
The original code can be fixed thus:
QString str{"Foo"};
for (int i = 0; i < 1000; ++i)
QtConcurrent::run([=]() mutable { str.append("bar"); });
This isn't to say that this code is very useful: the modified data is lost when the functor is destructed within the worker thread. But it serves to illustrate how to deal with Qt value types and multithreading. Here's why it works: copies of str are taken when each instance of the functor is constructed. This functor is then passed to a worker thread to execute, where its copy of the string is appended to. The copy initially shares data with the str instance in the originating thread, but QString will thread-safely duplicate the data. You could write out the functor explicitly to make it clear what happens:
QString str{"Foo"};
struct Functor {
QString str;
Functor(const QString & str) : str{str} {}
void operator()() {
str.append("bar");
}
};
for (int i = 0; i < 1000; ++i)
QtConcurrent::run(Functor(str));
How do we deal with passing data using Qt types in and out of a worker object? All communication with the object, when it is in the worker thread, must be done via signals/slots. Qt will automatically copy the data for us in a thread-safe manner so that each instance of a value is ever only accessed in one thread only. E.g.:
class ImageSource : public QObject {
QImage render() {
QImage image{...};
QPainter p{image};
...
return image;
}
public:
Q_SIGNAL newImage(const QImage & image);
void makeImage() {
QtConcurrent::run([this]{
emit newImage(render());
});
}
};
int main(int argc, char ** argv) {
QApplication app...;
ImageSource source;
QLabel label;
label.show();
connect(source, &ImageSource::newImage, &label, [&](const QImage & img){
label.setPixmap(QPixmap::fromImage(img));
});
source.makeImage();
return app.exec();
}
The connection between the source's signal and the label's thread context is automatic. The signal happens to be emitted in a worker thread in the default thread pool. At the time of signal emission, the source and target threads are compared, and if different, the functor will be wrapped in an event, the event posted the label, and the label's QObject::event will run the functor that sets the pixmap. This is all thread-safe and leverages Qt to make it almost effortless. The target thread context &label is critically important: without it, the functor would run in the worker thread, not the UI thread.
Note that we didn't even have to move the object to a worker thread: in fact, moving a QObject to a worker thread should be avoided unless the object does need to react to events and does more than merely generate a piece of data. You'd typically want to move e.g. objects that deal with communications, or complex application controllers that are abstracted from their UI. Mere generation of data can be usually done using QtConcurrent::run using a signal to abstract away the thread-safety magic of extracting the data from the worker thread to another thread.
In order to use Qt's mechanisms for passing data between threads with queues, you cannot call the object's function directly. You need to either use the signal/slot mechanism, or you can use the QMetaObject::invokeMethod call:
QMetaObject::invokeMethod(myObject, "mySlotFunction",
Qt::QueuedConnection,
Q_ARG(int, 42));
This will only work if both the sending and receiving objects have event queues running - i.e. a main or QThread based thread.
For the other part of your question, see the Qt docs section on reentrancy:
http://doc.qt.io/qt-4.8/threads-reentrancy.html#reentrant
Many Qt classes are reentrant, but they are not made thread-safe,
because making them thread-safe would incur the extra overhead of
repeatedly locking and unlocking a QMutex. For example, QString is
reentrant but not thread-safe. You can safely access different
instances of QString from multiple threads simultaneously, but you
can't safely access the same instance of QString from multiple threads
simultaneously (unless you protect the accesses yourself with a
QMutex).
#include<iostream>
#include<vector>
#include<thread>
#include<string>
using namespace std;
vector<string> s;
void add()
{
while(true)
{
getchar();
s.push_back("added");
}
}
void show()
{
while(true)
{
//cout<<"";
while(!s.empty())
{
cout<<(*s.begin())<<endl;
s.erase(s.begin());
}
}
}
int main()
{
thread one(add);
thread two(show);
one.join();
two.join();
}
In debug mode there is no such a problem. In release mode if the comment line is uncommented it works again. But with just like this, there is a problem. What is the problem?
std::vector (as any other std:: container) is not generally thread-safe. It means that concurrent modifying access to the same vector from multiple thread is generally not supported. What that means is that while you can call non-modifying functions of the vector from many threads at the same time (for instance, you can call begin() and end() with no problems), modification functions should have exclusive access to the vector object. To achieve this exclusivity, you need to use thread-synchronization primitives to 'signal' your intention to obtain exclusive access to the vector, perform your modification and than 'signal' that exclusive access is no longer need.
Note, this is not enough to perform that sort of routine when you modify (insert) data to the vector. You will also have to do the same dance when you read data from the vector, since modifications need exclusive access, and even the read will violate this exclusivity. The non-technical term I've used here, 'signalling', has a technical counterpart - it is called critical section. Here we say that you 'enter critical section' and 'leave critical section'.
There a more than one way to enter and leave critical section. The stapples of this are so-called mutexes, and they should be enough for your learning. Just keep in mind there are other ways as well, which you'll learn in the due course.
I'm writing a C++11 class Foo, and I want to give each instance its own thread-local storage of type Bar. That is to say, I want one Bar to be allocated per thread and per Foo instance.
If I were using pthreads, Foo would have a nonstatic member of type pthread_key_t, which Foo's constructor would initialize with pthread_key_create() and Foo's destructor would free with pthread_key_delete(). Or if I were writing for Microsoft Windows only, I could do something similar with TlsAlloc() and TlsFree(). Or if I were using Boost.Thread, Foo would have a nonstatic member of type boost::thread_specific_ptr.
In reality, however, I am trying to write portable C++11. C++11's thread_local keyword does not apply to nonstatic data members. So it's fine if you want one Bar per thread, but not if you want one Bar per thread per Foo.
So as far as I can tell, I need to define a thread-local map from Foos to Bars, and then deal with the question of how to clean up appropriately whenever a Foo is destroyed. But before I undertake that, I'm posting here in the hope that someone will stop me and say "There's an easier way."
(Btw, the reason I'm not using either pthread_key_create() or boost::thread_specific_ptr is because, if I understand correctly, they assume that all threads will be spawned using pthreads or Boost.Thread respectively. I don't want to make any assumptions about how the users of my code will spawn threads.)
You would like Foo to contain a thread_local variable of type Bar. Since, as noted, thread_local cannot apply to a data member, we have to do something more indirect. The underlying behavior will be for N instances of Bar to exist for each instance of Foo, where N is the number of threads in existence.
Here is a somewhat inefficient way of doing it. With more code, it could be made faster. Basically, each Foo will contain a TLS map.
#include <unordered_map>
class Bar { ... };
class Foo {
private:
static thread_local std::unordered_map<Foo*, Bar> tls;
public:
// All internal member functions must use this too.
Bar *get_bar() {
auto I = tls.find(this);
if (I != tls.end())
return &I->second;
auto II = tls.emplace(this, Bar()); // Could use std::piecewise_construct here...
return &II->second.second;
}
};
I'd like to know how are mutex created on Linux? I figured out, that pthread_mutex_init() doesn't change value of pthread_mutex_t variable, so how it "create" mutex?
Does it mark this variable as some kind of system resource or what?
I was implementing R-value constructor for class, which have a pthread_mutex_t field in it's body and I don't know how to move mutex frome one class to another...
You can see what pthread_mutex_init does here (warning, you brain will hurt).
It does memset() the mutex.
However, mutexes are implemented on top of the futex calls. This works on memory addresses, i.e.
the address of one of the pthread_mutex_t members is used as a system resource.
This means you cannot copy/move a pthread_mutex_t.
It seems like you want to pass ownership of the mutex to another class. Are you sure that is the correct way to solve your problem? If you absolutely need to do it though, you could create an auto_ptr to pass ownership around:
class A
{
A(const A & other) mutex(other.mutex) { /* ... */ }
auto_ptr<pthread_mutex_t> mutex;
}
Recently I tried to Access a textbox from a thread (other than the UI thread) and an exception was thrown. It said something about the "code not being thread safe" and so I ended up writing a delegate (sample from MSDN helped) and calling it instead.
But even so I didn't quite understand why all the extra code was necessary.
Update:
Will I run into any serious problems if I check
Controls.CheckForIllegalCrossThread..blah =true
Eric Lippert has a nice blog post entitled What is this thing you call "thread safe"? about the definition of thread safety as found of Wikipedia.
3 important things extracted from the links :
“A piece of code is thread-safe if it functions correctly during
simultaneous execution by multiple threads.”
“In particular, it must satisfy the need for multiple threads to
access the same shared data, …”
“…and the need for a shared piece of data to be accessed by only one
thread at any given time.”
Definitely worth a read!
In the simplest of terms threadsafe means that it is safe to be accessed from multiple threads. When you are using multiple threads in a program and they are each attempting to access a common data structure or location in memory several bad things can happen. So, you add some extra code to prevent those bad things. For example, if two people were writing the same document at the same time, the second person to save will overwrite the work of the first person. To make it thread safe then, you have to force person 2 to wait for person 1 to complete their task before allowing person 2 to edit the document.
Wikipedia has an article on Thread Safety.
This definitions page (you have to skip an ad - sorry) defines it thus:
In computer programming, thread-safe describes a program portion or routine that can be called from multiple programming threads without unwanted interaction between the threads.
A thread is an execution path of a program. A single threaded program will only have one thread and so this problem doesn't arise. Virtually all GUI programs have multiple execution paths and hence threads - there are at least two, one for processing the display of the GUI and handing user input, and at least one other for actually performing the operations of the program.
This is done so that the UI is still responsive while the program is working by offloading any long running process to any non-UI threads. These threads may be created once and exist for the lifetime of the program, or just get created when needed and destroyed when they've finished.
As these threads will often need to perform common actions - disk i/o, outputting results to the screen etc. - these parts of the code will need to be written in such a way that they can handle being called from multiple threads, often at the same time. This will involve things like:
Working on copies of data
Adding locks around the critical code
Opening files in the appropriate mode - so if reading, don't open the file for write as well.
Coping with not having access to resources because they're locked by other threads/processes.
Simply, thread-safe means that a method or class instance can be used by multiple threads at the same time without any problems occurring.
Consider the following method:
private int myInt = 0;
public int AddOne()
{
int tmp = myInt;
tmp = tmp + 1;
myInt = tmp;
return tmp;
}
Now thread A and thread B both would like to execute AddOne(). but A starts first and reads the value of myInt (0) into tmp. Now for some reason, the scheduler decides to halt thread A and defer execution to thread B. Thread B now also reads the value of myInt (still 0) into it's own variable tmp. Thread B finishes the entire method so in the end myInt = 1. And 1 is returned. Now it's Thread A's turn again. Thread A continues. And adds 1 to tmp (tmp was 0 for thread A). And then saves this value in myInt. myInt is again 1.
So in this case the method AddOne() was called two times, but because the method was not implemented in a thread-safe way the value of myInt is not 2, as expected, but 1 because the second thread read the variable myInt before the first thread finished updating it.
Creating thread-safe methods is very hard in non-trivial cases. And there are quite a few techniques. In Java you can mark a method as synchronized, this means that only one thread can execute that method at a given time. The other threads wait in line. This makes a method thread-safe, but if there is a lot of work to be done in a method, then this wastes a lot of space. Another technique is to 'mark only a small part of a method as synchronized' by creating a lock or semaphore, and locking this small part (usually called the critical section). There are even some methods that are implemented as lock-less thread-safe, which means that they are built in such a way that multiple threads can race through them at the same time without ever causing problems, this can be the case when a method only executes one atomic call. Atomic calls are calls that can't be interrupted and can only be done by one thread at a time.
In real world example for the layman is
Let's suppose you have a bank account with the internet and mobile banking and your account have only $10.
You performed transfer balance to another account using mobile banking, and the meantime, you did online shopping using the same bank account.
If this bank account is not threadsafe, then the bank allows you to perform two transactions at the same time and then the bank will become bankrupt.
Threadsafe means that an object's state doesn't change if simultaneously multiple threads try to access the object.
You can get more explanation from the book "Java Concurrency in Practice":
A class is thread‐safe if it behaves correctly when accessed from multiple threads, regardless of the scheduling or interleaving of the execution of those threads by the runtime environment, and with no additional synchronization or other coordination on the part of the calling code.
A module is thread-safe if it guarantees it can maintain its invariants in the face of multi-threaded and concurrence use.
Here, a module can be a data-structure, class, object, method/procedure or function. Basically scoped piece of code and related data.
The guarantee can potentially be limited to certain environments such as a specific CPU architecture, but must hold for those environments. If there is no explicit delimitation of environments, then it is usually taken to imply that it holds for all environments that the code can be compiled and executed.
Thread-unsafe modules may function correctly under mutli-threaded and concurrent use, but this is often more down to luck and coincidence, than careful design. Even if some module does not break for you under, it may break when moved to other environments.
Multi-threading bugs are often hard to debug. Some of them only happen occasionally, while others manifest aggressively - this too, can be environment specific. They can manifest as subtly wrong results, or deadlocks. They can mess up data-structures in unpredictable ways, and cause other seemingly impossible bugs to appear in other remote parts of the code. It can be very application specific, so it is hard to give a general description.
Thread safety: A thread safe program protects it's data from memory consistency errors. In a highly multi-threaded program, a thread safe program does not cause any side effects with multiple read/write operations from multiple threads on same objects. Different threads can share and modify object data without consistency errors.
You can achieve thread safety by using advanced concurrency API. This documentation page provides good programming constructs to achieve thread safety.
Lock Objects support locking idioms that simplify many concurrent applications.
Executors define a high-level API for launching and managing threads. Executor implementations provided by java.util.concurrent provide thread pool management suitable for large-scale applications.
Concurrent Collections make it easier to manage large collections of data, and can greatly reduce the need for synchronization.
Atomic Variables have features that minimize synchronization and help avoid memory consistency errors.
ThreadLocalRandom (in JDK 7) provides efficient generation of pseudorandom numbers from multiple threads.
Refer to java.util.concurrent and java.util.concurrent.atomic packages too for other programming constructs.
Producing Thread-safe code is all about managing access to shared mutable states. When mutable states are published or shared between threads, they need to be synchronized to avoid bugs like race conditions and memory consistency errors.
I recently wrote a blog about thread safety. You can read it for more information.
You are clearly working in a WinForms environment. WinForms controls exhibit thread affinity, which means that the thread in which they are created is the only thread that can be used to access and update them. That is why you will find examples on MSDN and elsewhere demonstrating how to marshall the call back onto the main thread.
Normal WinForms practice is to have a single thread that is dedicated to all your UI work.
I find the concept of http://en.wikipedia.org/wiki/Reentrancy_%28computing%29 to be what I usually think of as unsafe threading which is when a method has and relies on a side effect such as a global variable.
For example I have seen code that formatted floating point numbers to string, if two of these are run in different threads the global value of decimalSeparator can be permanently changed to '.'
//built in global set to locale specific value (here a comma)
decimalSeparator = ','
function FormatDot(value : real):
//save the current decimal character
temp = decimalSeparator
//set the global value to be
decimalSeparator = '.'
//format() uses decimalSeparator behind the scenes
result = format(value)
//Put the original value back
decimalSeparator = temp
To understand thread safety, read below sections:
4.3.1. Example: Vehicle Tracker Using Delegation
As a more substantial example of delegation, let's construct a version of the vehicle tracker that delegates to a thread-safe class. We store the locations in a Map, so we start with a thread-safe Map implementation, ConcurrentHashMap. We also store the location using an immutable Point class instead of MutablePoint, shown in Listing 4.6.
Listing 4.6. Immutable Point class used by DelegatingVehicleTracker.
class Point{
public final int x, y;
public Point() {
this.x=0; this.y=0;
}
public Point(int x, int y) {
this.x = x;
this.y = y;
}
}
Point is thread-safe because it is immutable. Immutable values can be freely shared and published, so we no longer need to copy the locations when returning them.
DelegatingVehicleTracker in Listing 4.7 does not use any explicit synchronization; all access to state is managed by ConcurrentHashMap, and all the keys and values of the Map are immutable.
Listing 4.7. Delegating Thread Safety to a ConcurrentHashMap.
public class DelegatingVehicleTracker {
private final ConcurrentMap<String, Point> locations;
private final Map<String, Point> unmodifiableMap;
public DelegatingVehicleTracker(Map<String, Point> points) {
this.locations = new ConcurrentHashMap<String, Point>(points);
this.unmodifiableMap = Collections.unmodifiableMap(locations);
}
public Map<String, Point> getLocations(){
return this.unmodifiableMap; // User cannot update point(x,y) as Point is immutable
}
public Point getLocation(String id) {
return locations.get(id);
}
public void setLocation(String id, int x, int y) {
if(locations.replace(id, new Point(x, y)) == null) {
throw new IllegalArgumentException("invalid vehicle name: " + id);
}
}
}
If we had used the original MutablePoint class instead of Point, we would be breaking encapsulation by letting getLocations publish a reference to mutable state that is not thread-safe. Notice that we've changed the behavior of the vehicle tracker class slightly; while the monitor version returned a snapshot of the locations, the delegating version returns an unmodifiable but “live” view of the vehicle locations. This means that if thread A calls getLocations and thread B later modifies the location of some of the points, those changes are reflected in the Map returned to thread A.
4.3.2. Independent State Variables
We can also delegate thread safety to more than one underlying state variable as long as those underlying state variables are independent, meaning that the composite class does not impose any invariants involving the multiple state variables.
VisualComponent in Listing 4.9 is a graphical component that allows clients to register listeners for mouse and keystroke events. It maintains a list of registered listeners of each type, so that when an event occurs the appropriate listeners can be invoked. But there is no relationship between the set of mouse listeners and key listeners; the two are independent, and therefore VisualComponent can delegate its thread safety obligations to two underlying thread-safe lists.
Listing 4.9. Delegating Thread Safety to Multiple Underlying State Variables.
public class VisualComponent {
private final List<KeyListener> keyListeners
= new CopyOnWriteArrayList<KeyListener>();
private final List<MouseListener> mouseListeners
= new CopyOnWriteArrayList<MouseListener>();
public void addKeyListener(KeyListener listener) {
keyListeners.add(listener);
}
public void addMouseListener(MouseListener listener) {
mouseListeners.add(listener);
}
public void removeKeyListener(KeyListener listener) {
keyListeners.remove(listener);
}
public void removeMouseListener(MouseListener listener) {
mouseListeners.remove(listener);
}
}
VisualComponent uses a CopyOnWriteArrayList to store each listener list; this is a thread-safe List implementation particularly suited for managing listener lists (see Section 5.2.3). Each List is thread-safe, and because there are no constraints coupling the state of one to the state of the other, VisualComponent can delegate its thread safety responsibilities to the underlying mouseListeners and keyListeners objects.
4.3.3. When Delegation Fails
Most composite classes are not as simple as VisualComponent: they have invariants that relate their component state variables. NumberRange in Listing 4.10 uses two AtomicIntegers to manage its state, but imposes an additional constraint—that the first number be less than or equal to the second.
Listing 4.10. Number Range Class that does Not Sufficiently Protect Its Invariants. Don't do this.
public class NumberRange {
// INVARIANT: lower <= upper
private final AtomicInteger lower = new AtomicInteger(0);
private final AtomicInteger upper = new AtomicInteger(0);
public void setLower(int i) {
//Warning - unsafe check-then-act
if(i > upper.get()) {
throw new IllegalArgumentException(
"Can't set lower to " + i + " > upper ");
}
lower.set(i);
}
public void setUpper(int i) {
//Warning - unsafe check-then-act
if(i < lower.get()) {
throw new IllegalArgumentException(
"Can't set upper to " + i + " < lower ");
}
upper.set(i);
}
public boolean isInRange(int i){
return (i >= lower.get() && i <= upper.get());
}
}
NumberRange is not thread-safe; it does not preserve the invariant that constrains lower and upper. The setLower and setUpper methods attempt to respect this invariant, but do so poorly. Both setLower and setUpper are check-then-act sequences, but they do not use sufficient locking to make them atomic. If the number range holds (0, 10), and one thread calls setLower(5) while another thread calls setUpper(4), with some unlucky timing both will pass the checks in the setters and both modifications will be applied. The result is that the range now holds (5, 4)—an invalid state. So while the underlying AtomicIntegers are thread-safe, the composite class is not. Because the underlying state variables lower and upper are not independent, NumberRange cannot simply delegate thread safety to its thread-safe state variables.
NumberRange could be made thread-safe by using locking to maintain its invariants, such as guarding lower and upper with a common lock. It must also avoid publishing lower and upper to prevent clients from subverting its invariants.
If a class has compound actions, as NumberRange does, delegation alone is again not a suitable approach for thread safety. In these cases, the class must provide its own locking to ensure that compound actions are atomic, unless the entire compound action can also be delegated to the underlying state variables.
If a class is composed of multiple independent thread-safe state variables and has no operations that have any invalid state transitions, then it can delegate thread safety to the underlying state variables.